Re: [PATCH 6.12.y] mm: folio_may_be_lru_cached() unless folio_test_large()

[Hugh Dickins <hughd@google.com>]

On Sun, 21 Sep 2025, Greg KH wrote:
> On Sun, Sep 21, 2025 at 11:41:34AM -0400, Sasha Levin wrote:
> > From: Hugh Dickins <hughd@google.com>
> > 
> > [ Upstream commit 2da6de30e60dd9bb14600eff1cc99df2fa2ddae3 ]
> > 
> > mm/swap.c and mm/mlock.c agree to drain any per-CPU batch as soon as a
> > large folio is added: so collect_longterm_unpinnable_folios() just wastes
> > effort when calling lru_add_drain[_all]() on a large folio.
> > 
> > But although there is good reason not to batch up PMD-sized folios, we
> > might well benefit from batching a small number of low-order mTHPs (though
> > unclear how that "small number" limitation will be implemented).
> > 
> > So ask if folio_may_be_lru_cached() rather than !folio_test_large(), to
> > insulate those particular checks from future change.  Name preferred to
> > "folio_is_batchable" because large folios can well be put on a batch: it's
> > just the per-CPU LRU caches, drained much later, which need care.
> > 
> > Marked for stable, to counter the increase in lru_add_drain_all()s from
> > "mm/gup: check ref_count instead of lru before migration".
> > 
> > Link: https://lkml.kernel.org/r/57d2eaf8-3607-f318-e0c5-be02dce61ad0@google.com
> > Fixes: 9a4e9f3b2d73 ("mm: update get_user_pages_longterm to migrate pages allocated from CMA region")
> > Signed-off-by: Hugh Dickins <hughd@google.com>
> > Suggested-by: David Hildenbrand <david@redhat.com>
> > Acked-by: David Hildenbrand <david@redhat.com>
> > Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
> > Cc: Axel Rasmussen <axelrasmussen@google.com>
> > Cc: Chris Li <chrisl@kernel.org>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Jason Gunthorpe <jgg@ziepe.ca>
> > Cc: Johannes Weiner <hannes@cmpxchg.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: Keir Fraser <keirf@google.com>
> > Cc: Konstantin Khlebnikov <koct9i@gmail.com>
> > Cc: Li Zhe <lizhe.67@bytedance.com>
> > Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
> > Cc: Peter Xu <peterx@redhat.com>
> > Cc: Rik van Riel <riel@surriel.com>
> > Cc: Shivank Garg <shivankg@amd.com>
> > Cc: Vlastimil Babka <vbabka@suse.cz>
> > Cc: Wei Xu <weixugc@google.com>
> > Cc: Will Deacon <will@kernel.org>
> > Cc: yangge <yangge1116@126.com>
> > Cc: Yuanchu Xie <yuanchu@google.com>
> > Cc: Yu Zhao <yuzhao@google.com>
> > Cc: <stable@vger.kernel.org>
> > Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> > [ adapted to drain_allow instead of drained ]
> > Signed-off-by: Sasha Levin <sashal@kernel.org>
> > ---
> 
> Does not apply as it conflicts with the other mm changes you sent right
> before this one :(

Thanks for grabbing all these, I'm sorry they are troublesome.

Though I'm usually able to work out what to do from the FAILED mails,
in this case I'd just be guessing without the full contexts.
So I'll wait until I see what goes into the various branches of
linux-stable-rc.git before checking and adjusting where necessary.

(As usual, I'll tend towards minimal change, where Sasha tends
towards maximal backporting of encroaching mods: we may disagree.)

The main commits contributing to the pinning failures that Will Deacon
reported were commits going into 5.18 and 6.11.  So although I stand
by my Fixes tag, I'm likely to conclude that 5.15 and 5.10 and 5.4
are better left stable without any of it.

Thanks,
Hugh