@@ -1349,7 +1349,19 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
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;
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@linux-foundation.org/ 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. Fixes: 2d6692e642e7 ("drm/i915: Start writeback from the shrinker") Signed-off-by: Hugh Dickins <hughd@google.com> Reviewed-by: Shakeel Butt <shakeelb@google.com> Acked-by: Yang Shi <shy828301@gmail.com> --- mm/shmem.c | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-)