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    Subject[PATCH 5.17 0106/1126] mm: madvise: return correct bytes advised with process_madvise
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    From: Charan Teja Kalla <quic_charante@quicinc.com>

    commit 5bd009c7c9a9e888077c07535dc0c70aeab242c3 upstream.

    Patch series "mm: madvise: return correct bytes processed with
    process_madvise", v2. With the process_madvise(), always choose to return
    non zero processed bytes over an error. This can help the user to know on
    which VMA, passed in the 'struct iovec' vector list, is failed to advise
    thus can take the decission of retrying/skipping on that VMA.

    This patch (of 2):

    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.

    Link: https://lkml.kernel.org/r/cover.1647008754.git.quic_charante@quicinc.com
    Link: https://lkml.kernel.org/r/125b61a0edcee5c2db8658aed9d06a43a19ccafc.1647008754.git.quic_charante@quicinc.com
    Fixes: ecb8ac8b1f14("mm/madvise: introduce process_madvise() syscall: an external memory hinting API")
    Signed-off-by: Charan Teja Kalla <quic_charante@quicinc.com>
    Cc: Suren Baghdasaryan <surenb@google.com>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: David Rientjes <rientjes@google.com>
    Cc: Stephen Rothwell <sfr@canb.auug.org.au>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Nadav Amit <nadav.amit@gmail.com>
    Cc: Michal Hocko <mhocko@suse.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    mm/madvise.c | 3 +--
    1 file changed, 1 insertion(+), 2 deletions(-)

    --- a/mm/madvise.c
    +++ b/mm/madvise.c
    @@ -1440,8 +1440,7 @@ SYSCALL_DEFINE5(process_madvise, int, pi
    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);

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