commit 198b62f83eef1d605d70eca32759c92cdcc14175 upstream
When a THP is removed from the page cache by reclaim, we replace it with a shadow entry that occupies all slots of the XArray previously occupied by the THP. If the user then accesses that page again, we only allocate a single page, but storing it into the shadow entry replaces all entries with that one page. That leads to bugs like
page dumped because: VM_BUG_ON_PAGE(page_to_pgoff(page) != offset) ------------[ cut here ]------------ kernel BUG at mm/filemap.c:2529!
https://bugzilla.kernel.org/show_bug.cgi?id=206569
This is hard to reproduce with mainline, but happens regularly with the THP patchset (as so many more THPs are created). This solution is take from the THP patchset. It splits the shadow entry into order-0 pieces at the time that we bring a new page into cache.
Fixes: 99cb0dbd47a1 ("mm,thp: add read-only THP support for (non-shmem) FS") Signed-off-by: Matthew Wilcox (Oracle) willy@infradead.org Signed-off-by: Andrew Morton akpm@linux-foundation.org Cc: Song Liu songliubraving@fb.com Cc: "Kirill A . Shutemov" kirill@shutemov.name Cc: Qian Cai cai@lca.pw Link: https://lkml.kernel.org/r/20200903183029.14930-4-willy@infradead.org Signed-off-by: Linus Torvalds torvalds@linux-foundation.org --- mm/filemap.c | 37 ++++++++++++++++++++++++++++--------- 1 file changed, 28 insertions(+), 9 deletions(-)
diff --git a/mm/filemap.c b/mm/filemap.c index db542b494883..c10e237cc2c6 100644 --- a/mm/filemap.c +++ b/mm/filemap.c @@ -856,7 +856,6 @@ noinline int __add_to_page_cache_locked(struct page *page, int huge = PageHuge(page); struct mem_cgroup *memcg; int error; - void *old;
VM_BUG_ON_PAGE(!PageLocked(page), page); VM_BUG_ON_PAGE(PageSwapBacked(page), page); @@ -872,21 +871,41 @@ noinline int __add_to_page_cache_locked(struct page *page, get_page(page); page->mapping = mapping; page->index = offset; + gfp_mask &= GFP_RECLAIM_MASK;
do { + unsigned int order = xa_get_order(xas.xa, xas.xa_index); + void *entry, *old = NULL; + + if (order > thp_order(page)) + xas_split_alloc(&xas, xa_load(xas.xa, xas.xa_index), + order, gfp_mask); xas_lock_irq(&xas); - old = xas_load(&xas); - if (old && !xa_is_value(old)) - xas_set_err(&xas, -EEXIST); + xas_for_each_conflict(&xas, entry) { + old = entry; + if (!xa_is_value(entry)) { + xas_set_err(&xas, -EEXIST); + goto unlock; + } + } + + if (old) { + if (shadowp) + *shadowp = old; + /* entry may have been split before we acquired lock */ + order = xa_get_order(xas.xa, xas.xa_index); + if (order > thp_order(page)) { + xas_split(&xas, old, order); + xas_reset(&xas); + } + } + xas_store(&xas, page); if (xas_error(&xas)) goto unlock;
- if (xa_is_value(old)) { + if (old) mapping->nrexceptional--; - if (shadowp) - *shadowp = old; - } mapping->nrpages++;
/* hugetlb pages do not participate in page cache accounting */ @@ -894,7 +913,7 @@ noinline int __add_to_page_cache_locked(struct page *page, __inc_node_page_state(page, NR_FILE_PAGES); unlock: xas_unlock_irq(&xas); - } while (xas_nomem(&xas, gfp_mask & GFP_RECLAIM_MASK)); + } while (xas_nomem(&xas, gfp_mask));
if (xas_error(&xas)) goto error;