From 7e09e738afd21ef99f047425fc0b0c9be8b03254 Mon Sep 17 00:00:00 2001 From: Hugh Dickins Date: Thu, 20 Mar 2014 21:52:17 -0700 Subject: 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 Reported-and-tested-by: Sasha Levin Signed-off-by: Hugh Dickins Signed-off-by: Linus Torvalds --- mm/migrate.c | 32 ++++++++++++++++++++++++++++++++ 1 file changed, 32 insertions(+) (limited to 'mm/migrate.c') diff --git a/mm/migrate.c b/mm/migrate.c index b494fdb9a636..bed48809e5d0 100644 --- a/mm/migrate.c +++ b/mm/migrate.c @@ -177,6 +177,37 @@ out: return SWAP_AGAIN; } +/* + * Congratulations to trinity for discovering this bug. + * mm/fremap.c's remap_file_pages() accepts any range within a single vma to + * convert that vma to VM_NONLINEAR; and generic_file_remap_pages() will then + * replace the specified range by file ptes throughout (maybe populated after). + * If page migration finds a page within that range, while it's still located + * by vma_interval_tree rather than lost to i_mmap_nonlinear list, no problem: + * zap_pte() clears the temporary migration entry before mmap_sem is dropped. + * But if the migrating page is in a part of the vma outside the range to be + * remapped, then it will not be cleared, and remove_migration_ptes() needs to + * deal with it. Fortunately, this part of the vma is of course still linear, + * so we just need to use linear location on the nonlinear list. + */ +static int remove_linear_migration_ptes_from_nonlinear(struct page *page, + struct address_space *mapping, void *arg) +{ + struct vm_area_struct *vma; + /* hugetlbfs does not support remap_pages, so no huge pgoff worries */ + pgoff_t pgoff = page->index << (PAGE_CACHE_SHIFT - PAGE_SHIFT); + unsigned long addr; + + list_for_each_entry(vma, + &mapping->i_mmap_nonlinear, shared.nonlinear) { + + addr = vma->vm_start + ((pgoff - vma->vm_pgoff) << PAGE_SHIFT); + if (addr >= vma->vm_start && addr < vma->vm_end) + remove_migration_pte(page, vma, addr, arg); + } + return SWAP_AGAIN; +} + /* * Get rid of all migration entries and replace them by * references to the indicated page. @@ -186,6 +217,7 @@ static void remove_migration_ptes(struct page *old, struct page *new) struct rmap_walk_control rwc = { .rmap_one = remove_migration_pte, .arg = old, + .file_nonlinear = remove_linear_migration_ptes_from_nonlinear, }; rmap_walk(new, &rwc); -- cgit v1.2.3