The patch titled
Subject: shmem: shmem_writepage() split unlikely i915 THP
has been added to the -mm tree. Its filename is
shmem-shmem_writepage-split-unlikely-i915-thp.patch
This patch should soon appear at
https://ozlabs.org/~akpm/mmots/broken-out/shmem-shmem_writepage-split-unlikely-i915-thp.patch
and later at
https://ozlabs.org/~akpm/mmotm/broken-out/shmem-shmem_writepage-split-unlikely-i915-thp.patch
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From: Hugh Dickins <hughd@xxxxxxxxxx>
Subject: shmem: shmem_writepage() split unlikely i915 THP
drivers/gpu/drm/i915/gem/i915_gem_shmem.c contains a shmem_writeback()
which calls shmem_writepage() from a shrinker: that usually works well
enough; but if /sys/kernel/mm/transparent_hugepage/shmem_enabled has been
set to "always" (intended to be usable) or "force" (forces huge everywhere
for easy testing), shmem_writepage() is surprised to be called with a huge
page, and crashes on the VM_BUG_ON_PAGE(PageCompound) (I did not find out
where the crash happens when CONFIG_DEBUG_VM is off).
LRU page reclaim always splits the shmem huge page first: I'd prefer not
to demand that of i915, so check and split compound in shmem_writepage().
Patch history: when first sent last year
http://lkml.kernel.org/r/alpine.LSU.2.11.2008301401390.5954@eggly.anvils
https://lore.kernel.org/linux-mm/20200919042009.bomzxmrg7%25akpm@xxxxxxxxxxxxxxxxxxxx/
Matthew Wilcox noticed that tail pages were wrongly left clean. This
version brackets the split with Set and Clear PageDirty as he suggested:
which works very well, even if it falls short of our aspirations. And
recently I realized that the crash is not limited to the testing option
"force", but affects "always" too: which is more important to fix.
Link: https://lkml.kernel.org/r/bac6158c-8b3d-4dca-cffc-4982f58d9794@xxxxxxxxxx
Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
Signed-off-by: Hugh Dickins <hughd@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
Acked-by: Yang Shi <shy828301@xxxxxxxxx>
Cc: "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx>
Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
Cc: Miaohe Lin <linmiaohe@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Cc: Rik van Riel <riel@xxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/shmem.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)
--- a/mm/shmem.c~shmem-shmem_writepage-split-unlikely-i915-thp
+++ a/mm/shmem.c
@@ -1344,7 +1344,19 @@ static int shmem_writepage(struct page *
swp_entry_t swap;
pgoff_t index;
- VM_BUG_ON_PAGE(PageCompound(page), page);
+ /*
+ * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "always" or
+ * "force", drivers/gpu/drm/i915/gem/i915_gem_shmem.c gets huge pages,
+ * and its shmem_writeback() needs them to be split when swapping.
+ */
+ if (PageTransCompound(page)) {
+ /* Ensure the subpages are still dirty */
+ SetPageDirty(page);
+ if (split_huge_page(page) < 0)
+ goto redirty;
+ ClearPageDirty(page);
+ }
+
BUG_ON(!PageLocked(page));
mapping = page->mapping;
index = page->index;
_
Patches currently in -mm which might be from hughd@xxxxxxxxxx are
fs-mm-fix-race-in-unlinking-swapfile.patch
huge-tmpfs-fix-fallocatevanilla-advance-over-huge-pages.patch
huge-tmpfs-fix-split_huge_page-after-falloc_fl_keep_size.patch
huge-tmpfs-remove-shrinklist-addition-from-shmem_setattr.patch
huge-tmpfs-revert-shmems-use-of-transhuge_vma_enabled.patch
huge-tmpfs-move-shmem_huge_enabled-upwards.patch
huge-tmpfs-sgp_noalloc-to-stop-collapse_file-on-race.patch
huge-tmpfs-shmem_is_hugevma-inode-index.patch
huge-tmpfs-decide-statst_blksize-by-shmem_is_huge.patch
shmem-shmem_writepage-split-unlikely-i915-thp.patch
