linux-kernel - Re: [PATCH V2,1/2] mm: madvise: return correct bytes advised with process

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Date:   Mon, 21 Mar 2022 16:18:18 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Charan Teja Kalla <quic_charante@...cinc.com>
Cc:     akpm@...ux-foundation.org, surenb@...gle.com, vbabka@...e.cz,
        rientjes@...gle.com, sfr@...b.auug.org.au, edgararriaga@...gle.com,
        minchan@...nel.org, nadav.amit@...il.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, "# 5 . 10+" <stable@...r.kernel.org>
Subject: Re: [PATCH V2,1/2] mm: madvise: return correct bytes advised with
 process_madvise

On Fri 11-03-22 20:59:05, Charan Teja Kalla wrote:
> The process_madvise() system call returns error even after processing
> some VMA's passed in the 'struct iovec' vector list which leaves the
> user confused to know where to restart the advise next. It is also
> against this syscall man page[1] documentation where it mentions that
> "return value may be less than the total number of requested bytes, if
> an error occurred after some iovec elements were already processed.".
> 
> Consider a user passed 10 VMA's in the 'struct iovec' vector list of
> which 9 are processed but one. Then it just returns the error caused on
> that failed VMA despite the first 9 VMA's processed, leaving the user
> confused about on which VMA it is failed. Returning the number of bytes
> processed here can help the user to know which VMA it is failed on and
> thus can retry/skip the advise on that VMA.
> 
> [1]https://man7.org/linux/man-pages/man2/process_madvise.2.html.
> 
> Fixes: ecb8ac8b1f14("mm/madvise: introduce process_madvise() syscall: an external memory hinting API")
> Cc: <stable@...r.kernel.org> # 5.10+
> Signed-off-by: Charan Teja Kalla <quic_charante@...cinc.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> Changes in V2:
>  -- Separated the ENOMEM handling and return bytes processed, as per Minchan comments.
>  -- This contains correcting return bytes processed with process_madvise().
> 
> Changes in V1:
>  -- Fixed the ENOMEM handling and return bytes processed by process_madvise.
>  -- https://patchwork.kernel.org/project/linux-mm/patch/1646803679-11433-1-git-send-email-quic_charante@quicinc.com/
> 
>  mm/madvise.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index 38d0f51..e97e6a9 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -1433,8 +1433,7 @@ SYSCALL_DEFINE5(process_madvise, int, pidfd, const struct iovec __user *, vec,
>  		iov_iter_advance(&iter, iovec.iov_len);
>  	}
>  
> -	if (ret == 0)
> -		ret = total_len - iov_iter_count(&iter);
> +	ret = (total_len - iov_iter_count(&iter)) ? : ret;
>  
>  release_mm:
>  	mmput(mm);
> -- 
> 2.7.4

-- 
Michal Hocko
SUSE Labs