mm: fix swapops.h:131 bug if remap_file_pages raced migration
Add remove_linear_migration_ptes_from_nonlinear(), to fix an interesting little include/linux/swapops.h:131 BUG_ON(!PageLocked) found by trinity: indicating that remove_migration_ptes() failed to find one of the migration entries that was temporarily inserted. The problem comes from remap_file_pages()'s switch from vma_interval_tree (good for inserting the migration entry) to i_mmap_nonlinear list (no good for locating it again); but can only be a problem if the remap_file_pages() range does not cover the whole of the vma (zap_pte() clears the range). remove_migration_ptes() needs a file_nonlinear method to go down the i_mmap_nonlinear list, applying linear location to look for migration entries in those vmas too, just in case there was this race. The file_nonlinear method does need rmap_walk_control.arg to do this; but it never needed vma passed in - vma comes from its own iteration. Reported-and-tested-by: Dave Jones <davej@redhat.com> Reported-and-tested-by: Sasha Levin <sasha.levin@oracle.com> Signed-off-by: Hugh Dickins <hughd@google.com> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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3 changed files with 36 additions and 4 deletions
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@ -250,8 +250,7 @@ struct rmap_walk_control {
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int (*rmap_one)(struct page *page, struct vm_area_struct *vma,
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unsigned long addr, void *arg);
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int (*done)(struct page *page);
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int (*file_nonlinear)(struct page *, struct address_space *,
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struct vm_area_struct *vma);
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int (*file_nonlinear)(struct page *, struct address_space *, void *arg);
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struct anon_vma *(*anon_lock)(struct page *page);
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bool (*invalid_vma)(struct vm_area_struct *vma, void *arg);
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};
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32
mm/migrate.c
32
mm/migrate.c
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@ -177,6 +177,37 @@ out:
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return SWAP_AGAIN;
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}
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/*
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* Congratulations to trinity for discovering this bug.
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* mm/fremap.c's remap_file_pages() accepts any range within a single vma to
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* convert that vma to VM_NONLINEAR; and generic_file_remap_pages() will then
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* replace the specified range by file ptes throughout (maybe populated after).
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* If page migration finds a page within that range, while it's still located
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* by vma_interval_tree rather than lost to i_mmap_nonlinear list, no problem:
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* zap_pte() clears the temporary migration entry before mmap_sem is dropped.
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* But if the migrating page is in a part of the vma outside the range to be
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* remapped, then it will not be cleared, and remove_migration_ptes() needs to
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* deal with it. Fortunately, this part of the vma is of course still linear,
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* so we just need to use linear location on the nonlinear list.
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*/
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static int remove_linear_migration_ptes_from_nonlinear(struct page *page,
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struct address_space *mapping, void *arg)
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{
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struct vm_area_struct *vma;
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/* hugetlbfs does not support remap_pages, so no huge pgoff worries */
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pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT);
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unsigned long addr;
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list_for_each_entry(vma,
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&mapping->i_mmap_nonlinear, shared.nonlinear) {
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addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT);
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if (addr >= vma->vm_start && addr < vma->vm_end)
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remove_migration_pte(page, vma, addr, arg);
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}
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return SWAP_AGAIN;
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}
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/*
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* Get rid of all migration entries and replace them by
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* references to the indicated page.
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@ -186,6 +217,7 @@ static void remove_migration_ptes(struct page *old, struct page *new)
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struct rmap_walk_control rwc = {
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.rmap_one = remove_migration_pte,
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.arg = old,
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.file_nonlinear = remove_linear_migration_ptes_from_nonlinear,
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};
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rmap_walk(new, &rwc);
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@ -1360,8 +1360,9 @@ static int try_to_unmap_cluster(unsigned long cursor, unsigned int *mapcount,
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}
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static int try_to_unmap_nonlinear(struct page *page,
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struct address_space *mapping, struct vm_area_struct *vma)
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struct address_space *mapping, void *arg)
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{
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struct vm_area_struct *vma;
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int ret = SWAP_AGAIN;
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unsigned long cursor;
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unsigned long max_nl_cursor = 0;
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@ -1663,7 +1664,7 @@ static int rmap_walk_file(struct page *page, struct rmap_walk_control *rwc)
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if (list_empty(&mapping->i_mmap_nonlinear))
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goto done;
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ret = rwc->file_nonlinear(page, mapping, vma);
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ret = rwc->file_nonlinear(page, mapping, rwc->arg);
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done:
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mutex_unlock(&mapping->i_mmap_mutex);
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