linux-kernel - Re: [PATCH 3/7] mm/gup: local lru_add_drain() to avoid lru_add_drain

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Message-ID: <e3922760-b074-4d5a-bf4a-9d7cf7f5f914@redhat.com>
Date: Mon, 1 Sep 2025 10:05:49 +0200
From: David Hildenbrand <david@...hat.com>
To: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Will Deacon <will@...nel.org>, Shivank Garg <shivankg@....com>,
 Matthew Wilcox <willy@...radead.org>, Christoph Hellwig <hch@...radead.org>,
 Keir Fraser <keirf@...gle.com>, Jason Gunthorpe <jgg@...pe.ca>,
 John Hubbard <jhubbard@...dia.com>, Frederick Mayle <fmayle@...gle.com>,
 Peter Xu <peterx@...hat.com>, "Aneesh Kumar K.V" <aneesh.kumar@...nel.org>,
 Johannes Weiner <hannes@...xchg.org>, Vlastimil Babka <vbabka@...e.cz>,
 Alexander Krabler <Alexander.Krabler@...a.com>, Ge Yang
 <yangge1116@....com>, Li Zhe <lizhe.67@...edance.com>,
 Chris Li <chrisl@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
 Axel Rasmussen <axelrasmussen@...gle.com>, Yuanchu Xie <yuanchu@...gle.com>,
 Wei Xu <weixugc@...gle.com>, Konstantin Khlebnikov <koct9i@...il.com>,
 linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 3/7] mm/gup: local lru_add_drain() to avoid
 lru_add_drain_all()

On 31.08.25 11:08, Hugh Dickins wrote:
> In many cases, if collect_longterm_unpinnable_folios() does need to
> drain the LRU cache to release a reference, the cache in question is
> on this same CPU, and much more efficiently drained by a preliminary
> local lru_add_drain(), than the later cross-CPU lru_add_drain_all().
> 
> Marked for stable, to counter the increase in lru_add_drain_all()s
> from "mm/gup: check ref_count instead of lru before migration".
> Note for clean backports: can take 6.16 commit a03db236aebf ("gup:
> optimize longterm pin_user_pages() for large folio") first.
> 
> Signed-off-by: Hugh Dickins <hughd@...gle.com>
> Cc: <stable@...r.kernel.org>
> ---
>   mm/gup.c | 2 ++
>   1 file changed, 2 insertions(+)
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 82aec6443c0a..9f7c87f504a9 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -2291,6 +2291,8 @@ static unsigned long collect_longterm_unpinnable_folios(
>   	struct folio *folio;
>   	long i = 0;
>   
> +	lru_add_drain();
> +
>   	for (folio = pofs_get_folio(pofs, i); folio;
>   	     folio = pofs_next_folio(folio, pofs, &i)) {
>   

Do we really want to drain all the time we enter 
collect_longterm_unpinnable_folios(), or only if we detect an actual 
problem? (unexpected reference?)

-- 
Cheers

David / dhildenb