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    SubjectRe: dm bufio: Reduce dm_bufio_lock contention
    On Thu 21-06-18 21:17:24, Mikulas Patocka wrote:
    [...]
    > > But seriously, isn't the best way around the throttling issue to use
    > > PF_LESS_THROTTLE?
    >
    > Yes - it could be done by setting PF_LESS_THROTTLE. But I think it would
    > be better to change it just in one place than to add PF_LESS_THROTTLE to
    > every block device driver (because adding it to every block driver results
    > in more code).

    Why would every block device need this? I thought we were talking about
    mempool allocator and the md variant of it. They are explicitly doing
    their own back off so PF_LESS_THROTTLE sounds appropriate to me.

    > What about this patch? If __GFP_NORETRY and __GFP_FS is not set (i.e. the
    > request comes from a block device driver or a filesystem), we should not
    > sleep.

    Why? How are you going to audit all the callers that the behavior makes
    sense and moreover how are you going to ensure that future usage will
    still make sense. The more subtle side effects gfp flags have the harder
    they are to maintain.

    > Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    > Cc: stable@vger.kernel.org
    >
    > Index: linux-2.6/mm/vmscan.c
    > ===================================================================
    > --- linux-2.6.orig/mm/vmscan.c
    > +++ linux-2.6/mm/vmscan.c
    > @@ -2674,6 +2674,7 @@ static bool shrink_node(pg_data_t *pgdat
    > * the LRU too quickly.
    > */
    > if (!sc->hibernation_mode && !current_is_kswapd() &&
    > + (sc->gfp_mask & (__GFP_NORETRY | __GFP_FS)) != __GFP_NORETRY &&
    > current_may_throttle() && pgdat_memcg_congested(pgdat, root))
    > wait_iff_congested(BLK_RW_ASYNC, HZ/10);
    >

    --
    Michal Hocko
    SUSE Labs

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