From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 B425C3EC2D0 for ; Mon, 11 May 2026 13:42:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778506968; cv=none; b=gp1eJ1iOW4dG+7IGEJApxI6ZAlF7g/qB+RWTEro2VX2u6ZxFP6YITRIA9R5M+htUq86K1LY5D7G6jP0DfJIC/Z3dw1alBspg9THHmcniolvoRA0nEHwZaPS3BbJjA8J1MePq7OdsqW33M3mPdn+/U8kPWYs9/tm2ogxWxLsHFzI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1778506968; c=relaxed/simple; bh=xOte3uNSZatG3qR1qsZN8Z4Fe1SxHgD1xOtRdYD/D38=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ig1BBG+L/x+kiNXp2VNg7aMk0sLKCTu9zp7uLBHO54PSNwQpdXyJqtKsvHDTy3TPWU6asR71Bi1WDo3UE/ARbX85YrimwB4l0fk7auCDPjnpX88JvnzTGXBdLFy19XrnDQHctdgL3AkhFRpbCGAngNhHKsFHfpxXfMLj3kbl8sU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=EBLtnSG5; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="EBLtnSG5" 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 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] 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 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