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SubjectRe: [PATCH v6 04/11] mm, compaction: don't recheck watermarks after COMPACT_SUCCESS
On Wed, Aug 10, 2016 at 11:12:19AM +0200, Vlastimil Babka wrote:
> Joonsoo has reminded me that in a later patch changing watermark checks
> throughout compaction I forgot to update checks in try_to_compact_pages() and
> compactd_do_work(). Closer inspection however shows that they are redundant now
> that compact_zone() reliably reports success with COMPACT_SUCCESS, as they just
> repeat (a subset) of checks that have just passed. So instead of checking
> watermarks again, just test the return value.

In fact, it's not redundant. Even if try_to_compact_pages() returns
!COMPACT_SUCCESS, watermark check could return true.
__compact_finished() calls find_suitable_fallback() and it's slightly
different with watermark check. Anyway, I don't think it is a big
problem.

Thanks.


>
> Also remove the stray "bool success" variable from kcompactd_do_work().
>
> Reported-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
> Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
> ---
> mm/compaction.c | 11 +++--------
> 1 file changed, 3 insertions(+), 8 deletions(-)
>
> diff --git a/mm/compaction.c b/mm/compaction.c
> index c355bf0d8599..a144f58f7193 100644
> --- a/mm/compaction.c
> +++ b/mm/compaction.c
> @@ -1698,9 +1698,8 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
> alloc_flags, ac_classzone_idx(ac));
> rc = max(status, rc);
>
> - /* If a normal allocation would succeed, stop compacting */
> - if (zone_watermark_ok(zone, order, low_wmark_pages(zone),
> - ac_classzone_idx(ac), alloc_flags)) {
> + /* The allocation should succeed, stop compacting */
> + if (status == COMPACT_SUCCESS) {
> /*
> * We think the allocation will succeed in this zone,
> * but it is not certain, hence the false. The caller
> @@ -1873,8 +1872,6 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> .ignore_skip_hint = true,
>
> };
> - bool success = false;
> -
> trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
> cc.classzone_idx);
> count_vm_event(KCOMPACTD_WAKE);
> @@ -1903,9 +1900,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
> return;
> status = compact_zone(zone, &cc);
>
> - if (zone_watermark_ok(zone, cc.order, low_wmark_pages(zone),
> - cc.classzone_idx, 0)) {
> - success = true;
> + if (status == COMPACT_SUCCESS) {
> compaction_defer_reset(zone, cc.order, false);
> } else if (status == COMPACT_PARTIAL_SKIPPED || status == COMPACT_COMPLETE) {
> /*
> --
> 2.9.2
>
> --
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