From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 84B5823D7F4 for ; Wed, 24 Jun 2026 10:24:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782296693; cv=none; b=K8blldxvfG4tBauqGqip2ye1Pq6JDeKCDoSWkjoWdHS5a3fr+nNAkpTsegYS1c9SFDyWsSQdIzwcvsdko4HCO73BY5BqMqtI/clOZSfOdxh1+q0D9r5MnbY8qS0lvUFKHmr8oobpLxjm/JItYEu0UQAC4uzo6hyMzv4OJ4XXQW4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782296693; c=relaxed/simple; bh=JgJrJFvhsBD0VnQZIwpI02/GUPS7PD+CfrpnhiPeaOo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=DgA3n2tOvqcf6u+PIHhJRvG59blgDycZuPuCm67+yGc6vsA2JTMQ2ukyT1AevhRvBT3j4M6jF/4yzY66WD8lXTbw6gx1cjx5UORn9yFLgcquD7qzt/H25e60fIvvx2T3Cq9nFI5PfsVKJN6ukkHg7BTzU8xzNM7oVkpg3tvqbS0= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=M3Ze81Ve; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="M3Ze81Ve" Received: by smtp.kernel.org (Postfix) with ESMTPSA id A99EE1F000E9; Wed, 24 Jun 2026 10:24:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782296692; bh=dbgZMRyxUhpQ3kzlaU3M0hhk9GF4/29S5UZlvh3+gjs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; 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 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: 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 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