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b=mRnkjc8PEzcdgLosKuDo9W5Ok4pnCQFFHZKl7jIH1MyWc5UZ6Z/M9+dpM8y0Loil5 Lf4IJRMSwIWFjqzPwylw3L5+M7v4ZbOBWytqAS+1Bz+vDriy2Q6fzncCPE1jIM03bz efxJk4g2FEqWtJMOCVNbvH8zHvjQqozYIQk6q0Z1jpuvxSjADF2gT1XmrBUKutHJSO cdPIwwKFEVz5UfiWVcaWSZL7N98QUM/BbGVlWIvZe8hA3/PJ++4PnJEK+fkDZtWk+A DyjrbAh5GLzSIpBbMsm0beKvzr3yfHjXJ2njrCoYddtASaeLL0cfkwt4rWBvYUcssC jUkZEpG13Phng== Message-ID: <6a3bfdb9-7839-4f39-ba5c-5a0c55055aee@kernel.org> Date: Thu, 30 Jul 2026 11:28:59 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 1/2] mm: allow smaller large folios to use lru_cache To: "Barry Song (Xiaomi)" Cc: akpm@linux-foundation.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, mhocko@suse.com, npache@redhat.com, rppt@kernel.org, ryan.roberts@arm.com, surenb@google.com, vbabka@kernel.org, ziy@nvidia.com References: <229c3079-8fde-41b4-9d6c-f0e34f780555@kernel.org> <20260729222651.31321-1-baohua@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 8bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: 89141180002 X-Stat-Signature: 7ywf1pzk4sf7opajiujr3h9dsychjhda X-Rspam-User: X-HE-Tag: 1785403746-29618 X-HE-Meta: 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 qxSjzB/m KGEYyAA7zvzPLylda5phcDFVYnaXMYmxSic6O7x6pzXZXLJK6h/6lWyOfdYX8dhAMcd1p0NZFdQNPMxe6gPAv1eabOTu12RNr9acRsQIpYUuWF2j+qtbH6qBrAdyXjHq8mtm7hwLRhbtqn2wuWPNJvramw/SK89s+dxMgTS23HOMrZQ1B9tsydn1l6dqXeJoprGbgFHrovXCqRLUqGLrtf35ylO3YKxjJQBl74NV09Pgzxk7wDWaRRs6TEeNF7W0RCA33lfvdf7SBbjghsd6u4keOkQGwH4rg4KpciK8LQP1xxsH0vBhxU10Ni+1sRcu1zJE94dKMgWdi9Yarl3ZwpelUiIVe959kxV2st8ziXEjP6FGwFB/e3jC9d0zQuk7iik/J Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 7/30/26 00:26, Barry Song (Xiaomi) wrote: > On Wed, Jul 29, 2026 at 8:13 PM David Hildenbrand (Arm) wrote: > [...] >>> diff --git a/include/linux/swap.h b/include/linux/swap.h >>> index 8d19be675baf..9c5f7a11c7b0 100644 >>> --- a/include/linux/swap.h >>> +++ b/include/linux/swap.h >>> @@ -316,9 +316,9 @@ static inline bool folio_may_be_lru_cached(struct folio *folio) >>> /* >>> * Holding PMD-sized folios in per-CPU LRU cache unbalances accounting. >>> * Holding small numbers of low-order mTHP folios in per-CPU LRU cache >>> - * will be sensible, but nobody has implemented and tested that yet. >>> + * will be sensible. >>> */ >>> - return !folio_test_large(folio); >>> + return folio_order(folio) < PAGE_ALLOC_COSTLY_ORDER; >>> } >>> >>> extern atomic_t lru_disable_count; >> >> Sashiko rightfully raises that split_folio() can now fail more easily. >> >> So we might want to proactively drain (earlier?) on some more of the split paths. > > Yes, this is a pain. I haven't tested it, but perhaps a > conceptual model could be something like this? > > diff --git a/mm/huge_memory.c b/mm/huge_memory.c > index 04e8a6b55343..fcd449dec96b 100644 > --- a/mm/huge_memory.c > +++ b/mm/huge_memory.c > @@ -4069,6 +4069,8 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > struct folio *new_folio, *next; > int nr_shmem_dropped = 0; > enum ttu_flags ttu_flags = 0; > + bool maybe_in_lru_cache; > + int expected_ref_count; > int ret; > pgoff_t end = 0; > > @@ -4154,7 +4156,15 @@ static int __folio_split(struct folio *folio, unsigned int new_order, > * Racy check if we can split the page, before unmap_folio() will > * split PMDs > */ > - if (folio_expected_ref_count(folio) != folio_ref_count(folio) - 1) { > + maybe_in_lru_cache = folio_may_be_lru_cached(folio) && !folio_test_lru(folio); > + expected_ref_count = folio_expected_ref_count(folio); > + if (expected_ref_count + maybe_in_lru_cache < folio_ref_count(folio) - 1) { > + ret = -EAGAIN; > + goto out_unlock; > + } > + if (maybe_in_lru_cache) > + lru_add_drain_all(); The drain-all is nasty. In collect_longterm_unpinnable_folios() we escalate the drainining. I think we recently stumbled into a similar problem in guest_memfd. Ah, yes, there it is: https://lore.kernel.org/r/20260728-gmem-inplace-conversion-v9-14-35f9aec2aed2@google.com -- Cheers, David