Re: [PATCH v5 9/9] mm: shmem: support large folio swap out

[Hugh Dickins <hughd@google.com>]

On Mon, 12 Aug 2024, Baolin Wang wrote:

> Shmem will support large folio allocation [1] [2] to get a better performance,
> however, the memory reclaim still splits the precious large folios when trying
> to swap out shmem, which may lead to the memory fragmentation issue and can not
> take advantage of the large folio for shmeme.
> 
> Moreover, the swap code already supports for swapping out large folio without
> split, hence this patch set supports the large folio swap out for shmem.
> 
> Note the i915_gem_shmem driver still need to be split when swapping, thus
> add a new flag 'split_large_folio' for writeback_control to indicate spliting
> the large folio.

Is that last paragraph a misunderstanding? The i915 THP splitting in
shmem_writepage() was to avoid mm VM_BUG_ONs and crashes when shmem.c
did not support huge page swapout: but now you are enabling that support,
and such VM_BUG_ONs and crashes are gone (so far as I can see: and this
is written on a laptop using the i915 driver).

I cannot think of why i915 itself would care how mm implements swapout
(beyond enjoying faster): I think all the wbc->split_large_folio you
introduce here should be reverted.  But you may know better!

I do need a further change to shmem_writepage() here: see fixup patch
below: that's written to apply on top of this 9/9, but I'd be glad to
see a replacement with wbc->split_large_folio gone, and just one
!IS_ENABLED(CONFIG_THP_SWAP) instead.

> 
> [1] https://lore.kernel.org/all/cover.1717495894.git.baolin.wang@linux.alibaba.com/
> [2] https://lore.kernel.org/all/20240515055719.32577-1-da.gomez@samsung.com/

I get "Not found" for that [2] link.

> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  drivers/gpu/drm/i915/gem/i915_gem_shmem.c |  1 +
>  include/linux/writeback.h                 |  4 +++
>  mm/shmem.c                                | 12 ++++++---
>  mm/vmscan.c                               | 32 ++++++++++++++++++-----
>  4 files changed, 38 insertions(+), 11 deletions(-)

[PATCH] mm: shmem: shmem_writepage() split folio at EOF before swapout

Working in a constrained (size= or nr_blocks=) huge=always tmpfs relies
on swapout to split a large folio at EOF, to trim off its excess before
hitting premature ENOSPC: shmem_unused_huge_shrink() contains no code to
handle splitting huge swap blocks, and nobody would want that to be added.

Signed-off-by: Hugh Dickins <hughd@google.com>
---
 mm/shmem.c | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 37c300f69baf..4dd0570962fa 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1459,6 +1459,7 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	swp_entry_t swap;
 	pgoff_t index;
 	int nr_pages;
+	bool split = false;
 
 	/*
 	 * Our capabilities prevent regular writeback or sync from ever calling
@@ -1480,8 +1481,20 @@ static int shmem_writepage(struct page *page, struct writeback_control *wbc)
 	 * 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.
+	 *
+	 * And shrinkage of pages beyond i_size does not split swap, so
+	 * swapout of a large folio crossing i_size needs to split too
+	 * (unless fallocate has been used to preallocate beyond EOF).
 	 */
-	if (wbc->split_large_folio && folio_test_large(folio)) {
+	if (folio_test_large(folio)) {
+		split = wbc->split_large_folio;
+		index = shmem_fallocend(inode,
+			DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE));
+		if (index > folio->index && index < folio_next_index(folio))
+			split = true;
+	}
+
+	if (split) {
 try_split:
 		/* Ensure the subpages are still dirty */
 		folio_test_set_dirty(folio);
-- 
2.35.3