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

[Minchan Kim]

On Thu, Jan 28, 2021 at 08:53:25AM +0100, Michal Hocko wrote:
> On Wed 27-01-21 12:42:45, Minchan Kim wrote:
> > On Tue, Jan 26, 2021 at 08:44:49AM +0100, Michal Hocko wrote:
> > > On Mon 25-01-21 11:33:36, Minchan Kim wrote:
> > > > 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.
> > > 
> > > OK, I have missed that. This is a tricky code. ASYNC mode should be
> > > completely orthogonal to the retries count. Those are different things.
> > > Page allocator does an explicit bail out based on __GFP_NORETRY. You
> > > should be doing the same.
> > 
> > Before sending next revision, let me check this part again.
> > 
> > I want to use __GFP_NORETRY to indicate "opportunistic-easy-to-fail attempt"
> > and I want to use ASYNC migrate_mode to help the goal.
> > 
> > Do you see the problem?
> 
> No, as I've said. This is a normal NORETRY policy. And ASYNC migration
> is a mere implementation detail you do not have bother your users about.
> This is the semantic view. From the implementation POV it should be the
> gfp mask to drive decisions rather than a random (ASYNC) flag to control
> retries as you did here.

Make sense.

Let me cook next revision.

Thanks for the review, Michal.