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b=VKt0ev36BT3N/oyYml6Oz7KOswtKD+GwJbS+MB6y6jrMJ+wF3KAt7UZyAlbvo1C5M 7tFIFsyU+ADcWjdUgj3L9M7uFyh9P2nUrSSCXBvfzr1lU0MjRGZQuz1z3pz9XfjZC8 64EZuAA4GSX8Rwxv9uX/xRUeBK+FWkq2Q1s/IZmWD40+Lij6oih+mKsYRQn3xutP0b 0XO99QYvfgazYuG5GRS6S7fcEuINPkHSMLd4FHfTDUgzSjDO96na35fVJjfiEP6KXw IK0NBNoyw6Iku/4PH7vVdJQ/0HpOqFGNNT2HlVX72JugeJB1MtSHw/8clpznUWsM40 rNFOrhXuHB/Hw== Message-ID: Date: Mon, 3 Aug 2026 10:03:29 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v2 2/2] mm: improve large folio reuse for LRU-cached folios To: Barry Song Cc: akpm@linux-foundation.org, linux-mm@kvack.org, ljs@kernel.org, baolin.wang@linux.alibaba.com, dev.jain@arm.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, npache@redhat.com, ryan.roberts@arm.com, ziy@nvidia.com, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com References: <20260709081536.82768-1-baohua@kernel.org> <20260709081536.82768-3-baohua@kernel.org> <8f74bd51-11f2-4837-9463-d016444e45cc@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: D053040011 X-Stat-Signature: aj59bu4yo7kgeo4o3qqndqa7kpen4n7g X-Rspam-User: X-HE-Tag: 1785744217-207219 X-HE-Meta: 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 SRtt4jUt y/mnbT/CEua00c5WQhAULx/m5CB+DCMF+7jGYFvjvOHY6BDhsAjlW/7YMysuaGt8hShzf6bL28o4GLeehdyts9TBII8qh3/XdE5HrGgzxxcEjkrglQNCs0/0xYm6xyyViWGkCGSwjYFac7RJgLuXTB4OeOdP/RBgtz6+wOF1bbeld4eCDHc+2D2kRK4e6uF0IfeROMJZfizNzGIEjXTnud1Tu3KX9QQpcUh2l1+aPMQn4kxsDGPqnvuXK+sGeVNpn0e4vYiYmVDOmTISjxI3soPHk2PCfpVLcv0sPhGvmiMUC9PdOMYlmiCfnT5eH7qdAubqiEP0CgnMggiJ8LqJayJzcuRm62+ppkQ/m Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 8/3/26 09:24, Barry Song wrote: > On Wed, Jul 29, 2026 at 8:11 PM David Hildenbrand (Arm) > wrote: >> >> On 7/9/26 10:15, Barry Song (Xiaomi) wrote: >>> Large folios may now reside in the per-CPU LRU cache. Before >>> attempting to reuse them, drain the local LRU cache, which >>> can still be beneficial in cases where the folios are likely >>> to remain in this CPU's LRU cache: >>> >>> int main(int argc, char *argv[]) >>> { >>> int i; >>> while (1) { >>> volatile int *p = mmap(0, SIZE, PROT_READ | PROT_WRITE, >>> MAP_PRIVATE | MAP_ANONYMOUS, >>> -1, 0); >>> for (int i = 0; i < SIZE / sizeof(int); i++) >>> p[i] = i; >>> madvise((void *)p, SIZE, MADV_PAGEOUT); >>> if (!fork()) >>> _exit(0); >>> for (int i = 0; i < SIZE / sizeof(int); i++) >>> p[i] = i; >>> munmap((void *)p, SIZE); >>> } >>> return 0; >>> } >>> >>> Signed-off-by: Barry Song (Xiaomi) >>> --- >>> mm/memory.c | 13 +++++++++++++ >>> 1 file changed, 13 insertions(+) >>> >>> diff --git a/mm/memory.c b/mm/memory.c >>> index 5689b7cff76c..1d08ed5ba99b 100644 >>> --- a/mm/memory.c >>> +++ b/mm/memory.c >>> @@ -4136,6 +4136,19 @@ static bool __wp_can_reuse_large_anon_folio(struct folio *folio, >>> folio_unlock(folio); >>> } >>> >>> + if (folio_may_be_lru_cached(folio) && !folio_test_lru(folio)) { >>> + if (folio_ref_count(folio) > folio_large_mapcount(folio) + 1) >> >> I assume that can just be a "!=". > > I was trying to guard against the case where a folio is not on > the LRU but also does not hold an extra reference. For example, > the folio is off the LRU, yet > folio_ref_count(folio) == folio_large_mapcount(folio). > > In that case, we could simply reuse !folio_test_lru(folio). > > Maybe such a case cannot actually happen, since paths like > isolate_folio() take an extra reference before removing the > folio from the LRU. > > So we should be ok to just use folio_ref_count(folio) != > folio_large_mapcount(folio) +1 I just stumbled into something nasty last week: https://lore.kernel.org/r/20260731-check_and_migrate_movable_folios-v1-1-e0002d7b791e@kernel.org For an mTHP, if we end up pinning multiple pages, calculating pin_refs is problematic and might require us to batch over multiple pages. But there are some nasty corner cases when just batching contiguous pages. -- Cheers, David