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 BD42E3EF64A; Mon, 17 Aug 2026 18:10:32 +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=1786990233; cv=none; b=nK9GdZHQKpKVPhD3m4IA5v8pxiOTIUW0wJ4Hj35uIlsBEXwMJgsH/hD1Caa1nByMSRtLZLpeELXkmbGp3kWoVeGiMiEXPs+il05sNDIhW8wxUukdfCHVBKgSn5FgQXN4bBHZ6Q+T/INvYBhr5ndh4UC2wHjTzSdPtN7pEoqg5HE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786990233; c=relaxed/simple; bh=6QgeDgzn9BEo49hI+J7da/FfwJvAG7Qe0G1CZQN5rQQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=WR2r+OJQC4nzJa4rJYaqVJucshQFAlkss7qm+iwawDXSq3P+E37+LCphUz1ZpUdVtCqyDbnIDp/x+4XZpL6KL+CqXiwVCXH5iT+t073q0eXpb63cxe6VkawBxaJz1gHJJ7Hyk20WI3vIkvAatyCyrbl84b4Ec9yI8a6/3jrVTFY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=P6u3cyJO; 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="P6u3cyJO" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 7283B1F000E9; Mon, 17 Aug 2026 18:10:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1786990232; bh=IWschY6PJIJj78TPAK68wuF6XfNqvRhD4s+H+yde0ec=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=P6u3cyJOWqTGgmW2GpD2BfOMO+B6rcLUTroOW0CnhXHdzCA9hhbVgBoQyJnIMYQln ixmfd+ipl+0hMRrTCQiawq5EZ4qv193Fdt/sAatEejaoei9OExsjZVIXOVxID4VkuY KquJ+6JlpsFLMatZHvmkhsSVsjCPOWirDHXQ31UPChw/MI7AzicEq10JTfjGVF04Xc eqVWKErNMzostAW2c98rp/7g0HtznoiY7HfuAfJBD9Z7XFVhpzJCd4CdOApzkFxM95 KhB1y5lCFli53BwAp9WLWlIcxVmmW4Lg5cFVsqHGeuIDQo0/SqGXb7C4eCfabHvMS5 r/7/IUcLjvNjw== Message-ID: <04f3ce2f-67f5-4829-9551-a69b9df29854@kernel.org> Date: Mon, 17 Aug 2026 20:10:26 +0200 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] bpf: account ring buffer backing pages separately from Lost RAM To: Xiang Gao , Andrii Nakryiko Cc: Alexei Starovoitov , Daniel Borkmann , Andrew Morton , yinchuang1@xiaomi.com, bpf@vger.kernel.org, linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Xiang Gao , "Lorenzo Stoakes (Arm)" , Steven Rostedt References: <20260815091819.3651099-1-gaoxiang17@xiaomi.com> 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 8/15/26 11:18, Xiang Gao wrote: > Hi, Hi, > > I would like to discuss accounting BPF ring buffer backing pages in > system-wide memory reports. > > BPF ring buffers allocate their data and metadata as order-0 pages directly > from the buddy allocator, and then map those pages with vmap(). I assume there is a reason the slab isn't used, right? Are these pages mapped into user space such that page->mapcount would get used? Can you point me at relevant code? > > Because vmap() maps caller-owned pages, these backing pages are not counted > by VmallocUsed. They are also not slab pages. As a result, most BPF ring > buffer memory is not represented by an existing named /proc/meminfo category > and appears as Lost RAM in Android memory reports. > > We measured this on an Android 6.18 kernel. > > Test case: > > 32 BPF ring buffer maps > 16 MiB data area per map > 512 MiB total data area > > Observed changes: > > Lost RAM: approximately +529 MiB > VmallocUsed: approximately +2 MiB > Slab: approximately unchanged > > After destroying all maps, the values returned close to baseline. > > The question is whether the kernel should expose the unique physical backing > pages of live BPF ring buffers through a dedicated global counter and a > /proc/meminfo entry, for example: > > BpfRingbuf: This looks a bit too specific for my taste. And I think we should try to no inflate these statistics here too much. > > The proposed counter would include: > > * ring buffer data pages; > * metadata pages; > * consumer and producer position pages. > > It would exclude: > > * the second virtual mapping of data pages; > * the pages[] pointer array; > * map metadata allocations; > * vmap page tables. > > The goal is to account for the currently unclassified direct backing pages. > Slab- and vmalloc-backed auxiliary allocations are already represented by > existing memory categories and should not be counted again. > > A possible implementation is an NR_BPF_RINGBUF vmstat counter maintained by > the ring buffer allocation and free paths, with the aggregate exposed through > /proc/meminfo. > > Questions: > > 1. Is a dedicated BPF ring buffer counter appropriate? I don't think so. See [1] where we just had the same discussion for tracing buffers. For them, Steve [2] had an idea on how to expose them more fine-grained and tracing specific. [1] https://lore.kernel.org/r/20260810094025.136705-1-gaoxiang17@xiaomi.com [2] https://lore.kernel.org/r/20260810105710.6ee5e493@gandalf.local.home > 2. Should this be represented as an NR_* vmstat counter? I don't think so. > 3. Is /proc/meminfo an acceptable interface for this information? Again, I don't think so. "Lost RAM" really is just "excessive memory allocated by some other subsystem". I agree that some users might want to figure out what is consuming that much memory, but I don't think growing /proc/meminfo in that way is really what we want. > 4. Is counting only unique physical backing pages the correct accounting unit? I'd assume the "It would exclude" part above should not be accounted there, if that's what you mean. -- Cheers, David