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b=M3Ze81Ve0Kwa/dryGMXeQhXmDgrwhoCyK+B5Y5hD3wFt6wncgbaBqE/mDZ8y0jFse NT30GzTzMZeeI4CWpy7Y5G9081RtewnhASHQxJvwpbckO7SNxE1+c59dEol03T6YAX JfRNP8nj6VWjpmX8ORTBYyBoZ/ERSCgrUWUlnSakxefxUKnmDoe/AwB6Y7A0nMb6Qu wlkOZLiGG8BenrqjR7FmkfYGVEJjncKxM25GnFdPv6ChvM4HlV2qC2fU0y5xNPPMxA jYtU+0Lw/MQlPQzWHqzEd8sbynjrY5ELjWSz9qHP9bCeO8TyPMxrCLb/9R+BtxkJ1T VPyqJ1Iy54Uvg== Message-ID: Date: Wed, 24 Jun 2026 12:24:44 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 0/2] mm: improve folio refcount scalability To: Ilya Gladyshev , Matthew Wilcox Cc: Linus Torvalds , Andrew Morton , ivgorbunov@me.com, Liam.Howlett@oracle.com, apopple@nvidia.com, artem.kuzin@huawei.com, baolin.wang@linux.alibaba.com, foxido@foxido.dev, harry.yoo@oracle.com, linux-kernel@vger.kernel.org, linux-mm@kvack.org, lorenzo.stoakes@oracle.com, mhocko@suse.com, muchun.song@linux.dev, rppt@kernel.org, surenb@google.com, vbabka@suse.cz, yuzhao@google.com, ziy@nvidia.com, pfalcato@suse.de, kirill@shutemov.name References: <20260608154734.8e4115fde4e2e14a3b6892fb@linux-foundation.org> <839a2ea2755fdddf5af773e006237b07c9e261df@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: mx7ajficz3i8oyqfk5soet5frocypwd9 X-Rspamd-Queue-Id: 4614840007 X-Rspam-User: X-Rspamd-Server: rspam01 X-HE-Tag: 1782296693-348164 X-HE-Meta: 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 1rbQrz9J zoKBizISkySkcueNz8vqyUXe0+dqHuXD8VZ5GFXzlIBc7fMyJoTKjsiEYXp/izSZIWkaB1uZoh++wMsC1L64ymib+vb4rAS6kUFjLurG7yxXo1bejI0SYJSnrnN6/yc1Fc0X2tdClU5n+3dgJ6vJ8Vp4yns5Fb90FfucK0Nm7YXd5hKyM5xMpbaxiTMQbEHinpK+zUhr1Rc0B3xAr+/+MvTWc4Aduw80ZS9HPmbmNw22MI0xpaVjfchsXnlodU8ZEjS+gvTzjWV4tzHE= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 6/23/26 23:23, Ilya Gladyshev wrote: >> >> On Sun, Jun 21, 2026 at 09:34:47PM +0000, Gladyshev Ilya wrote: >> >>> >>> June 21, 2026 at 7:46 AM, Linus Torvalds wrote: >>> >>> On Sat, 20 Jun 2026 at 11:19, wrote: >>> >>> > >>> > T2: optimistic get() [0 -> 1] >>> > T2: put page back [1 -> 0] >>> > T2: calls dtor for type X, returns into the allocator >>> > >>> Which optimistic getter does this? >>> >>> If I understood you correctly, you are talking about the scenario where >>> an optimistic getter took a refcount on the stolen page, so the validity >>> check in the XArray will fail. And this scenario does indeed work normally. >>> >>> This "ABA" happens if the optimistic getter successfully gets a refcount >>> on a valid page, so the full T2 execution looks like this: >>> >>> T2: optimistic get() [0 -> 1] >>> T2: re-checks page [OK] >>> >> I don't think that can happen. Or maybe it can and we need to add >> some barriers. The page is always removed from visibility (whether >> we're talking about a page cache lookup or a page table lookup), then >> the refcount is decremented. I hope we have enough barriers in place >> to ensure that the refcount decrement is observed after the removal of >> the PTE entry or the XArray entry. >> >> But I'm not sure why the folio_put() after a speculative get avoids this >> problem; why do we need the recheck to be successful to hit this race? > > After additional thought, this race doesn't require speculative gets at all > and is more about two parallel `folio_puts()`: one successfully deallocates > the page, and one sleeps for a long time and then calls __folio_put() on a > "logically new" page. > > And as everybody pointed out, this race isn't something specific to this patch. > So folio_put() was always ready to be called on non-folio objects due to > optimistic try_get() + put() in the filecache. We only care about calling > folio_put() once, and nothing breaks with this patch. > > So thanks to you, Linus, and David, for helping to clarify this :) I'll post > v5 with more or less cosmetic fixes from the AI review then. Note that I found an example where I think it might indeed be problematic (the deconstructor thingy) that might require another look. -- Cheers, David