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Message-ID: <16327571-3031-4758-a401-2b5f19c3196b@linux.alibaba.com>
Date: Tue, 27 Aug 2024 14:58:40 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: 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 2024/8/26 07:14, Hugh Dickins wrote:
> 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).
Thanks for the history, and I understand.
> 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.
Sure. After Andrew queuing your fixes, I can send a proper fix patch to
remove the 'wbc->split_large_folio'.
>> [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.
Weird, I can access the link, not sure why.
>
>> 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);
Thanks for the fix, Hugh. Very appreciated for your reviewing.
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