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From: Baoquan He To: Barry Song Cc: Shakeel Butt , akpm@linux-foundation.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, david@kernel.org, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, chrisl@kernel.org, kasong@tencent.com, shikemeng@huaweicloud.com, nphamcs@gmail.com, youngjun.park@lge.com, jp.kobryn@linux.dev, usama.arif@linux.dev Subject: Re: [RFC PATCH 1/3] mm: avoid unnecessary lru drain for wp_can_reuse_anon_folio() Message-ID: References: <20260611105124.98668-1-baohua@kernel.org> <20260611105124.98668-2-baohua@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Migadu-Flow: FLOW_OUT On 06/12/26 at 09:57am, Barry Song wrote: > On Fri, Jun 12, 2026 at 9:08 AM Baoquan He wrote: > > > > On 06/11/26 at 11:17am, Shakeel Butt wrote: > > > On Thu, Jun 11, 2026 at 11:09:43AM -0700, Shakeel Butt wrote: > > > > On Thu, Jun 11, 2026 at 06:51:22PM +0800, Barry Song (Xiaomi) wrote: > > > > > We always unconditionally drain the LRU before retrying anon folio > > > > > reuse in wp_can_reuse_anon_folio(). Instead, assume !LRU anon folios > > > > > are in lru_cache, and use the refcount to avoid many unnecessary LRU > > > > > drains. > > > > > > > > > > Signed-off-by: Barry Song (Xiaomi) > > > > > --- > > > > > mm/memory.c | 8 +++++++- > > > > > 1 file changed, 7 insertions(+), 1 deletion(-) > > > > > > > > > > diff --git a/mm/memory.c b/mm/memory.c > > > > > index 56be920c56d7..487a34377a7b 100644 > > > > > --- a/mm/memory.c > > > > > +++ b/mm/memory.c > > > > > @@ -4193,12 +4193,18 @@ static bool wp_can_reuse_anon_folio(struct folio *folio, > > > > > */ > > > > > if (folio_test_ksm(folio) || folio_ref_count(folio) > 3) > > > > > return false; > > > > > - if (!folio_test_lru(folio)) > > > > > + if (!folio_test_lru(folio)) { > > > > > + /* > > > > > + * Assume folio is on lru_cache and holds a cache reference. > > > > > + */ > > > > > + if (folio_ref_count(folio) > 2 + folio_test_swapcache(folio)) > > > > > + return false; > > > > > > > > In your experiments, how much amount of drains were reduced due to this specific > > > > check? > > > > > > > > I wonder if that data can motivate to introduce lru_add_drain_folio(folio) which > > > > only drains if the given folio is in the local lru_add cache. > > > > > > Actually if we can peek into lru_add cache and folio_ref_count(folio) is exactly > > > equal to (2 + folio_test_swapcache(folio)) and folio is not on LRU then we can > > > just reuse folio if it is in lru_add cache without draining, right? > > > > Sounds more reasonable if we can only touch the wanted folio if it > > exists in pvec. Believe it can improve efficiency more than the > > current patch. > > > > Technically yes, but in practice it seems quite hard. As I explained > to Shakeel, we don't have an easy way to determine whether a folio is > sitting in the lru_cache. !folio_test_lru(folio) only tells us that > the folio may either have been removed from the LRU or still be > sitting in the lru_cache; it does not distinguish between the two. > Unless we want to scan the lru_cache to look up the folio, we may need > a dedicated folio flag, similar to PG_lru, to indicate lru_cache > membership - > I guess people would be quite unhappy about adding a new flag? > > On the other hand, the folio may be sitting in the lru_cache of > another CPU, which the current drain cannot flush. As things stand > today, the drain only succeeds if the folio happens to be queued on > the current CPU's lru_cache, giving it roughly a 1/nr_cpus chance of > working. drain_all would clearly be too expensive to avoid. BAM, sitting in other CPU's pvec is a real barrier. If it's in local cpu, it definitely deserves a try as Shakeel said, when refcount = (2 + folio_test_swapcache(folio)). Then the current solution is optimal at present. > > So another possibility is to drop this drain as well. The folio can > be released later, at the cost of missing some opportunities for > reuse. > > Best Regards > Barry