Re: [PATCH v3 2/4] mm: failfast mode with __GFP_NORETRY in alloc_contig_range

David Hildenbrand
2021-01-13T08:39:39Z

On 13.01.21 02:21, Minchan Kim wrote:
> Contiguous memory allocation can be stalled due to waiting
> on page writeback and/or page lock which causes unpredictable
> delay. It's a unavoidable cost for the requestor to get *big*
> contiguous memory but it's expensive for *small* contiguous
> memory(e.g., order-4) because caller could retry the request
> in diffrent range where would have easy migratable pages
> without stalling.

s/diffrent/different/

>
> This patch introduce __GFP_NORETRY as compaction gfp_mask in
> alloc_contig_range so it will fail fast without blocking
> when it encounters pages needed waitting.

s/waitting/waiting/

>
> Signed-off-by: Minchan Kim <minchan@kernel.org>
> ---
> mm/page_alloc.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5b3923db9158..ff41ceb4db51 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -8489,12 +8489,16 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> unsigned int nr_reclaimed;
> unsigned long pfn = start;
> unsigned int tries = 0;
> + unsigned int max_tries = 5;
> int ret = 0;
> struct migration_target_control mtc = {
> .nid = zone_to_nid(cc->zone),
> .gfp_mask = GFP_USER | __GFP_MOVABLE | __GFP_RETRY_MAYFAIL,
> };
>
> + if (cc->alloc_contig && cc->mode == MIGRATE_ASYNC)
> + max_tries = 1;
> +
> migrate_prep();
>
> while (pfn < end || !list_empty(&cc->migratepages)) {
> @@ -8511,7 +8515,7 @@ static int __alloc_contig_migrate_range(struct compact_control *cc,
> break;
> }
> tries = 0;
> - } else if (++tries == 5) {
> + } else if (++tries == max_tries) {
> ret = ret < 0 ? ret : -EBUSY;
> break;
> }
> @@ -8562,7 +8566,7 @@ int alloc_contig_range(unsigned long start, unsigned long end,
> .nr_migratepages = 0,
> .order = -1,
> .zone = page_zone(pfn_to_page(start)),
> - .mode = MIGRATE_SYNC,
> + .mode = gfp_mask & __GFP_NORETRY ? MIGRATE_ASYNC : MIGRATE_SYNC,
> .ignore_skip_hint = true,
> .no_set_skip_hint = true,
> .gfp_mask = current_gfp_context(gfp_mask),
>

I'm fine with using gfp flags (e.g., __GFP_NORETRY) as long as they
don't enable other implicit behavior (e.g., move draining X to the
caller) that's hard to get from the flag name.

IMHO, if we ever want to move draining to the caller, or change the
behavior of alloc_contig_range() in different ways (e.g., disable PCP),
we won't get around introducing a separate set of flags for
alloc_contig_range().

Let's see what Michal thinks. Thanks!

--
Thanks,

David / dhildenb