Re: [patch 04/15] shmem: shmem_writepage() split unlikely i915 THP

On Fri, Sep 18, 2020 at 09:20:09PM -0700, Andrew Morton wrote:

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().

Sorry for not checking this earlier, but I don't think this is right.

for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) { ... if (!page_mapped(page) && clear_page_dirty_for_io(page)) { ... ret = mapping->a_ops->writepage(page, &wbc);

so we cleared the dirty bit on the entire hugepage, but then split it after clearing the dirty bit, so the subpages are now not dirty. I think we'll lose writes as a result? At least we won't swap pages out that deserve to be paged out.

• VM_BUG_ON_PAGE(PageCompound(page), page);

• /*

• * If /sys/kernel/mm/transparent_hugepage/shmem_enabled is "force",
  

• * then 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))

• if (split_huge_page(page) < 0)
  

• 	goto redirty;
  

• BUG_ON(!PageLocked(page)); mapping = page->mapping; index = page->index;

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