On Wed 27-01-21 12:42:45, Minchan Kim wrote:

On Tue, Jan 26, 2021 at 08:44:49AM +0100, Michal Hocko wrote:

quoted

On Mon 25-01-21 11:33:36, Minchan Kim wrote:

quoted

On Mon, Jan 25, 2021 at 02:12:00PM +0100, Michal Hocko wrote:

quoted

On Thu 21-01-21 09:55:00, Minchan Kim wrote:

quoted

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.

-- 
Michal Hocko
SUSE Labs