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

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Message-ID: <b2aca727-ecb8-5429-e418-e8bcbc8df070@google.com>
Date: Mon, 8 Sep 2025 03:53:48 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: David Hildenbrand <david@...hat.com>
cc: Hugh Dickins <hughd@...gle.com>, Andrew Morton <akpm@...ux-foundation.org>, 
    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 Mon, 1 Sep 2025, David Hildenbrand wrote:
> 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?)

It looked nice and simple to me (hmm, where's the blank line before
lru_add_drain() gone? weird, something wrong with my mail setup),
I've never avoided an lru_add_drain() before; but you're right,
we dom't need to do that every time, fixed in v2 - thanks.

Hugh