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 225802264CA for ; Thu, 25 Jun 2026 07:50:03 +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=1782373805; cv=none; b=tUbhFivpVPEtLbjYkY1iLUZv6C4CDAIPj/UpxNOBSgPbOMRf0si1LhC8NWn5Eexx0+6fBE00fFqJT07jNZNWh/MT8KCu93pP0TGWs9zTeMWcOVF7j1CNwv/p+4lF/oU/k9KK0bsvBMfpG/Jd2v7I7C6VHWGvlTUKwRBNuqdnVjI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782373805; c=relaxed/simple; bh=W8x8+9CPYMLcdGOebD23KXEmv/vwXYJB/OFPtub8TXA=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=mE77VQ09jGwOWhJppCy9NYXL7c/e3me0XlPE7ZWg/Qv9IQekfRtMDNlDKqjXX6E9puaLRV6bPVkYpM/puKy0ztk5RTjQj3bKnnIOmteW5ZbP8Ctk3S7Y1yFg49fYn3GNzd1FEywE3/UHtwQY2a3e6i1kbKB32u+TKljn+Kb14L4= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=SG1WT17A; 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="SG1WT17A" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 780981F000E9; Thu, 25 Jun 2026 07:49:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782373803; bh=3mjvDm6wc662Q3j5xc6LNyuyos/WPV/sprSHNbnLMDY=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=SG1WT17AXH8H0rBfsv8gpBfHL8MfVexRe92YQELgaeGghMfk6UU7i/qi+DOwjV4Uj 93Ojq9qtIuVps3MoODxWL5pKNGNlY9SxjJ6pSoIhGv1Si1QXBDjfnqSyH4EOmHjslN EIYlUrMFZk3X8KcmcyXR6fqSKRl+jEOZmi5Ksg7Kp2kzLdjIHFc/zY7AeWGhAq2s87 Jrzkm+2iIAoLDgCTCp0Xddc2HXewVRKofU2fNQcpZjQm/yOfxcE76fejz+885oPU6H amN8pNhDuScXBv3XqRf0Bizynr/SqIjrHF0ym1fdEKbQIPIq/St65i656qyfXcHZe9 PS8wbC71cNomQ== Message-ID: Date: Thu, 25 Jun 2026 09:49:56 +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] mm: Avoiding split large folios if swap has no space To: Barry Song Cc: akpm@linux-foundation.org, axelrasmussen@google.com, baolin.wang@linux.alibaba.com, dev.jain@arm.com, kasong@tencent.com, lance.yang@linux.dev, liam@infradead.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, ljs@kernel.org, npache@redhat.com, qi.zheng@linux.dev, ryan.roberts@arm.com, shakeel.butt@linux.dev, weixugc@google.com, yuanchu@google.com, zhaonanzhe@xiaomi.com, ziy@nvidia.com, Johannes Weiner , Michal Hocko , Roman Gushchin References: <5790c4a4-d502-4180-82f5-47de5809a4fe@kernel.org> <20260620081017.89085-1-baohua@kernel.org> <4aa8350e-712f-4380-b3bf-2ff06cf2a35d@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 >> >> But now I wonder whether we would also want to check "is there any free swap >> space", not just "is there any swap". > > I don't quite understand you. get_nr_swap_pages() returns > nr_swap_pages, which increases or decreases as swap is allocated or > freed. I guess it just reflects how many swaps we currently have > available? Indeed, I was confused by the function name it's "free swap pages". So all goof :) > >> >> >> Essentially, try returning -E2BIG if there is the chance to swap out after >> split, and -ENOSPC / -ENOMEM if a split wouldn't help. >> >>> } >>> >>> again: >>> @@ -1769,11 +1772,13 @@ int folio_alloc_swap(struct folio *folio) >>> } >>> >>> /* Need to call this even if allocation failed, for MEMCG_SWAP_FAIL. */ >>> - if (unlikely(mem_cgroup_try_charge_swap(folio))) >>> + if (unlikely(mem_cgroup_try_charge_swap(folio))) { >>> swap_cache_del_folio(folio); >>> + return -ENOMEM; >> >> Here we wouldn't have the information whether we could charge after a split. >> >> So that would require a rework to signal this more cleanly to the caller. > > Yep. The tricky part is that mem_cgroup_try_charge_swap() cannot > return how much swap quota is available in the memcg. Do you prefer to > add an output argument to mem_cgroup_try_charge_swap() to expose > that That would probably be cleanest, if that is easily possible. We would want to get memcg maintainer feedback on that. @memcg folks: we'd like to know whether splitting a large folio would make mem_cgroup_try_charge_swap() succeed on a split (smaller) part, to distinguish "there is no way we can swap out anything, don't split" vs. "we could swap out, split". What's the best way to obtain that information? -- Cheers, David