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Fri, 19 Dec 2025 19:18:58 +0300 Message-ID: <9822c658-c2f0-4b1c-9eef-9ffa865e44f7@h-partners.com> Date: Fri, 19 Dec 2025 19:18:53 +0300 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 2/2] mm: implement page refcount locking via dedicated bit To: Kiryl Shutsemau CC: , , , , , , , , , , , , , , , , , , , , , , , , References: <81e3c45f49bdac231e831ec7ba09ef42fbb77930.1766145604.git.gladyshev.ilya1@h-partners.com> Content-Language: en-US From: Gladyshev Ilya In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.123.123.67] X-ClientProxiedBy: lhrpeml100010.china.huawei.com (7.191.174.197) To mscpeml500003.china.huawei.com (7.188.49.51) X-Rspam-User: X-Rspamd-Queue-Id: 0700C4000A X-Rspamd-Server: rspam10 X-Stat-Signature: t4iutz6jmtpu5yt5zebmqcx3q16neu5a X-HE-Tag: 1766161145-991175 X-HE-Meta: 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 TBI1eiPQ yiPu/SZ2qNDfgPvwXc1VHJkRtAX0OvEFD/4pQpm8rO2WjJ63ik29Lnv4bSdiafPObUH2hQVuPRHvji6VCz1yoC7LDRyHFrdkFKMsZEoYPp4CvrHQiHSea0hk9rIml/9ZTzOeSL8AbXXZS7XYljrVw5ZwhLw7GerMHprU2an1uq7WbW84d9YFVRWoPxVl4yeFMuF1VK9YAOMHgrZWSb25KTidsSYFI1wjaU9+p X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 12/19/2025 5:50 PM, Kiryl Shutsemau wrote: > On Fri, Dec 19, 2025 at 12:46:39PM +0000, Gladyshev Ilya wrote: >> The current atomic-based page refcount implementation treats zero >> counter as dead and requires a compare-and-swap loop in folio_try_get() >> to prevent incrementing a dead refcount. This CAS loop acts as a >> serialization point and can become a significant bottleneck during >> high-frequency file read operations. >> >> This patch introduces FOLIO_LOCKED_BIT to distinguish between a > > s/FOLIO_LOCKED_BIT/PAGEREF_LOCKED_BIT/ Ack, thanks >> (temporary) zero refcount and a locked (dead/frozen) state. Because now >> incrementing counter doesn't affect it's locked/unlocked state, it is >> possible to use an optimistic atomic_fetch_add() in >> page_ref_add_unless_zero() that operates independently of the locked bit. >> The locked state is handled after the increment attempt, eliminating the >> need for the CAS loop. > > I don't think I follow. > > Your trick with the PAGEREF_LOCKED_BIT helps with serialization against > page_ref_freeze(), but I don't think it does anything to serialize > against freeing the page under you. > > Like, if the page in the process of freeing, page allocator sets its > refcount to zero and your version of page_ref_add_unless_zero() > successfully acquirees reference for the freed page. > > How is it safe? Page is freed only after a successful page_ref_dec_and_test() call, which will set LOCKED_BIT. This bit will persist until set_page_count(1) is called somewhere in the allocation path [alloc_pages()], and effectively block any "use after free" users.