On Fri, Sep 18, 2020 at 10:44:32PM -0700, Hugh Dickins wrote:
> It behaves a lot better with this patch in than without it; but you're
> right, only the head will get written to swap, and the tails left in
> memory; with dirty cleared, so they may be left indefinitely (I've
> not yet looked to see when if ever PageDirty might get set later).
>
> Hmm. It may just be a matter of restyling the i915 code with
>
> if (!page_mapped(page)) {
> clear_page_dirty_for_io(page);
>
> but I don't want to rush to that conclusion - there might turn
> out to be a good way of doing it at the shmem_writepage() end, but
> probably only hacks available. I'll mull it over: it deserves some
> thought about what would suit, if a THP arrived here some other way.
I think the ultimate solution is to do as I have done for iomap and make
->writepage handle arbitrary sized pages. However, I don't know the
swap I/O path particularly well, and I would rather not learn it just yet.
How about this for a band-aid until we sort that out properly? Just mark
the page as dirty before splitting it so subsequent iterations see the
subpages as dirty. Arguably, we should use set_page_dirty() instead of
SetPageDirty, but I don't think i915 cares. In particular, it uses
an untagged iteration instead of searching for PAGECACHE_TAG_DIRTY.
diff --git a/mm/shmem.c b/mm/shmem.c
index 271548ca20f3..6231207ab1eb 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1362,8 +1362,21 @@ 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);
BUG_ON(!PageLocked(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)) {
+ /* Ensure the subpages are still dirty */
+ SetPageDirty(page);
+ if (split_huge_page(page) < 0)
+ goto redirty;
+ ClearPageDirty(page);
+ }
+
mapping = page->mapping;
index = page->index;
inode = mapping->host;
