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/[email protected]
https://lore.kernel.org/linux-mm/20200919042009.bomzxmrg7%[email protected]/
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/[email protected]
Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker")
Signed-off-by: Hugh Dickins <[email protected]>
Reviewed-by: Shakeel Butt <[email protected]>
Acked-by: Yang Shi <[email protected]>
Cc: "Kirill A. Shutemov" <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Mike Kravetz <[email protected]>
Cc: Rik van Riel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

0 files changed, 0 insertions, 0 deletions