From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 02D502C21F4 for ; Fri, 20 Feb 2026 16:50:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771606251; cv=none; b=LdZUibfXrcy+xJDeLHCGNqFozDZ7WPDvAQOexZtrc6wsGku16QAAhT9AkdRnBe108VPm0PXOuc+6LBKEzI57yaI5IYVsS00hnM0uchg9MDmh/kaweRcP/S3PrsBwkvzDTYm0Pyvuy+tU68Do0wmHZKGbybjYHcXD/JLubsU50p4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771606251; c=relaxed/simple; bh=+Uij/0xoD2fksuZzZkZLFNcIAKJYICDzDr0TGD6sxow=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=gHjOGxxawylrE2JLeuo9pLkG1q3hde8twd+CFdAwB03djZGh/bYP+MTRXjADc86MjS1374l8vUnbxyhMfmOIaw325/f86oW5uEkYD5oujl3J1477iqeIMDQczk06LVTGY3FOBFICcjNqi8hLntGK4hN21MQ9RuNcK0Q3YT0bfq4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=d5W1HDLz; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="d5W1HDLz" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 35BC8C116C6; Fri, 20 Feb 2026 16:50:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771606250; bh=+Uij/0xoD2fksuZzZkZLFNcIAKJYICDzDr0TGD6sxow=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=d5W1HDLzSBMx3frw2niVk0GR2O9vcnpmuDh3nVZ09DxzHczZ5uzY2u225TKsjUTua bZnCGRvMT7WmR5K5UJUVsUJk/Rt1oUCOQozAkKZYIbZ5sd1vuDPYRD+oXzRT//KPRH DvrTY8DWFMcDHceh3SgIZQCVBHw+KKQ9PO9czG3ta2WIyETUS5IG2NxNpHIC333463 3fjyC5hLQVzbEqVqV9OP0tgNsDBH5aj4jjeUmOX8aBQDVWuDn+VTfFHZSlo2clBzd6 ViGv4WtWdnCqQdApp6geo/GRYIvEtQRzpZfwufEoU/00iPUg7GvvN2ngvmfIGhopfE ecrZkicAFaTMA== Message-ID: <5f61a5fd-ab38-4635-aab8-5de515c4015a@kernel.org> Date: Fri, 20 Feb 2026 17:50:44 +0100 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: [LSF/MM/BPF TOPIC] Per-process page size To: Matthew Wilcox Cc: Dev Jain , lsf-pc@lists.linux-foundation.org, ryan.roberts@arm.com, catalin.marinas@arm.com, will@kernel.org, ardb@kernel.org, hughd@google.com, baolin.wang@linux.alibaba.com, akpm@linux-foundation.org, lorenzo.stoakes@oracle.com, Liam.Howlett@oracle.com, vbabka@suse.cz, rppt@kernel.org, surenb@google.com, mhocko@suse.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org References: <20260217145026.3880286-1-dev.jain@arm.com> <2e68ef61-dcf2-46b2-913f-14980a104faf@kernel.org> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 2/20/26 05:49, Matthew Wilcox wrote: > On Tue, Feb 17, 2026 at 04:30:59PM +0100, David Hildenbrand (Arm) wrote: >> In a private conversation I also raised that some situations might make it >> impossible/hard to drop+re-read. >> >> One example I cam up with if a folio is simply long-term R/O pinned. But I >> am also not quite sure how mlock might interfere here. >> >> So yes, I think the page cache is likely the one of the most >> problematic/messy thing to handle. > > So what if we convert to max-supported-order the first time somebody > calls mmap on a given file? Most files are never mmaped, so it won't > affect them. Yes! > And files that are mmaped are generally not written to. Well, let's say many mmaped files are not written to. :) > So there should not be much in the page cache for the common case. You'd assume many files to either get mmaped or read/written, yes. > And if no pages from the file have been mmaped yet, they cannot be pinned > or mlocked. Is there some other way for someone to block a page from getting evicted from the pagecache? We have this memfd_pin_folios() thing, but I don't think we have something comparable for ordinary pagecache files. ... putting them into a pipe and never reading from the pipe maybe (I assume that's what splice() does, but not sure if it actually places the pages in there or whether it creates a copy first)? But yes, doing the conversion on first mmap() should handle mlock+gup. -- Cheers, David