Re: [PATCH 06/14] mm, migrate: Immediately fail migration of a page with no migration handler

Yang Shi
2018-12-20T19:45:27Z

On Fri, Dec 14, 2018 at 3:03 PM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> Pages with no migration handler use a fallback hander which sometimes
> works and sometimes persistently fails such as blockdev pages. Migration

A minor correction. The above statement sounds not accurate anymore
since Jan Kara had patch series (blkdev: avoid migration stalls for
blkdev pages) have blockdev use its own migration handler.

Thanks,
Yang

> will retry a number of times on these persistent pages which is wasteful
> during compaction. This patch will fail migration immediately unless the
> caller is in MIGRATE_SYNC mode which indicates the caller is willing to
> wait while being persistent.
>
> This is not expected to help THP allocation success rates but it does
> reduce latencies slightly.
>
> 1-socket thpfioscale
> 4.20.0-rc6 4.20.0-rc6
> noreserved-v1r4 failfast-v1r4
> Amean fault-both-1 0.00 ( 0.00%) 0.00 * 0.00%*
> Amean fault-both-3 2276.15 ( 0.00%) 3867.54 * -69.92%*
> Amean fault-both-5 4992.20 ( 0.00%) 5313.20 ( -6.43%)
> Amean fault-both-7 7373.30 ( 0.00%) 7039.11 ( 4.53%)
> Amean fault-both-12 11911.52 ( 0.00%) 11328.29 ( 4.90%)
> Amean fault-both-18 17209.42 ( 0.00%) 16455.34 ( 4.38%)
> Amean fault-both-24 20943.71 ( 0.00%) 20448.94 ( 2.36%)
> Amean fault-both-30 22703.00 ( 0.00%) 21655.07 ( 4.62%)
> Amean fault-both-32 22461.41 ( 0.00%) 21415.35 ( 4.66%)
>
> The 2-socket results are not materially different. Scan rates are
> similar as expected.
>
> Signed-off-by: Mel Gorman <mgorman@techsingularity.net>
> ---
> mm/migrate.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index df17a710e2c7..0e27a10429e2 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -885,7 +885,7 @@ static int fallback_migrate_page(struct address_space *mapping,
> */
> if (page_has_private(page) &&
> !try_to_release_page(page, GFP_KERNEL))
> - return -EAGAIN;
> + return mode == MIGRATE_SYNC ? -EAGAIN : -EBUSY;
>
> return migrate_page(mapping, newpage, page, mode);
> }
> --
> 2.16.4
>