path: root/mm
diff options
authorHugh Dickins <>2014-07-23 21:00:13 (GMT)
committerGreg Kroah-Hartman <>2014-07-28 15:00:03 (GMT)
commit7dc7fb432bc92a988afb49e948218de575b7eb3f (patch)
treee48dce23ed22c9f526dcaae595217210cef882f0 /mm
parent887675c981bcefc567bd1f18352238d7ce1cf47a (diff)
shmem: fix splicing from a hole while it's punched
commit b1a366500bd537b50c3aad26dc7df083ec03a448 upstream. shmem_fault() is the actual culprit in trinity's hole-punch starvation, and the most significant cause of such problems: since a page faulted is one that then appears page_mapped(), needing unmap_mapping_range() and i_mmap_mutex to be unmapped again. But it is not the only way in which a page can be brought into a hole in the radix_tree while that hole is being punched; and Vlastimil's testing implies that if enough other processors are busy filling in the hole, then shmem_undo_range() can be kept from completing indefinitely. shmem_file_splice_read() is the main other user of SGP_CACHE, which can instantiate shmem pagecache pages in the read-only case (without holding i_mutex, so perhaps concurrently with a hole-punch). Probably it's silly not to use SGP_READ already (using the ZERO_PAGE for holes): which ought to be safe, but might bring surprises - not a change to be rushed. shmem_read_mapping_page_gfp() is an internal interface used by drivers/gpu/drm GEM (and next by uprobes): it should be okay. And shmem_file_read_iter() uses the SGP_DIRTY variant of SGP_CACHE, when called internally by the kernel (perhaps for a stacking filesystem, which might rely on holes to be reserved): it's unclear whether it could be provoked to keep hole-punch busy or not. We could apply the same umbrella as now used in shmem_fault() to shmem_file_splice_read() and the others; but it looks ugly, and use over a range raises questions - should it actually be per page? can these get starved themselves? The origin of this part of the problem is my v3.1 commit d0823576bf4b ("mm: pincer in truncate_inode_pages_range"), once it was duplicated into shmem.c. It seemed like a nice idea at the time, to ensure (barring RCU lookup fuzziness) that there's an instant when the entire hole is empty; but the indefinitely repeated scans to ensure that make it vulnerable. Revert that "enhancement" to hole-punch from shmem_undo_range(), but retain the unproblematic rescanning when it's truncating; add a couple of comments there. Remove the "indices[0] >= end" test: that is now handled satisfactorily by the inner loop, and mem_cgroup_uncharge_start()/end() are too light to be worth avoiding here. But if we do not always loop indefinitely, we do need to handle the case of swap swizzled back to page before shmem_free_swap() gets it: add a retry for that case, as suggested by Konstantin Khlebnikov; and for the case of page swizzled back to swap, as suggested by Johannes Weiner. Signed-off-by: Hugh Dickins <> Reported-by: Sasha Levin <> Suggested-by: Vlastimil Babka <> Cc: Konstantin Khlebnikov <> Cc: Johannes Weiner <> Cc: Lukas Czerner <> Cc: Dave Jones <> Signed-off-by: Andrew Morton <> Signed-off-by: Linus Torvalds <> Signed-off-by: Greg Kroah-Hartman <>
Diffstat (limited to 'mm')
1 files changed, 15 insertions, 9 deletions
diff --git a/mm/shmem.c b/mm/shmem.c
index 3d26fed..16cc1d7 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -534,22 +534,19 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
index = start;
- for ( ; ; ) {
+ while (index < end) {
cond_resched(); = shmem_find_get_pages_and_swap(mapping, index,
min(end - index, (pgoff_t)PAGEVEC_SIZE),
pvec.pages, indices);
if (! {
- if (index == start || unfalloc)
+ /* If all gone or hole-punch or unfalloc, we're done */
+ if (index == start || end != -1)
+ /* But if truncating, restart to make sure all gone */
index = start;
- if ((index == start || unfalloc) && indices[0] >= end) {
- shmem_deswap_pagevec(&pvec);
- pagevec_release(&pvec);
- break;
- }
for (i = 0; i < pagevec_count(&pvec); i++) {
struct page *page = pvec.pages[i];
@@ -561,8 +558,12 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
if (radix_tree_exceptional_entry(page)) {
if (unfalloc)
- nr_swaps_freed += !shmem_free_swap(mapping,
- index, page);
+ if (shmem_free_swap(mapping, index, page)) {
+ /* Swap was replaced by page: retry */
+ index--;
+ break;
+ }
+ nr_swaps_freed++;
@@ -571,6 +572,11 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
if (page->mapping == mapping) {
truncate_inode_page(mapping, page);
+ } else {
+ /* Page was replaced by swap: retry */
+ unlock_page(page);
+ index--;
+ break;