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Mon, 11 May 2026 13:42:51 +0000 (UTC) X-FDA: 84755254542.05.697862E Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf07.hostedemail.com (Postfix) with ESMTP id 4CE324000E for ; Mon, 11 May 2026 13:42:49 +0000 (UTC) Authentication-Results: imf07.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=EBLtnSG5; spf=pass (imf07.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1778506969; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=zZCQkOBH2aMemBU8J9OZ7T3Pp5tnAYYcq5hyraC71qI=; b=tDxmn82cWFtT9ooyWom2WegfxpNburK6ionORAec2VAMRQ4XHGjzVUdwd8M4Nli9JjzUTl 8OJ1F1U/KqUGCx+JyJnlObhnHgVhwgrhyBYysVKzZTSmG930gHoqgUSrElS/vK5S1ZQsFM NHaZ7KdVKRTWB05oyxFt5LCHjmvLJ/I= ARC-Authentication-Results: i=1; imf07.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20201202 header.b=EBLtnSG5; spf=pass (imf07.hostedemail.com: domain of david@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=david@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1778506969; a=rsa-sha256; cv=none; b=ZX2+zUDQRNlhqcRogSzK36HvZoEM9znNBryk9mLGXLHkJh2qXcu9USaYySmrIaKreBz8GI JDvXvt3WrNFIZ9N2PG17OoRZXUo27PYZGmEqznf75Sjj00FvRooGLQKEjxJebvecnjQgWE 7MWarOwdkL8l/v7fKS9By6alnh1thig= Received: from smtp.kernel.org (transwarp.subspace.kernel.org [100.75.92.58]) by tor.source.kernel.org (Postfix) with ESMTP id B054F60123; Mon, 11 May 2026 13:42:48 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id A2D28C2BCC9; Mon, 11 May 2026 13:42:45 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1778506968; bh=xOte3uNSZatG3qR1qsZN8Z4Fe1SxHgD1xOtRdYD/D38=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=EBLtnSG5OM1F0I5eqdeN2Hg0EcB9aE5nXbkRXs9IiJxcokJFzbChdymWZnhpLeDEJ 5du2csfjyaOxZTFEoJDRI1E2ELCNM/Gs3UJOxuGftMkiILC+eDFWS1zdAdoRU31xTv j2xwAbh78Iykw+KGTFGPyOpIlE/6Wexj+jgVU0d6oTUlqqDCc7JPMSecz9jYydpJS6 nxDtjxWFm4Wnvy7JPIUcQpsoSDkbMQVtSK2TfsXhoW/4HQ1sLb2p9iC4HjhFcm+DKA J9Cf8RFN5fCGguro3Gnr2g2BTQD3nj9rmCLgZaaFUxRDk3IFTmubN6RvtGE1MPnxRf bt60BA0Qa/y6Q== Message-ID: <574fc329-bf2d-4686-9f15-b1709432326e@kernel.org> Date: Mon, 11 May 2026 15:42:44 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC] mm: restrict zero-page remapping to underused THP splits To: Usama Arif Cc: Nico Pache , linux-mm@kvack.org, linux-kernel@vger.kernel.org, yuzhao@google.com, usamaarif642@gmail.com, lance.yang@linux.dev, baohua@kernel.org, dev.jain@arm.com, ryan.roberts@arm.com, liam@infradead.org, baolin.wang@linux.alibaba.com, ziy@nvidia.com, ljs@kernel.org, akpm@linux-foundation.org References: <20260510114001.600681-1-usama.arif@linux.dev> <8838114e-5b6a-4f3d-932c-9e97e51216ae@kernel.org> <608bef55-44d1-47f1-a201-4a6bd7be137d@linux.dev> From: "David Hildenbrand (Arm)" Content-Language: en-US Autocrypt: addr=david@kernel.org; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspam-User: X-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 4CE324000E X-Stat-Signature: iok6m51hpujh46miy1dq8sob94u454di X-HE-Tag: 1778506969-148873 X-HE-Meta: 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 XscMfpfa Mf+ebOaNrmiyU9djosFYjCvBQzAewYTQ2LZn6c5tR05Pr/9mk1MwvEVyK7Y4mnbr6FvCQYoqZEqKyDFFaC0Xq0pkVGeCFGbk7LJhHRCNs833yfXckdwOxMRnI9D3Jk4/cOjluec6VGR2EFhB6Yt1ugUJkLaMvo35FsJDslei+9P0yW8oiEwzrGsv/q5uXp0c8C1muSrQVFFfoDwCnW1lN/4m+7BOzVPLoLRVEqs86zvlx5/JTEc5egogMbD+0yzQEMpNl Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 5/11/26 15:10, Usama Arif wrote: > > > On 11/05/2026 07:36, David Hildenbrand (Arm) wrote: >> >>> >>> Hello! >> >> >> Hi! >> >>> >>> I think (3) definitely makes sense. >>> >>> I have not had a deep look at KSM up until just now, so might be dumb >>> to say all of below.. :) >>> >>> What I see is that KSM scans THPs as 512 individual 4K subpages and splits the >>> THP whenever it actually wants to merge a single 4K chunk. That seems like a >>> lot of work for a single 4K? >> >> Yes, but that's what the users ask for: if there is a chance to deduplicate >> memory, it shall be deduplicated asap. >> >>> >>> One thing that came to my mind is to have a separate tree for THPs and only >>> merge the THPs that have the same content, but the possibility of encoutering >>> 2M pages with same content is extremely low? so this is probably a bad idea. >> >> Right, the probability is low, and it would change existing semantics, breaking >> existing users. >> >> In addition, we would have to add large folio support for KSM, which I rather >> would avoid. >> >>> >>> An alternative is, does it even make sense to process and split THPs by KSM >>> in the way it works now? IMO this is a lot of work for a single 4K merge. >>> Shrinker is designed to release memory when its needed, i.e. reclaim, at >>> which point IMO free memory is more important than performance. But KSM runs >>> all the time.. so constantly splitting THPs everytime a single 4K can be >>> merged just hurts performance all the time. >> >> Right, but that's what you get with KSM: bad performance if there is a chance to >> deduplicate :) >> >> (and bad performance from scanning overhead) >> >>> If someone cares about memory, >>> they should be running the shrinker. >> >> It's not just the zero page, but really any page content. The zero page is >> currently only "special" after we added conditional support to deduplicate to >> the shared zeropage in KSM. The shrinker doesn't help for any other page content >> besides zero-filled. >> >> Further, the shrinker is something system-wide, whereby KSM is usually only >> enabled for selected VMAs (with some exceptions nowadays). >> >> Also note that KSM deduplicates independent of the folio size: not just THPs, >> but really any (large) folio. Yes, it splits large folios, but that's really >> just to keep the T in THP. >> >>> Is a better alternative that KSM skips >>> THPs, THP shrinker splits THPs into 4K subpages when memory is needed, and >>> only then KSM gets those 4K subpages? >>> >>> Above sounds like reworking KSM, but just wanted to put it out there. >> >> Right, and it makes KSM more THP aware. Which is something I would avoid right now. >> >>> >>> (2) + (3) sounds like a good solution, but I wonder if above alternative >>> of KSM just skipping THPs might be better? >> >> That would change the semantics where, for example, where we expect that memory >> was deduplicated after a KSM run. >> >> VMs (where KSM is usually employed) are expected to be mostly backed by THPs: >> except where we can deduplicate memory. Skipping THPs would essentially break >> the main use case for KSM :) >> >> Does that make sense? >> > > Yes, all of above makes sense. But I feel like this means someone should not > set THP policy to always and enable KSM together. IIRC, QEMU will, as default, set MADV_HUGEPAGE and MADV_MERGEABLE :) (KSM itself later has to be enabled manually on a system level) > In general I feel like KSM > is not something that should be run on big servers, as hopefully you are > not managing memory as 4K chunks for big machines and using a lot of THPs. Right. But the 4k chunks are movable and compaction can move them around to create THPs elsewhere. -- Cheers, David