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

[Baolin Wang <baolin.wang@linux.alibaba.com>]

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@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);

Thanks for the fix, Hugh. Very appreciated for your reviewing.