[091/212] shmem: shmem_writepage() split unlikely i915 THP

Commit Message

Andrew Morton Sept. 2, 2021, 9:54 p.m. UTC

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

 mm/shmem.c |   14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

Patch

--- 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;