Re: [PATCH v4 2/4] mm: failfast mode with __GFP_NORETRY in alloc_contig_range

[Minchan Kim <minchan@kernel.org>]

On Mon, Jan 25, 2021 at 02:12:00PM +0100, Michal Hocko wrote:
> On Thu 21-01-21 09:55:00, 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 different range where would have easy migratable pages
> > without stalling.
> > 
> > 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 waiting.
> 
> I am not against controling how hard this allocator tries with gfp mask
> but this changelog is rather void on any data and any user.
> 
> It is also rather dubious to have retries when then caller says to not
> retry.

Since max_tries is 1 with ++tries, it shouldn't retry.

> 
> Also why didn't you consider GFP_NOWAIT semantic for non blocking mode?

GFP_NOWAIT seems to be low(specific) flags rather than the one I want to
express. Even though I said only page writeback/lock in the description,
the goal is to avoid costly operations we might find later so such
"failfast", I thought GFP_NORETRY would be good fit.

> 
> > 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 b031a5ae0bd5..1cdc3ee0b22e 100644
> > --- a/mm/page_alloc.c
> > +++ b/mm/page_alloc.c
> > @@ -8491,12 +8491,16 @@ 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)) {
> > @@ -8513,7 +8517,7 @@ 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;
> >  		}
> > @@ -8564,7 +8568,7 @@ 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),
> > -- 
> > 2.30.0.296.g2bfb1c46d8-goog
> 
> -- 
> Michal Hocko
> SUSE Labs