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Message-ID: <aef55f8d-6040-692d-65e3-16150cce4440@google.com>
Date: Sun, 25 Aug 2024 16:14:17 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>,
Baolin Wang <baolin.wang@...ux.alibaba.com>
cc: hughd@...gle.com, willy@...radead.org, david@...hat.com,
wangkefeng.wang@...wei.com, chrisl@...nel.org, ying.huang@...el.com,
21cnbao@...il.com, ryan.roberts@....com, shy828301@...il.com,
ziy@...dia.com, ioworker0@...il.com, da.gomez@...sung.com,
p.raghav@...sung.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 9/9] mm: shmem: support large folio swap out
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@...ux.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@...gle.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
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