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Linux/mm/mremap.c

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  1 // SPDX-License-Identifier: GPL-2.0
  2 /*
  3  *      mm/mremap.c
  4  *
  5  *      (C) Copyright 1996 Linus Torvalds
  6  *
  7  *      Address space accounting code   <alan@lxorguk.ukuu.org.uk>
  8  *      (C) Copyright 2002 Red Hat Inc, All Rights Reserved
  9  */
 10 
 11 #include <linux/mm.h>
 12 #include <linux/hugetlb.h>
 13 #include <linux/shm.h>
 14 #include <linux/ksm.h>
 15 #include <linux/mman.h>
 16 #include <linux/swap.h>
 17 #include <linux/capability.h>
 18 #include <linux/fs.h>
 19 #include <linux/swapops.h>
 20 #include <linux/highmem.h>
 21 #include <linux/security.h>
 22 #include <linux/syscalls.h>
 23 #include <linux/mmu_notifier.h>
 24 #include <linux/uaccess.h>
 25 #include <linux/mm-arch-hooks.h>
 26 #include <linux/userfaultfd_k.h>
 27 
 28 #include <asm/cacheflush.h>
 29 #include <asm/tlbflush.h>
 30 
 31 #include "internal.h"
 32 
 33 static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
 34 {
 35         pgd_t *pgd;
 36         p4d_t *p4d;
 37         pud_t *pud;
 38         pmd_t *pmd;
 39 
 40         pgd = pgd_offset(mm, addr);
 41         if (pgd_none_or_clear_bad(pgd))
 42                 return NULL;
 43 
 44         p4d = p4d_offset(pgd, addr);
 45         if (p4d_none_or_clear_bad(p4d))
 46                 return NULL;
 47 
 48         pud = pud_offset(p4d, addr);
 49         if (pud_none_or_clear_bad(pud))
 50                 return NULL;
 51 
 52         pmd = pmd_offset(pud, addr);
 53         if (pmd_none(*pmd))
 54                 return NULL;
 55 
 56         return pmd;
 57 }
 58 
 59 static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
 60                             unsigned long addr)
 61 {
 62         pgd_t *pgd;
 63         p4d_t *p4d;
 64         pud_t *pud;
 65         pmd_t *pmd;
 66 
 67         pgd = pgd_offset(mm, addr);
 68         p4d = p4d_alloc(mm, pgd, addr);
 69         if (!p4d)
 70                 return NULL;
 71         pud = pud_alloc(mm, p4d, addr);
 72         if (!pud)
 73                 return NULL;
 74 
 75         pmd = pmd_alloc(mm, pud, addr);
 76         if (!pmd)
 77                 return NULL;
 78 
 79         VM_BUG_ON(pmd_trans_huge(*pmd));
 80 
 81         return pmd;
 82 }
 83 
 84 static void take_rmap_locks(struct vm_area_struct *vma)
 85 {
 86         if (vma->vm_file)
 87                 i_mmap_lock_write(vma->vm_file->f_mapping);
 88         if (vma->anon_vma)
 89                 anon_vma_lock_write(vma->anon_vma);
 90 }
 91 
 92 static void drop_rmap_locks(struct vm_area_struct *vma)
 93 {
 94         if (vma->anon_vma)
 95                 anon_vma_unlock_write(vma->anon_vma);
 96         if (vma->vm_file)
 97                 i_mmap_unlock_write(vma->vm_file->f_mapping);
 98 }
 99 
100 static pte_t move_soft_dirty_pte(pte_t pte)
101 {
102         /*
103          * Set soft dirty bit so we can notice
104          * in userspace the ptes were moved.
105          */
106 #ifdef CONFIG_MEM_SOFT_DIRTY
107         if (pte_present(pte))
108                 pte = pte_mksoft_dirty(pte);
109         else if (is_swap_pte(pte))
110                 pte = pte_swp_mksoft_dirty(pte);
111 #endif
112         return pte;
113 }
114 
115 static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
116                 unsigned long old_addr, unsigned long old_end,
117                 struct vm_area_struct *new_vma, pmd_t *new_pmd,
118                 unsigned long new_addr, bool need_rmap_locks)
119 {
120         struct mm_struct *mm = vma->vm_mm;
121         pte_t *old_pte, *new_pte, pte;
122         spinlock_t *old_ptl, *new_ptl;
123         bool force_flush = false;
124         unsigned long len = old_end - old_addr;
125 
126         /*
127          * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
128          * locks to ensure that rmap will always observe either the old or the
129          * new ptes. This is the easiest way to avoid races with
130          * truncate_pagecache(), page migration, etc...
131          *
132          * When need_rmap_locks is false, we use other ways to avoid
133          * such races:
134          *
135          * - During exec() shift_arg_pages(), we use a specially tagged vma
136          *   which rmap call sites look for using is_vma_temporary_stack().
137          *
138          * - During mremap(), new_vma is often known to be placed after vma
139          *   in rmap traversal order. This ensures rmap will always observe
140          *   either the old pte, or the new pte, or both (the page table locks
141          *   serialize access to individual ptes, but only rmap traversal
142          *   order guarantees that we won't miss both the old and new ptes).
143          */
144         if (need_rmap_locks)
145                 take_rmap_locks(vma);
146 
147         /*
148          * We don't have to worry about the ordering of src and dst
149          * pte locks because exclusive mmap_sem prevents deadlock.
150          */
151         old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
152         new_pte = pte_offset_map(new_pmd, new_addr);
153         new_ptl = pte_lockptr(mm, new_pmd);
154         if (new_ptl != old_ptl)
155                 spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
156         flush_tlb_batched_pending(vma->vm_mm);
157         arch_enter_lazy_mmu_mode();
158 
159         for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
160                                    new_pte++, new_addr += PAGE_SIZE) {
161                 if (pte_none(*old_pte))
162                         continue;
163 
164                 pte = ptep_get_and_clear(mm, old_addr, old_pte);
165                 /*
166                  * If we are remapping a valid PTE, make sure
167                  * to flush TLB before we drop the PTL for the
168                  * PTE.
169                  *
170                  * NOTE! Both old and new PTL matter: the old one
171                  * for racing with page_mkclean(), the new one to
172                  * make sure the physical page stays valid until
173                  * the TLB entry for the old mapping has been
174                  * flushed.
175                  */
176                 if (pte_present(pte))
177                         force_flush = true;
178                 pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
179                 pte = move_soft_dirty_pte(pte);
180                 set_pte_at(mm, new_addr, new_pte, pte);
181         }
182 
183         arch_leave_lazy_mmu_mode();
184         if (force_flush)
185                 flush_tlb_range(vma, old_end - len, old_end);
186         if (new_ptl != old_ptl)
187                 spin_unlock(new_ptl);
188         pte_unmap(new_pte - 1);
189         pte_unmap_unlock(old_pte - 1, old_ptl);
190         if (need_rmap_locks)
191                 drop_rmap_locks(vma);
192 }
193 
194 #ifdef CONFIG_HAVE_MOVE_PMD
195 static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
196                   unsigned long new_addr, unsigned long old_end,
197                   pmd_t *old_pmd, pmd_t *new_pmd)
198 {
199         spinlock_t *old_ptl, *new_ptl;
200         struct mm_struct *mm = vma->vm_mm;
201         pmd_t pmd;
202 
203         if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK)
204             || old_end - old_addr < PMD_SIZE)
205                 return false;
206 
207         /*
208          * The destination pmd shouldn't be established, free_pgtables()
209          * should have release it.
210          */
211         if (WARN_ON(!pmd_none(*new_pmd)))
212                 return false;
213 
214         /*
215          * We don't have to worry about the ordering of src and dst
216          * ptlocks because exclusive mmap_sem prevents deadlock.
217          */
218         old_ptl = pmd_lock(vma->vm_mm, old_pmd);
219         new_ptl = pmd_lockptr(mm, new_pmd);
220         if (new_ptl != old_ptl)
221                 spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
222 
223         /* Clear the pmd */
224         pmd = *old_pmd;
225         pmd_clear(old_pmd);
226 
227         VM_BUG_ON(!pmd_none(*new_pmd));
228 
229         /* Set the new pmd */
230         set_pmd_at(mm, new_addr, new_pmd, pmd);
231         flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
232         if (new_ptl != old_ptl)
233                 spin_unlock(new_ptl);
234         spin_unlock(old_ptl);
235 
236         return true;
237 }
238 #endif
239 
240 unsigned long move_page_tables(struct vm_area_struct *vma,
241                 unsigned long old_addr, struct vm_area_struct *new_vma,
242                 unsigned long new_addr, unsigned long len,
243                 bool need_rmap_locks)
244 {
245         unsigned long extent, next, old_end;
246         struct mmu_notifier_range range;
247         pmd_t *old_pmd, *new_pmd;
248 
249         old_end = old_addr + len;
250         flush_cache_range(vma, old_addr, old_end);
251 
252         mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
253                                 old_addr, old_end);
254         mmu_notifier_invalidate_range_start(&range);
255 
256         for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
257                 cond_resched();
258                 next = (old_addr + PMD_SIZE) & PMD_MASK;
259                 /* even if next overflowed, extent below will be ok */
260                 extent = next - old_addr;
261                 if (extent > old_end - old_addr)
262                         extent = old_end - old_addr;
263                 old_pmd = get_old_pmd(vma->vm_mm, old_addr);
264                 if (!old_pmd)
265                         continue;
266                 new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
267                 if (!new_pmd)
268                         break;
269                 if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
270                         if (extent == HPAGE_PMD_SIZE) {
271                                 bool moved;
272                                 /* See comment in move_ptes() */
273                                 if (need_rmap_locks)
274                                         take_rmap_locks(vma);
275                                 moved = move_huge_pmd(vma, old_addr, new_addr,
276                                                     old_end, old_pmd, new_pmd);
277                                 if (need_rmap_locks)
278                                         drop_rmap_locks(vma);
279                                 if (moved)
280                                         continue;
281                         }
282                         split_huge_pmd(vma, old_pmd, old_addr);
283                         if (pmd_trans_unstable(old_pmd))
284                                 continue;
285                 } else if (extent == PMD_SIZE) {
286 #ifdef CONFIG_HAVE_MOVE_PMD
287                         /*
288                          * If the extent is PMD-sized, try to speed the move by
289                          * moving at the PMD level if possible.
290                          */
291                         bool moved;
292 
293                         if (need_rmap_locks)
294                                 take_rmap_locks(vma);
295                         moved = move_normal_pmd(vma, old_addr, new_addr,
296                                         old_end, old_pmd, new_pmd);
297                         if (need_rmap_locks)
298                                 drop_rmap_locks(vma);
299                         if (moved)
300                                 continue;
301 #endif
302                 }
303 
304                 if (pte_alloc(new_vma->vm_mm, new_pmd))
305                         break;
306                 next = (new_addr + PMD_SIZE) & PMD_MASK;
307                 if (extent > next - new_addr)
308                         extent = next - new_addr;
309                 move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
310                           new_pmd, new_addr, need_rmap_locks);
311         }
312 
313         mmu_notifier_invalidate_range_end(&range);
314 
315         return len + old_addr - old_end;        /* how much done */
316 }
317 
318 static unsigned long move_vma(struct vm_area_struct *vma,
319                 unsigned long old_addr, unsigned long old_len,
320                 unsigned long new_len, unsigned long new_addr,
321                 bool *locked, struct vm_userfaultfd_ctx *uf,
322                 struct list_head *uf_unmap)
323 {
324         struct mm_struct *mm = vma->vm_mm;
325         struct vm_area_struct *new_vma;
326         unsigned long vm_flags = vma->vm_flags;
327         unsigned long new_pgoff;
328         unsigned long moved_len;
329         unsigned long excess = 0;
330         unsigned long hiwater_vm;
331         int split = 0;
332         int err;
333         bool need_rmap_locks;
334 
335         /*
336          * We'd prefer to avoid failure later on in do_munmap:
337          * which may split one vma into three before unmapping.
338          */
339         if (mm->map_count >= sysctl_max_map_count - 3)
340                 return -ENOMEM;
341 
342         /*
343          * Advise KSM to break any KSM pages in the area to be moved:
344          * it would be confusing if they were to turn up at the new
345          * location, where they happen to coincide with different KSM
346          * pages recently unmapped.  But leave vma->vm_flags as it was,
347          * so KSM can come around to merge on vma and new_vma afterwards.
348          */
349         err = ksm_madvise(vma, old_addr, old_addr + old_len,
350                                                 MADV_UNMERGEABLE, &vm_flags);
351         if (err)
352                 return err;
353 
354         new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
355         new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
356                            &need_rmap_locks);
357         if (!new_vma)
358                 return -ENOMEM;
359 
360         moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
361                                      need_rmap_locks);
362         if (moved_len < old_len) {
363                 err = -ENOMEM;
364         } else if (vma->vm_ops && vma->vm_ops->mremap) {
365                 err = vma->vm_ops->mremap(new_vma);
366         }
367 
368         if (unlikely(err)) {
369                 /*
370                  * On error, move entries back from new area to old,
371                  * which will succeed since page tables still there,
372                  * and then proceed to unmap new area instead of old.
373                  */
374                 move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
375                                  true);
376                 vma = new_vma;
377                 old_len = new_len;
378                 old_addr = new_addr;
379                 new_addr = err;
380         } else {
381                 mremap_userfaultfd_prep(new_vma, uf);
382                 arch_remap(mm, old_addr, old_addr + old_len,
383                            new_addr, new_addr + new_len);
384         }
385 
386         /* Conceal VM_ACCOUNT so old reservation is not undone */
387         if (vm_flags & VM_ACCOUNT) {
388                 vma->vm_flags &= ~VM_ACCOUNT;
389                 excess = vma->vm_end - vma->vm_start - old_len;
390                 if (old_addr > vma->vm_start &&
391                     old_addr + old_len < vma->vm_end)
392                         split = 1;
393         }
394 
395         /*
396          * If we failed to move page tables we still do total_vm increment
397          * since do_munmap() will decrement it by old_len == new_len.
398          *
399          * Since total_vm is about to be raised artificially high for a
400          * moment, we need to restore high watermark afterwards: if stats
401          * are taken meanwhile, total_vm and hiwater_vm appear too high.
402          * If this were a serious issue, we'd add a flag to do_munmap().
403          */
404         hiwater_vm = mm->hiwater_vm;
405         vm_stat_account(mm, vma->vm_flags, new_len >> PAGE_SHIFT);
406 
407         /* Tell pfnmap has moved from this vma */
408         if (unlikely(vma->vm_flags & VM_PFNMAP))
409                 untrack_pfn_moved(vma);
410 
411         if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
412                 /* OOM: unable to split vma, just get accounts right */
413                 vm_unacct_memory(excess >> PAGE_SHIFT);
414                 excess = 0;
415         }
416         mm->hiwater_vm = hiwater_vm;
417 
418         /* Restore VM_ACCOUNT if one or two pieces of vma left */
419         if (excess) {
420                 vma->vm_flags |= VM_ACCOUNT;
421                 if (split)
422                         vma->vm_next->vm_flags |= VM_ACCOUNT;
423         }
424 
425         if (vm_flags & VM_LOCKED) {
426                 mm->locked_vm += new_len >> PAGE_SHIFT;
427                 *locked = true;
428         }
429 
430         return new_addr;
431 }
432 
433 static struct vm_area_struct *vma_to_resize(unsigned long addr,
434         unsigned long old_len, unsigned long new_len, unsigned long *p)
435 {
436         struct mm_struct *mm = current->mm;
437         struct vm_area_struct *vma = find_vma(mm, addr);
438         unsigned long pgoff;
439 
440         if (!vma || vma->vm_start > addr)
441                 return ERR_PTR(-EFAULT);
442 
443         /*
444          * !old_len is a special case where an attempt is made to 'duplicate'
445          * a mapping.  This makes no sense for private mappings as it will
446          * instead create a fresh/new mapping unrelated to the original.  This
447          * is contrary to the basic idea of mremap which creates new mappings
448          * based on the original.  There are no known use cases for this
449          * behavior.  As a result, fail such attempts.
450          */
451         if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
452                 pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
453                 return ERR_PTR(-EINVAL);
454         }
455 
456         if (is_vm_hugetlb_page(vma))
457                 return ERR_PTR(-EINVAL);
458 
459         /* We can't remap across vm area boundaries */
460         if (old_len > vma->vm_end - addr)
461                 return ERR_PTR(-EFAULT);
462 
463         if (new_len == old_len)
464                 return vma;
465 
466         /* Need to be careful about a growing mapping */
467         pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
468         pgoff += vma->vm_pgoff;
469         if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
470                 return ERR_PTR(-EINVAL);
471 
472         if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
473                 return ERR_PTR(-EFAULT);
474 
475         if (vma->vm_flags & VM_LOCKED) {
476                 unsigned long locked, lock_limit;
477                 locked = mm->locked_vm << PAGE_SHIFT;
478                 lock_limit = rlimit(RLIMIT_MEMLOCK);
479                 locked += new_len - old_len;
480                 if (locked > lock_limit && !capable(CAP_IPC_LOCK))
481                         return ERR_PTR(-EAGAIN);
482         }
483 
484         if (!may_expand_vm(mm, vma->vm_flags,
485                                 (new_len - old_len) >> PAGE_SHIFT))
486                 return ERR_PTR(-ENOMEM);
487 
488         if (vma->vm_flags & VM_ACCOUNT) {
489                 unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
490                 if (security_vm_enough_memory_mm(mm, charged))
491                         return ERR_PTR(-ENOMEM);
492                 *p = charged;
493         }
494 
495         return vma;
496 }
497 
498 static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
499                 unsigned long new_addr, unsigned long new_len, bool *locked,
500                 struct vm_userfaultfd_ctx *uf,
501                 struct list_head *uf_unmap_early,
502                 struct list_head *uf_unmap)
503 {
504         struct mm_struct *mm = current->mm;
505         struct vm_area_struct *vma;
506         unsigned long ret = -EINVAL;
507         unsigned long charged = 0;
508         unsigned long map_flags;
509 
510         if (offset_in_page(new_addr))
511                 goto out;
512 
513         if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
514                 goto out;
515 
516         /* Ensure the old/new locations do not overlap */
517         if (addr + old_len > new_addr && new_addr + new_len > addr)
518                 goto out;
519 
520         /*
521          * move_vma() need us to stay 4 maps below the threshold, otherwise
522          * it will bail out at the very beginning.
523          * That is a problem if we have already unmaped the regions here
524          * (new_addr, and old_addr), because userspace will not know the
525          * state of the vma's after it gets -ENOMEM.
526          * So, to avoid such scenario we can pre-compute if the whole
527          * operation has high chances to success map-wise.
528          * Worst-scenario case is when both vma's (new_addr and old_addr) get
529          * split in 3 before unmaping it.
530          * That means 2 more maps (1 for each) to the ones we already hold.
531          * Check whether current map count plus 2 still leads us to 4 maps below
532          * the threshold, otherwise return -ENOMEM here to be more safe.
533          */
534         if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
535                 return -ENOMEM;
536 
537         ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
538         if (ret)
539                 goto out;
540 
541         if (old_len >= new_len) {
542                 ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
543                 if (ret && old_len != new_len)
544                         goto out;
545                 old_len = new_len;
546         }
547 
548         vma = vma_to_resize(addr, old_len, new_len, &charged);
549         if (IS_ERR(vma)) {
550                 ret = PTR_ERR(vma);
551                 goto out;
552         }
553 
554         map_flags = MAP_FIXED;
555         if (vma->vm_flags & VM_MAYSHARE)
556                 map_flags |= MAP_SHARED;
557 
558         ret = get_unmapped_area(vma->vm_file, new_addr, new_len, vma->vm_pgoff +
559                                 ((addr - vma->vm_start) >> PAGE_SHIFT),
560                                 map_flags);
561         if (IS_ERR_VALUE(ret))
562                 goto out1;
563 
564         ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, uf,
565                        uf_unmap);
566         if (!(offset_in_page(ret)))
567                 goto out;
568 out1:
569         vm_unacct_memory(charged);
570 
571 out:
572         return ret;
573 }
574 
575 static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
576 {
577         unsigned long end = vma->vm_end + delta;
578         if (end < vma->vm_end) /* overflow */
579                 return 0;
580         if (vma->vm_next && vma->vm_next->vm_start < end) /* intersection */
581                 return 0;
582         if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
583                               0, MAP_FIXED) & ~PAGE_MASK)
584                 return 0;
585         return 1;
586 }
587 
588 /*
589  * Expand (or shrink) an existing mapping, potentially moving it at the
590  * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
591  *
592  * MREMAP_FIXED option added 5-Dec-1999 by Benjamin LaHaise
593  * This option implies MREMAP_MAYMOVE.
594  */
595 SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
596                 unsigned long, new_len, unsigned long, flags,
597                 unsigned long, new_addr)
598 {
599         struct mm_struct *mm = current->mm;
600         struct vm_area_struct *vma;
601         unsigned long ret = -EINVAL;
602         unsigned long charged = 0;
603         bool locked = false;
604         bool downgraded = false;
605         struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
606         LIST_HEAD(uf_unmap_early);
607         LIST_HEAD(uf_unmap);
608 
609         /*
610          * There is a deliberate asymmetry here: we strip the pointer tag
611          * from the old address but leave the new address alone. This is
612          * for consistency with mmap(), where we prevent the creation of
613          * aliasing mappings in userspace by leaving the tag bits of the
614          * mapping address intact. A non-zero tag will cause the subsequent
615          * range checks to reject the address as invalid.
616          *
617          * See Documentation/arm64/tagged-address-abi.rst for more information.
618          */
619         addr = untagged_addr(addr);
620 
621         if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE))
622                 return ret;
623 
624         if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
625                 return ret;
626 
627         if (offset_in_page(addr))
628                 return ret;
629 
630         old_len = PAGE_ALIGN(old_len);
631         new_len = PAGE_ALIGN(new_len);
632 
633         /*
634          * We allow a zero old-len as a special case
635          * for DOS-emu "duplicate shm area" thing. But
636          * a zero new-len is nonsensical.
637          */
638         if (!new_len)
639                 return ret;
640 
641         if (down_write_killable(&current->mm->mmap_sem))
642                 return -EINTR;
643 
644         if (flags & MREMAP_FIXED) {
645                 ret = mremap_to(addr, old_len, new_addr, new_len,
646                                 &locked, &uf, &uf_unmap_early, &uf_unmap);
647                 goto out;
648         }
649 
650         /*
651          * Always allow a shrinking remap: that just unmaps
652          * the unnecessary pages..
653          * __do_munmap does all the needed commit accounting, and
654          * downgrades mmap_sem to read if so directed.
655          */
656         if (old_len >= new_len) {
657                 int retval;
658 
659                 retval = __do_munmap(mm, addr+new_len, old_len - new_len,
660                                   &uf_unmap, true);
661                 if (retval < 0 && old_len != new_len) {
662                         ret = retval;
663                         goto out;
664                 /* Returning 1 indicates mmap_sem is downgraded to read. */
665                 } else if (retval == 1)
666                         downgraded = true;
667                 ret = addr;
668                 goto out;
669         }
670 
671         /*
672          * Ok, we need to grow..
673          */
674         vma = vma_to_resize(addr, old_len, new_len, &charged);
675         if (IS_ERR(vma)) {
676                 ret = PTR_ERR(vma);
677                 goto out;
678         }
679 
680         /* old_len exactly to the end of the area..
681          */
682         if (old_len == vma->vm_end - addr) {
683                 /* can we just expand the current mapping? */
684                 if (vma_expandable(vma, new_len - old_len)) {
685                         int pages = (new_len - old_len) >> PAGE_SHIFT;
686 
687                         if (vma_adjust(vma, vma->vm_start, addr + new_len,
688                                        vma->vm_pgoff, NULL)) {
689                                 ret = -ENOMEM;
690                                 goto out;
691                         }
692 
693                         vm_stat_account(mm, vma->vm_flags, pages);
694                         if (vma->vm_flags & VM_LOCKED) {
695                                 mm->locked_vm += pages;
696                                 locked = true;
697                                 new_addr = addr;
698                         }
699                         ret = addr;
700                         goto out;
701                 }
702         }
703 
704         /*
705          * We weren't able to just expand or shrink the area,
706          * we need to create a new one and move it..
707          */
708         ret = -ENOMEM;
709         if (flags & MREMAP_MAYMOVE) {
710                 unsigned long map_flags = 0;
711                 if (vma->vm_flags & VM_MAYSHARE)
712                         map_flags |= MAP_SHARED;
713 
714                 new_addr = get_unmapped_area(vma->vm_file, 0, new_len,
715                                         vma->vm_pgoff +
716                                         ((addr - vma->vm_start) >> PAGE_SHIFT),
717                                         map_flags);
718                 if (IS_ERR_VALUE(new_addr)) {
719                         ret = new_addr;
720                         goto out;
721                 }
722 
723                 ret = move_vma(vma, addr, old_len, new_len, new_addr,
724                                &locked, &uf, &uf_unmap);
725         }
726 out:
727         if (offset_in_page(ret)) {
728                 vm_unacct_memory(charged);
729                 locked = 0;
730         }
731         if (downgraded)
732                 up_read(&current->mm->mmap_sem);
733         else
734                 up_write(&current->mm->mmap_sem);
735         if (locked && new_len > old_len)
736                 mm_populate(new_addr + old_len, new_len - old_len);
737         userfaultfd_unmap_complete(mm, &uf_unmap_early);
738         mremap_userfaultfd_complete(&uf, addr, new_addr, old_len);
739         userfaultfd_unmap_complete(mm, &uf_unmap);
740         return ret;
741 }
742 

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