Re: [PATCH mm-unstable v3 1/3] mm/page_alloc: ignore the exact initial compaction result

[Michal Hocko]

On Tue 06-01-26 12:52:36, Vlastimil Babka wrote:
> For allocations that are of costly order and __GFP_NORETRY (and can
> perform compaction) we attempt direct compaction first. If that fails,
> we continue with a single round of direct reclaim+compaction (as for
> other __GFP_NORETRY allocations, except the compaction is of lower
> priority), with two exceptions that fail immediately:
> 
> - __GFP_THISNODE is specified, to prevent zone_reclaim_mode-like
>   behavior for e.g. THP page faults
> 
> - compaction failed because it was deferred (i.e. has been failing
>   recently so further attempts are not done for a while) or skipped,
>   which means there are insufficient free base pages to defragment to
>   begin with
> 
> Upon closer inspection, the second condition has a somewhat flawed
> reasoning. If there are not enough base pages and reclaim could create
> them, we instead fail. When there are enough base pages and compaction
> has already ran and failed, we proceed and hope that reclaim and the
> subsequent compaction attempt will succeed. But it's unclear why they
> should and whether it will be as inexpensive as intended.
> 
> It might make therefore more sense to just fail unconditionally after
> the initial compaction attempt. However that would change the semantics
> of __GFP_NORETRY to attempt reclaim at least once.
> 
> Alternatively we can remove the compaction result checks and proceed
> with the single reclaim and (lower priority) compaction attempt, leaving
> only the __GFP_THISNODE exception for failing immediately.
> 
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>
Thanks!

> ---
>  mm/page_alloc.c | 34 ++++++----------------------------
>  1 file changed, 6 insertions(+), 28 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ac8a12076b00..b06b1cb01e0e 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -4805,44 +4805,22 @@ __alloc_pages_slowpath(gfp_t gfp_mask, unsigned int order,
>  		 * includes some THP page fault allocations
>  		 */
>  		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
> -			/*
> -			 * If allocating entire pageblock(s) and compaction
> -			 * failed because all zones are below low watermarks
> -			 * or is prohibited because it recently failed at this
> -			 * order, fail immediately unless the allocator has
> -			 * requested compaction and reclaim retry.
> -			 *
> -			 * Reclaim is
> -			 *  - potentially very expensive because zones are far
> -			 *    below their low watermarks or this is part of very
> -			 *    bursty high order allocations,
> -			 *  - not guaranteed to help because isolate_freepages()
> -			 *    may not iterate over freed pages as part of its
> -			 *    linear scan, and
> -			 *  - unlikely to make entire pageblocks free on its
> -			 *    own.
> -			 */
> -			if (compact_result == COMPACT_SKIPPED ||
> -			    compact_result == COMPACT_DEFERRED)
> -				goto nopage;
> -
>  			/*
>  			 * THP page faults may attempt local node only first,
>  			 * but are then allowed to only compact, not reclaim,
>  			 * see alloc_pages_mpol().
>  			 *
> -			 * Compaction can fail for other reasons than those
> -			 * checked above and we don't want such THP allocations
> -			 * to put reclaim pressure on a single node in a
> -			 * situation where other nodes might have plenty of
> -			 * available memory.
> +			 * Compaction has failed above and we don't want such
> +			 * THP allocations to put reclaim pressure on a single
> +			 * node in a situation where other nodes might have
> +			 * plenty of available memory.
>  			 */
>  			if (gfp_mask & __GFP_THISNODE)
>  				goto nopage;
>  
>  			/*
> -			 * Looks like reclaim/compaction is worth trying, but
> -			 * sync compaction could be very expensive, so keep
> +			 * Proceed with single round of reclaim/compaction, but
> +			 * since sync compaction could be very expensive, keep
>  			 * using async compaction.
>  			 */
>  			compact_priority = INIT_COMPACT_PRIORITY;
> 
> -- 
> 2.52.0

-- 
Michal Hocko
SUSE Labs