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From: Shakeel Butt To: Barry Song Cc: 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, baoquan.he@linux.dev, 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 Fri, Jun 12, 2026 at 09:35:04AM +0800, Barry Song wrote: > On Fri, Jun 12, 2026 at 2:18 AM 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? > > > > The real problem is that !folio_test_lru(folio) does not tell us > whether the folio is still sitting in lru_cache, and determining that > is unlikely to be cheap. We could either scan the lru_cache or add a > separate flag to indicate lru_cache membership, but neither option > sounds particularly appealing. I was thinking of lru_cache scanning. I see this can be 4 cache misses, so might be expensive. Though if we are draining non-full cache most of the time then it might be cheaper. Anyways if you don't mind checking the cost that would be awesome.