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 8DE313E276C for ; Mon, 18 May 2026 08:24:32 +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=1779092672; cv=none; b=GPbp7Eos1+YTNljXraL8EMLHHhVjqAGXK4s9fbeNT+0Kf4uYdWOPGrUE3J+t0nhqYcLcjCm/KHSoSusCBXhQKcq/yL2GlyUfw28Iiel/u5dqr/WfOeX/hI+R4yVhiDwSDtQb5jSkCpo0A+9HR3ajCV+jD+870/OcYSjleGsKvCs= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1779092672; c=relaxed/simple; bh=dP02SKNit0+Jm7JhPU/U/F270rPRC3qrNHHL6WhTXKM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Mj+yLwp8XlgqGPA9UOXL85TP00JYjJrI1mPm1S2IMutu5odsaI7RQHhuct6EIploLyx6wWUe6C4MFcYDtQVEv/qBvRJNmx5lrpPLgPtcuO7mm2UqJnNKzdC7h4qpGTN7sQqMSHjyituyi+j/AnCvHwrPSQyc2wC9zas9SqOISLs= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=GNf23WV0; 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="GNf23WV0" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EFE34C2BCC6; Mon, 18 May 2026 08:24:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1779092672; bh=dP02SKNit0+Jm7JhPU/U/F270rPRC3qrNHHL6WhTXKM=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=GNf23WV0JvPZwY2QElh33775c5c7VrdM0vLY6vGAqD/IBOzeeqkNLn37QCi/smmNP rx8j9UJnzdJ+dlb8eGgw3Vi+Kq5Lq2tMGA0Np2pJ1eT7+5/iij2ErPyEgCLybG0//v JOa7f9DWs5x+gfU3Y0IaIS823SZlaaXJP7Wu5+kF2xZXyg6QMPRDM9fBeP+euwqGfq +ssstIRq9JZFl+KS8weZjpdJxjdc5ihfGa1qrIsJk80GbPGT0furHR7YZ9KnsmLQM3 njhWi6y8s4Pqrrq5/O1aDN4HqOEMSyh2ysuqUTdOhN04+m2tw5K/GwUoKYOxILAGy7 JFoiEGxYQDLFA== Message-ID: Date: Mon, 18 May 2026 10:24:26 +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: Lance Yang , npache@redhat.com Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, yuzhao@google.com, usamaarif642@gmail.com, 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: <20260514081130.7199-1-lance.yang@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: 8bit On 5/14/26 10:11, Lance Yang wrote: > > On Tue, May 12, 2026 at 09:02:38PM +0200, David Hildenbrand (Arm) wrote: >> On 5/12/26 20:36, Nico Pache wrote: >>> On Tue, May 12, 2026 at 1:05 AM David Hildenbrand (Arm) >>> wrote: >>> >>> I meant before b1f202060afe ("mm: remap unused subpages to shared >>> zeropage when splitting isolated thp"). Sorry, I should have been more >>> specific. >>> >>> As in before the underutilized shrinker (and commit b1f202060afe), we >>> had the exact problem you describe above and no way to handle it. >>> Correct? >> >> Right. That's why the underused shrinker was added, to directly free pages that >> have been over-allocated with a THP, but are actually never "used" (remain zero). >> >>> >>> I guess at reclaim THPs would be split and we would swap out a zero filled page? >> We don't necessarily split during swapout, we try to swap out the THP if >> supported. But yes, that can happen. >> >> I'm more thinking about other reasons to split a THP (e.g., migration, partial >> MADV_PAGEOUT/MADV_COLD/MADV_FREE), where the behavior would be changed. > > Yeah. Today any successful anon split gets TTU_USE_SHARED_ZEROPAGE. Would > it be better to make KSM opt out, so other split paths keep the behavior > they have today? > > That way we do not have to worry about changing anything else at the > same time :D This is (1) here: https://lore.kernel.org/all/04ea0e68-de56-49c4-8c9f-1734139d5e7f@kernel.org/ There will still be interaction between both mechanisms. So I'd prefer that in VM_MERGEABLE sections with KSM enabled, only KSM would take care of deduplicating. -- Cheers, David