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SubjectRe: [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@vger.kernel.org> # 5.10+
> Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>

Acked-by: Michal Hocko <mhocko@suse.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

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