linux-kernel - Re: [PATCH] mm/shmem: add mTHP swpout fallback statistics in shmem

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Message-ID: <3c86d612-aab7-4e80-bc6a-3ccf2bab9c34@arm.com>
Date: Tue, 16 Dec 2025 12:04:02 +0530
From: Anshuman Khandual <anshuman.khandual@....com>
To: Weilin Tong <tongweilin@...ux.alibaba.com>,
 Hugh Dickins <hughd@...gle.com>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>,
 Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/shmem: add mTHP swpout fallback statistics in
 shmem_writeout()



On 15/12/25 8:16 AM, Weilin Tong wrote:
> Currently, when shmem mTHPs are split and swapped out via shmem_writeout(),
> there are no unified statistics to trace these mTHP swpout fallback events.
> This makes it difficult to analyze the prevalence of mTHP splitting and
> fallback during swap operations, which is important for memory diagnostics.
> 
> Here we add statistics counting for mTHP fallback to small pages
> when splitting and swapping out in shmem_writeout().
> 
> Signed-off-by: Weilin Tong <tongweilin@...ux.alibaba.com>
> Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
>  mm/shmem.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 3f194c9842a8..aa624c447358 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1593,11 +1593,23 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
>  	}
>  
>  	if (split) {
> +		int order;
> +

Could this be moved up near the 'nr_pages' declaration instead ?

>  try_split:
> +		order = folio_order(folio);
>  		/* Ensure the subpages are still dirty */
>  		folio_test_set_dirty(folio);
>  		if (split_folio_to_list(folio, folio_list))
>  			goto redirty;
> +
> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> +		if (order >= HPAGE_PMD_ORDER) {
> +			count_memcg_folio_events(folio, THP_SWPOUT_FALLBACK, 1);
> +			count_vm_event(THP_SWPOUT_FALLBACK);
> +		}
> +#endif
> +		count_mthp_stat(order, MTHP_STAT_SWPOUT_FALLBACK);
> +
>  		folio_clear_dirty(folio);
>  	}
>  

Otherwise LGTM.