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 B7483376A1E; Mon, 12 Oct 2026 13:34:36 +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=1791812077; cv=none; b=gAjsu8RTa/eaXYHoPLl6rjNjM/sDqaRVPk84ZCqZCw1zptDBIv3AiFoWHVWMGimKKdFtP69fV2YoFaej8sYYv/DxOFVLDH8EG8fj3JrWKGIruO6u6saIxY42coNqirIDonA1ciYs+4VHshQIZV1y/H9ezMBlou9y51naFNDBRhg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791812077; c=relaxed/simple; bh=fJGw0cpjBTi5/7bzqDmqo6fj6jl7Cb/zKCJc7n6Iuf4=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=FB/LnBMNcW4qvKzBd1rVemcPyHoFqQILhsyqNk0TfiJP9c9gsVKMQOe9n72MhF6wf/yporMeccKuoKqu1diJpWDzRBwN7GRQyuXXIHxZVfdv/2N+cSkSTGSMX23RVrVzGxgSNvI8Qg563nN4orb7dHHpXeALLeK9re3pHtUhHlU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Pb2qwS6e; 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="Pb2qwS6e" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2D2831F0089C; Mon, 12 Oct 2026 13:34:33 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791812076; bh=BJoKG/6UOVhJtQlfQRanKoyfmf6h8PW7vA7hrTi9Uzs=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Pb2qwS6eWHO6MPz6e6Cjv4IRYSsPioj8ppCtOqUDgtF5oADW4amg4Rm2d7JZotQw+ GFRMFOZog5JGPM/vi3cITAw2xXHYkWcOPUPNdygjRLcToiGgwezrK2esxqasNlm41c 1la4YDWxCXqPasGkTAzsjlnXTL7QG0XFmaUTZCWQM5YHTijpXSoU7Muk8T1pOm9B7l ZZHzIWI0J0W7qyl17QBn7c43TRFfYVV3Wlb3cuiMiz0WsECUXjk40joq6b92W/zoFC 2Ha28M5aDVhTHoRDPbawlXzi8d0o6txkkqde30XTk7KbDZoIMKa3RK1ttNOfJtAfJL esL/Yl33fruSQ== Message-ID: <01349eb9-8833-4cf4-bb42-b1d5a608c83d@kernel.org> Date: Mon, 12 Oct 2026 15:34:32 +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: [RFC PATCH 0/1] mm: page_frag: account page_frag pages in vmstat and meminfo To: Pedro Falcato Cc: Yifan Li , akpm@linux-foundation.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox , Vlastimil Babka , mhocko@suse.com, netdev@vger.kernel.org References: <20261010025941.1253-1-liyifan21@xiaomi.com> <40fc5d04-2a20-42c4-9557-7277bc981172@kernel.org> 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 10/12/26 13:27, Pedro Falcato wrote: > On Mon, Oct 12, 2026 at 12:36:35PM +0200, David Hildenbrand (Arm) wrote: >> On 10/10/26 04:59, Yifan Li wrote: >>> This is an RFC. >>> >>> Pages held by the page_frag cache are not accounted by any memory >>> statistics counter, so on systems that derive "LostRAM" as MemTotal >>> minus the sum of accounted memory they inflate LostRAM and lead to >> >> Sorry, what exactly is subtracted from MemTotal? >> >> How would any other kernel allocations be treated? > > I suppose those would mostly show up as Slab:. But yes, I agree the metric > is a little dubious. > >> >>> misleading "leak" conclusions, even though the memory is legitimately >>> used by the network path. >> >> Assuming "leak" in such scenarios is questionable, sorry. >> >> Aren't most page_frag allocations short-lived? Or is this more about the >> page_frag cache? > > page_frag allocations are kinda arbitrarily lived. If you queue (send) some > data to a socket to send over the wire, they are more likely than not > short-lived, unless the protocol/qdisc says no (for example, you're hitting > the TCP congestion window and as such can't send more data over the wire). > If you send it over loopback/veth you get arbitrarily livetimes as sockets > can keep the page frag alive if the application never reads them. > > On the regular RX side you can also (AIUI) keep page frags randomly pinned > as long as you don't read them. > > (note that all of these allocations are bounded by each protocol's memory > limits) > > > In any case, I remember an off-list discussion with mhocko regarding a > downstream bug, where I suggested adding a counter for this, and he said > networking is somewhat sensitive to extra accounting on the fast paths. > Added netdev in case they have comments. That makes sense. >> Certainly no page flag. >> >> A page type might be the right choice. I don't think Willy planned for that [1], >> IIUC, the bump allocator was something different, but another page type >> shouldn't hurt at this point. > > A page type might not work, these pages _may_ need to be mappable. I know > that was an issue for page_pool (Yet Another Networking Page Allocator) + TCP > zero-copy RX using mmap. I'll have to trust on networking experts, I was reading: (requires that page_frag pages are never mapped into userspace; true for the current RX paths). > > Of course, if you have a page frag under PAGE_SIZE or misaligned then you can't map that. It would be best if we could disallow that entirely :) (of if required, map them through a PFNMAP or similar that does not care about page metadata ... but the interactions with GUP are tricky) -- Cheers, David