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