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 4E59E33A70A; Mon, 12 Oct 2026 10:36:39 +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=1791801400; cv=none; b=LnhknALpSRkV+YEvxRKW9IZ0mjmcPRjj35UTundDwg5sQA04xXrfCZyU3t3jmQeriKhYxwVVw4ouffE0wRFZ1ymEctJhs+5hS72YuXanCFkRhfPl3xLNnuLMDCxKjyhia3S8i5/m6DiL6ERQjauQa7jCgwt6DP2Y7HCYE7OMPgc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1791801400; c=relaxed/simple; bh=CshdtUReocBu3d8xSBl4wYMU2IFjDNEUiLwOHv4hnKw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=aUZZsfZnteRigHiylAWB1TIA0nK9NQU+zvdK/UcilcT7UGcoiAuodq/v2t504/WLhvb7vjQJHTWsmHdqxGJb733geEjwoLOmwhPwhamtlpLScq4npsax0ekfTdyNZlFRPx1IlnQT6ryQLjV7FbRWqaDUi6bU9IdCVowEBjohYEQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=nBgMd6hF; 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="nBgMd6hF" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 0F1BD1F0089B; Mon, 12 Oct 2026 10:36:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1791801399; bh=d0Ru+UULBjDJTakg+6YSc+KSC2SkZhPc3GekW03A8bg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=nBgMd6hFMwuCIMEM9dpBw5Ng/bXWnZQf8qC5gP/NgD3E+kjXB414nVqvhz58wZRxn /XRcRinw9FqmQV2kYqX0W/Ac/4AuGlZbLpQvpWRPn0Jx85m8QyrziqQULFEGRSB7DD oPteniB8ChUBmZFVRijAy4y9UMXV/UPnsW2GUn4o9K5uNuyg/sz6PohKdZzjTv3PJg qQKZyBbuDuQNdkRUnsc6hrQ3glx9ALYzYMj+pM7p4A/aAGIPo2pqjXAxEE+j9+XUgw e2lk3wGbqFGapkvO4e/efpchStfSbSxvWWApO6ut/64OwO/Wb/DSSUWTwIlYhfWQow Din+DVEpP32BA== Message-ID: <40fc5d04-2a20-42c4-9557-7277bc981172@kernel.org> Date: Mon, 12 Oct 2026 12:36:35 +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: Yifan Li , akpm@linux-foundation.org Cc: linux-mm@kvack.org, linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, Matthew Wilcox , Vlastimil Babka , Pedro Falcato References: <20261010025941.1253-1-liyifan21@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 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? > 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? > > The page_frag allocator (page_frag_alloc / napi_alloc_frag / > netdev_alloc_frag) backs sk_buff->head and the frags portion of > skb_shared_info. Unlike slab (NR_SLAB_*) or page_pool, page_frag > pages have no counter. A preliminary measurement on a 4K-page > arm64 Android device shows 14432 kB (3608 pages) of page_frag > pages already in use at boot, and the memory footprint grows > larger under heavy load (e.g. sustained traffic through the > virtio_net / tun receive paths). > > Patch 1/1 adds an NR_PAGE_FRAG node counter and exposes it as > nr_page_frag in /proc/vmstat and PageFrag in /proc/meminfo. Pages > enter the cache through a single entry point > __page_frag_cache_refill(), so the increment side is complete; the > decrements sit on the paths that free the cache page. > > The remaining gap is on the release side, which is open: a cache page > handed to an skb frags array (skb_add_rx_frag() / > skb_fill_page_desc(), without an extra get_page) is freed via > put_page() (kfree_skb -> __skb_frag_unref -> put_netmem -> put_page), > bypassing page_frag_free(), so the counter can over-count under > network load. > > Closing that needs the page to carry a marker checked at the common > free path, e.g. either > > 1. a page flag: PAGEFLAG(PageFrag, page_frag, PF_HEAD), accounted > and cleared in __free_pages_prepare(); or > > 2. a page type: PGTY_page_frag reusing _mapcount, accounted in > free_frozen_pages() (requires that page_frag pages are never > mapped into userspace; true for the current RX paths). > 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. [1] https://kernelnewbies.org/MatthewWilcox/Memdescs -- Cheers, David