linux-kernel - Re: [PATCH] shm: Fix unlikely() test of info->seals to test only for WRITE and GROW

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Date:   Sat, 4 Feb 2017 14:51:55 -0800 (PST)
From:   Hugh Dickins <hughd@...gle.com>
To:     "Steven Rostedt (VMware)" <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Herrmann <dh.herrmann@...il.com>,
        Hugh Dickins <hughd@...gle.com>
Subject: Re: [PATCH] shm: Fix unlikely() test of info->seals to test only
 for WRITE and GROW

On Fri, 3 Feb 2017, Steven Rostedt (VMware) wrote:

> From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> 
> Running my likely/unlikely profiler, I discovered that the test in
> shmem_write_begin() that tests for info->seals as unlikely, is
> always incorrect. This is because shmem_get_inode() sets info->seals to
> have F_SEAL_SEAL set by default, and it is unlikely to be cleared when
> shmem_write_begin() is called. Thus, the if statement is very likely.
> 
> But as the if statement block only cares about F_SEAL_WRITE and
> F_SEAL_GROW, change the test to only test those two bits.
> 
> Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

Acked-by: Hugh Dickins <hughd@...gle.com>

> ---
>  mm/shmem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index bb53285..ef4cdbb 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2194,7 +2194,7 @@ shmem_write_begin(struct file *file, struct address_space *mapping,
>  	pgoff_t index = pos >> PAGE_SHIFT;
>  
>  	/* i_mutex is held by caller */
> -	if (unlikely(info->seals)) {
> +	if (unlikely(info->seals & (F_SEAL_WRITE | F_SEAL_GROW))) {
>  		if (info->seals & F_SEAL_WRITE)
>  			return -EPERM;
>  		if ((info->seals & F_SEAL_GROW) && pos + len > inode->i_size)
> -- 
> 2.9.3