Re: [PATCH] mm/msync: exit early when the flags is an MS_ASYNC and start < vm_start

From: Vlastimil Babka <vbabka@suse.cz>
To: Nikita Ermakov <sh1r4s3@mail.si-head.nl>, linux-mm@kvack.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] mm/msync: exit early when the flags is an MS_ASYNC and start < vm_start
Date: Tue, 20 Oct 2020 12:19:30 +0200	[thread overview]
Message-ID: <aaaaadff-977a-0e53-afe5-044889455fd6@suse.cz> (raw)
In-Reply-To: <20201012160911.6282-1-sh1r4s3@mail.si-head.nl>

On 10/12/20 6:09 PM, Nikita Ermakov wrote:
> Exit from the loop over the VMA in the case when the flags
> contain only an MS_ASYNC and start < vm_start. In this case msync()
> would return with -ENOMEM anyway so make it return early.
> 
> Signed-off-by: Nikita Ermakov <sh1r4s3@mail.si-head.nl>

AFAICS it can still return -EBUSY if there's MS_INVALIDATE and a mlocked vma. 
This is all subtle and I don't think we should risk breaking something for this 
optimization.

> ---
>   mm/msync.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/msync.c b/mm/msync.c
> index 69c6d2029531..ed20c3621d4c 100644
> --- a/mm/msync.c
> +++ b/mm/msync.c
> @@ -69,6 +69,8 @@ SYSCALL_DEFINE3(msync, unsigned long, start, size_t, len, int, flags)
>   			goto out_unlock;
>   		/* Here start < vma->vm_end. */
>   		if (start < vma->vm_start) {
> +			if (flags == MS_ASYNC)
> +				goto out_unlock;
>   			start = vma->vm_start;
>   			if (start >= end)
>   				goto out_unlock;
> 
> base-commit: 6824a8a9b4861d7df7ee132a952bdf6f84a99cb8
> 