Re: [PATCH RFC 2/2] mm, page_alloc: fail costly __GFP_NORETRY allocations faster

[Johannes Weiner <hannes@cmpxchg.org>]

On Tue, Dec 16, 2025 at 04:54:22PM +0100, Vlastimil Babka wrote:
> It might make therefore more sense to just fail unconditionally after
> the initial compaction attempt, so do that instead. Costly allocations
> that do want the reclaim/compaction to happen at least once can omit
> __GFP_NORETRY, or even specify __GFP_RETRY_MAYFAIL for more than one
> attempt.
>
> There is a slight potential unfairness in that costly __GFP_NORETRY
> allocations that can't perform direct compaction (i.e. lack __GFP_IO)
> will still be allowed to direct reclaim, while those that can direct
> compact will now never attempt direct reclaim. However, in cases of
> memory pressure causing compaction to be skipped due to insufficient
> base pages, direct reclaim was already not done before, so there should
> be no functional regressions from this change.

Hm, kind of. There could be enough basepages for compaction_suitable()
but compaction odds are still higher with more free pages. So there
might be cases it regresses.

__GFP_NORETRY semantics say it'll try reclaim at least once. We should
be able to keep that and still simplify, no?

>  		if (costly_order && (gfp_mask & __GFP_NORETRY)) {
> -			if (gfp_mask & __GFP_THISNODE)
> -				goto nopage;
> +			goto nopage;

IOW, maybe directly select for the NUMA-THP special case here?

		/* Optimistic node-local huge page - only compact once */
		if (costly_order &&
                    ((gfp_mask & (__GFP_NORETRY|__GFP_THISNODE)) ==
                     (__GFP_NORETRY|__GFP_THISNODE)))
			goto nopage;

and then let other __GFP_NORETRY fall through.