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 BB3B932AAA3 for ; Thu, 25 Jun 2026 13:46:04 +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=1782395165; cv=none; b=Y2SLj3vOgMYIz81YgoxCuAEpAi92w4wCKSB17OBodMAZqM1EiE6yJ+aSLjcqdIFDDHFmBGcUbEbnqX7VU60/F7//oXPSST1W9+9oCPGrOwxIcFu7DrZaYcaB2qYkOKipo3BgLCyZPFSrOj+cgbxwJDI+kPwythLcRWgJZ7XO/1g= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782395165; c=relaxed/simple; bh=l2mt7VHyaZqlC4Itc4uxQV3tAGxl8t3/T8f9re4EFpE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=GKfOR5Bht8SVtTnGZW6ppNLnHOEFTL06sEOhVuFJLZKjahLgwaOXz0Z9olemcB0oh1EEdHnrS8TCzCvmclScmHVD58x4qmGFu8Bu/HcjGEb9AmNzgh1ZVYION7jWj2/FtRnHy8EgHm6yGDqpviBUuaJNCv429JUCyC6wt+t76ik= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=Avj/XC3r; 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="Avj/XC3r" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 4F7F41F000E9; Thu, 25 Jun 2026 13:45:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782395164; bh=j7vCmlcktu/OIt286nnCz+UC1Ri3OuGM7sGaYKvlhpg=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=Avj/XC3r0JOxKdtvE2KK5ni2zOCBC6VC9iuKOzh7w0PvIotLT/ZkBBpRkBBwV0Q0Y 0ajS1EJviOxR03Qr2eRKs6UttA7u/nNrskxDz+b5cl6dqkOs68ZnKPFteFqOggO7gd Se4F+1NBBgTIss9OgyCE5bb4WYtE1WCwlds3W75Ma+uJYVy0W8axyk6o8iIDPv02wr std9SB3G/CBDB0Z3nCUsYYXSbIUIpAArTsfeFqc4BurNFSsqZsxgGfoQ8EVF012eNu K5DDQ0/6n6snlPg6brWF0CD+lc2o5mC/nIL5UgXJVkeBEZNXbdqFbd4cdqzkGcv8t/ TdbOzjeKrJh/g== Message-ID: <3af41c23-2365-4ac1-b1b9-076b6ef7ae9e@kernel.org> Date: Thu, 25 Jun 2026 15:45:56 +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: Johannes Weiner Cc: Barry Song , 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, 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 On 6/25/26 15:36, Johannes Weiner wrote: > On Thu, Jun 25, 2026 at 09:49:56AM +0200, David Hildenbrand (Arm) wrote: >>> >>> 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 :) >> >>> >>> >>> 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". > > It's technically doable, but is this worth the bother? The remaining > headroom is less than a large folio. You can split this one, but you > cannot even swap out all of its subpages anymore? I was asking myself the same, but when we think in terms of THPs on arm64 64k we're in the range of double-digit MiBs. > From the cgroup > side, we don't need the limit to be obeyed this rigidly. We overcharge > temporarily in other places if it's convenient to do so. A fuzz factor > around the limit is acceptable. Thanks for that information. > > But if you still want to do it, here is how: > > The page_counter_try_charge() in __mem_cgroup_try_charge_swap() walks > the hierarchy upwards. If it fails, it will store the first level that > failed against its limit. You can do the mem_cgroup_margin() math > against this counter to determine headroom. An ancestor *could* be > more restrictive, so you need to finish the hierarchy walk to the root > and use the min() of all the swap.max - page_counter_read(swap). Then > return that in a return argument from __mem_cgroup_try_charge_swap(). Thanks! @Barry, up to you if we want to implement that right away or if we're simply going to assume that if charging fails, not worth splitting (changing the existing handling IIUC). -- Cheers, David