On Tue 26-01-21 11:10:18, 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.

A concern with __GFP_NOWAIT is regardless of flags passed to cma_alloc,
internal implementation of alloc_contig_range inside will use blockable
operation. See __alloc_contig_migrate_range.

Yes it is now. But nothing should prevent from making it non blockable.

If we go with __GFP_NOWAIT, we should propagate the gfp_mask inside of
__alloc_contig_migrate_range to make cma_alloc consistent with alloc_pages.

Absolutely. You should be doing that anyway. As I've said above you
shouldn't rely on side effects like ASYNC mode.

(IIUC, that's what you want - make gfp_mask consistent between cma_alloc
and alloc_pages) but I am worry about the direction will make complicate
situation since cma invovles migration context as well as target page
allocation context. Sometime, the single gfp flag could be trouble
to express both contexts all at once. 

I am not sure I see your concern.

quoted

quoted

quoted

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.

I suspect you are too focused on implementation details here. Think
about the indended semantic. Callers of this functionality will not
think about those (I hope because if they rely on these details then the
whole thing will become unmaintainable because any change would require
an audit of all existing users). All you should be caring about is to
control how expensive the call can be. GFP_NOWAIT is not really low
level from that POV. It gives you a very lightweight non-sleeping
attempt to allocate. GFP_NORETRY will give you potentially sleeping but
an opportunistic-easy-to-fail attempt. And so on. See how that is
absolutely free of any page writeback or any specific locking.

With above reason I mentioned, I wanted to express __GFP_NORETRY as 
"opportunistic-easy-to-fail attempt" to support cma_alloc as "failfast"
for migration context.

Yes that is fine. And please note that I do not push for NOWAIT
semantic. If there is no user for that now then fine.
-- 
Michal Hocko
SUSE Labs