Re: [RFC PATCH] mm: Drain PCP during direct reclaim

[Johannes Weiner <hannes@cmpxchg.org>]

On Fri, Jun 06, 2025 at 02:59:30PM +0800, Wupeng Ma wrote:
> Memory retained in Per-CPU Pages (PCP) caches can prevent hugepage
> allocations from succeeding despite sufficient free system memory. This
> occurs because:
> 1. Hugepage allocations don't actively trigger PCP draining
> 2. Direct reclaim path fails to trigger drain_all_pages() when:
>    a) All zone pages are free/hugetlb (!did_some_progress)
>    b) Compaction skips due to costly order watermarks (COMPACT_SKIPPED)

This doesn't sound quite right. Direct reclaim skips when compaction
is suitable. Compaction says COMPACT_SKIPPED when it *isn't* suitable.

So if direct reclaim didn't drain, presumably compaction ran but
returned COMPLETE or PARTIAL_SKIPPED because the freelist checks in
__compact_finished() never succeed due to the pcp?

> @@ -4137,28 +4137,22 @@ __alloc_pages_direct_reclaim(gfp_t gfp_mask, unsigned int order,
>  {
>  	struct page *page = NULL;
>  	unsigned long pflags;
> -	bool drained = false;
>  
>  	psi_memstall_enter(&pflags);
>  	*did_some_progress = __perform_reclaim(gfp_mask, order, ac);
> -	if (unlikely(!(*did_some_progress)))
> -		goto out;
> -
> -retry:
> -	page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
> +	if (likely(*did_some_progress))
> +		page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);
>  
>  	/*
>  	 * If an allocation failed after direct reclaim, it could be because
>  	 * pages are pinned on the per-cpu lists or in high alloc reserves.
>  	 * Shrink them and try again
>  	 */
> -	if (!page && !drained) {
> +	if (!page) {
>  		unreserve_highatomic_pageblock(ac, false);
>  		drain_all_pages(NULL);
> -		drained = true;
> -		goto retry;
> +		page = get_page_from_freelist(gfp_mask, order, alloc_flags, ac);

This seems like the wrong place to fix the issue.

Kcompactd has a drain_all_pages() call. Move that to compact_zone(),
so that it also applies to the try_to_compact_pages() path?