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 C69593246E8 for ; Wed, 18 Feb 2026 09:15:22 +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=1771406122; cv=none; b=u2K0xbHJDATXkWRLcpDFFvc6jmdvWFc54Js4aK4rgUzvLRia0lEKb5noYrx9IgUjwtEW2gUKFIYm9iPEdEXTeuGzP3o1iVsv9AbxsTA+5g6VDaNKvquPk7z98oMEGVBX+OYGqMpW1cjF6kQZXe+UdLzkwia++o40QQBs/HEQiIU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1771406122; c=relaxed/simple; bh=facfh3Fnqd0pqqYAih484vRudCiY9z5cjTo3bywqak0=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=P2w/c/KOQdIkTh9aHW3IvuRIBzMtRJrncXqVfY3UQbk7hmWhXzDnMwlCYLu/asC127inGIpkZD5SOYSibhIW8n9d0MYw27wPDZY0WjdxsFxR55msAgQVFnwhycdH+If38ciuKRvyr72eVkYEx20JJXIUpsUuP47GzNPMJy4E0C8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=J2k6dAhE; 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="J2k6dAhE" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 161D4C19421; Wed, 18 Feb 2026 09:15:16 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1771406122; bh=facfh3Fnqd0pqqYAih484vRudCiY9z5cjTo3bywqak0=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=J2k6dAhERQMegIXafbmog4+yaSYLVdbQ3XnAiFP4bTsUoWTf3JvD2HEjMLMAc3Vg5 mnW+p+xrbT3Ew0S+ATK3csCYE+oxe7B+SMFTkmbue4OI8DB8GkrrdlcvjiDDKn9xyJ lHPxxGW6MKcYQ9Qht2ZWUWb+2eGZ/bqpQ3m59YUjNEwAli7YbFVxuvvOnvV9LhjKBh aA1J1TtFHfkE4e3fnCwrzuQKf8FUxubMRb1c9KFKRdhFsA+eG42rfNcBLSOV8BGX1u tlykWL4EWyrz207wMgTmcZXcbtQX4EgDSiRrewzzDC3N2WugbEqMvMRAfnoC4JV2yF k/CSo5QbcQVNw== Message-ID: <3a5179bf-fecb-498c-be3a-6a4635f3eb1a@kernel.org> Date: Wed, 18 Feb 2026 10:15:14 +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: Dev Jain , Matthew Wilcox Cc: 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> 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 2/18/26 09:58, Dev Jain wrote: > > On 18/02/26 2:09 pm, Dev Jain wrote: >> On 17/02/26 8:52 pm, Matthew Wilcox wrote: >>> Please don't use the term "enlighten". Tht's used to describe something >>> something or other with hypervisors. Come up with a new term or use one >>> that already exists. >> Sure. >> >>> That's going to be messy. I don't have a good idea for solving this >>> problem, but the page cache really isn't set up to change minimum folio >>> order while the inode is in use. >> Holding mapping->invalidate_lock, bumping mapping->min_folio_order and >> dropping-rereading the range suffers from a race - filemap_fault operating >> on some other partially populated 64K range will observe in filemap_get_folio >> that nothing is in the pagecache. Then, it will read the updated min_order >> in __filemap_get_folio, then use filemap_add_folio to add a 64K folio, but since >> the 64K range is partially populated, we get stuck in an infinite loop due to -EEXIST. >> >> So I figured that deleting the entire pagecache is simpler. We will also bail >> out early in __filemap_add_folio if the folio order asked by the caller to >> create is less than mapping_min_folio_order. Eventually the caller is going >> to read the correct min order. This algorithm avoids the race above, however... >> >> my assumption here was that we are synchronized on mapping->invalidate_lock. >> The kerneldoc above read_cache_folio() and some other comments convinced me >> of that, but I just checked with a VM_WARN_ON(!is_rwsem_locked()) in >> __filemap_add_folio and this doesn't seem to be the case for all code paths... >> If the algorithm sounds reasonable, I wonder what is the correct synchronization >> mechanism here. > > I may have been vague here... to avoid the race I described above, we must > ensure that after all folios have been dropped from pagecache, and min order > is bumped up, no other code path remembers the old order and partially > populates a 64K range. For this we need synchronization. And I don't think you can reliably do that when other processes might be using the files concurrently. It's best to start like Ryan suggested: lifting min_order on these systems for now and leaving dynamically switching the min order as future work. -- Cheers, David