linux-kernel - Re: [PATCH v9 1/6] filemap: check compound_head(page)->mapping in filemap

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Date:   Wed, 10 Jul 2019 13:51:31 -0400
From:   Johannes Weiner <hannes@...xchg.org>
To:     Song Liu <songliubraving@...com>
Cc:     linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, matthew.wilcox@...cle.com,
        kirill.shutemov@...ux.intel.com, kernel-team@...com,
        william.kucharski@...cle.com, akpm@...ux-foundation.org,
        hdanton@...a.com
Subject: Re: [PATCH v9 1/6] filemap: check compound_head(page)->mapping in
 filemap_fault()

On Mon, Jun 24, 2019 at 05:12:41PM -0700, Song Liu wrote:
> Currently, filemap_fault() avoids trace condition with truncate by
> checking page->mapping == mapping. This does not work for compound
> pages. This patch let it check compound_head(page)->mapping instead.
> 
> Acked-by: Rik van Riel <riel@...riel.com>
> Signed-off-by: Song Liu <songliubraving@...com>
> ---
>  mm/filemap.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index df2006ba0cfa..f5b79a43946d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2517,7 +2517,7 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>  		goto out_retry;
>  
>  	/* Did it get truncated? */
> -	if (unlikely(page->mapping != mapping)) {
> +	if (unlikely(compound_head(page)->mapping != mapping)) {

There is another check like these in pagecache_get_page(), which is
used by find_lock_page() and thus the truncate code (partial page
truncate calls, but this could happen against read-only cache).