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Date: Tue, 26 Mar 2024 14:05:33 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Soma Nakata <soma.nakata01@gmail.com>
Cc: linux-mm@kvack.org, "Matthew Wilcox (Oracle)" <willy@infradead.org>,
 linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm/filemap: set folio->mapping to NULL before
 xas_store()
Message-Id: <20240326140533.a0d0041371e21540dd934722@linux-foundation.org>
In-Reply-To: <20240322210455.3738-1-soma.nakata01@gmail.com>
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On Sat, 23 Mar 2024 06:04:54 +0900 Soma Nakata <soma.nakata01@gmail.com> wrote:

> Functions such as __filemap_get_folio() check the truncation of
> folios based on the mapping field. Therefore setting this field to NULL
> earlier prevents unnecessary operations on already removed folios.
> 
> ...
>
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -139,11 +139,12 @@ static void page_cache_delete(struct address_space *mapping,
>  
>  	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
>  
> +	folio->mapping = NULL;
> +	/* Leave page->index set: truncation lookup relies upon it */
> +
>  	xas_store(&xas, shadow);
>  	xas_init_marks(&xas);
>  
> -	folio->mapping = NULL;
> -	/* Leave page->index set: truncation lookup relies upon it */
>  	mapping->nrpages -= nr;
>  }

Seems at least harmless, but I wonder if it can really make any
difference.  Don't readers of folio->mapping lock the folio first?