linux-kernel - Re: [PATCH 5/9] mm/shmem, swap: remove redundant error handling for replacing folio

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Message-ID: <0fc6e083-b7be-4144-a50c-d1a7a2e1c3a5@linux.alibaba.com>
Date: Mon, 25 Aug 2025 11:02:47 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Kairui Song <kasong@...cent.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Matthew Wilcox <willy@...radead.org>, Hugh Dickins <hughd@...gle.com>,
 Chris Li <chrisl@...nel.org>, Barry Song <baohua@...nel.org>,
 Baoquan He <bhe@...hat.com>, Nhat Pham <nphamcs@...il.com>,
 Kemeng Shi <shikemeng@...weicloud.com>,
 Ying Huang <ying.huang@...ux.alibaba.com>,
 Johannes Weiner <hannes@...xchg.org>, David Hildenbrand <david@...hat.com>,
 Yosry Ahmed <yosryahmed@...gle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Zi Yan <ziy@...dia.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/9] mm/shmem, swap: remove redundant error handling for
 replacing folio



On 2025/8/23 03:20, Kairui Song wrote:
> From: Kairui Song <kasong@...cent.com>
> 
> Shmem may replace a folio in the swap cache if the cached one doesn't
> fit the swapin's GFP zone. When doing so, shmem has already double
> checked that the swap cache folio is locked, still has the swap cache
> flag set, and contains the wanted swap entry. So it is impossible to
> fail due to an Xarray mismatch. There is even a comment for that.
> 
> Delete the defensive error handling path, and add a WARN_ON instead:
> if that happened, something has broken the basic principle of how the
> swap cache works, we should catch and fix that.
> 
> Signed-off-by: Kairui Song <kasong@...cent.com>
> ---
>   mm/shmem.c | 28 +++-------------------------
>   1 file changed, 3 insertions(+), 25 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index b4d39f2a1e0a..e03793cc5169 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2158,35 +2158,13 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
>   	/* Swap cache still stores N entries instead of a high-order entry */
>   	xa_lock_irq(&swap_mapping->i_pages);
>   	for (i = 0; i < nr_pages; i++) {
> -		void *item = xas_load(&xas);
> -
> -		if (item != old) {
> -			error = -ENOENT;
> -			break;
> -		}
> -
> -		xas_store(&xas, new);
> +		WARN_ON_ONCE(xas_store(&xas, new));
>   		xas_next(&xas);
>   	}
> -	if (!error) {
> -		mem_cgroup_replace_folio(old, new);
> -		shmem_update_stats(new, nr_pages);
> -		shmem_update_stats(old, -nr_pages);
> -	}

It looks like the shmem statistics update was mistakenly deleted?

( Continue to understand the whole series:) )

>   	xa_unlock_irq(&swap_mapping->i_pages);
>   
> -	if (unlikely(error)) {
> -		/*
> -		 * Is this possible?  I think not, now that our callers
> -		 * check both the swapcache flag and folio->private
> -		 * after getting the folio lock; but be defensive.
> -		 * Reverse old to newpage for clear and free.
> -		 */
> -		old = new;
> -	} else {
> -		folio_add_lru(new);
> -		*foliop = new;
> -	}
> +	folio_add_lru(new);
> +	*foliop = new;
>   
>   	folio_clear_swapcache(old);
>   	old->private = NULL;