Re: [PATCH] mm/filemap: set folio->mapping to NULL before xas_store()

[Andrew Morton <akpm@linux-foundation.org>]

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?