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 9459E302742 for ; Wed, 24 Jun 2026 14:59:03 +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=1782313144; cv=none; b=IK6/MCpyd7PgYWLwSYBj85Ej0fO6HpT3MspmDD2lH4/ZRQxFCBQnQesf/4PEVjyhXaq+dbuog2qcx+AsJ5DwocEqmcq3xCYkcfvKSp+knPPn9R28GY52q+waAJQK0OO9bABxYlOiEq3NneKOucQL5Gt600hLBPAs4RvT6fkjpeU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782313144; c=relaxed/simple; bh=xU7MMbgbU+szi0a7Tor44IiGn/b1D9+m9R0uxQO0Lsk=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=MsfxCmyiE4X2itO0VzEbakB9fkOW3I8aUnQoNsXNZonR8ve0GVGe2XE3s42apYVmP1MYH3/z8QurHx0PYl7txKKCqY42+Da9753LTBlaw46zaYvmsznWD65LBu6Dl0lzuI6wOj6fX4Y8TV35Yj3gQ9bztHxuBNj4B60n3B9k+AQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=or1meLht; 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="or1meLht" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D13511F000E9; Wed, 24 Jun 2026 14:58:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1782313143; bh=A7Zra2DC/SjAzfu14QInVXQE1b9FL7wyAOkZas/Tzpo=; h=Date:Subject:To:Cc:References:From:In-Reply-To; b=or1meLhtDcCuNkUq36sAapMVjJdTDs4HJxfOQqEFdBpVvFlCHH+AomXPIXsmCnK2z pq0ey6DIp/5q23UtD4xXezg382Y++9bRkl0BLB0vP3YR6nOe1lbdLh6SXfs3kGdmPY cJC5ttYdxg9sNHh+nXfTYoLLl+LNb6hjl15GWrIoWRGY+o3BupGa1gPujhMEYb9DZN vkKVCiGzsYMBlqcy93yS5/n3s9fcHDrfJB6GLEk2iqFy3LHLRA3IcOgPyVsxMRX7sI 803nztF9X1IxroKsWrcUUMTPMUfiakrjph90GS8vN+h9WtbSQV6CeAqIi+ScZbMzQM CqHGVYzWmsSvg== Message-ID: <3523f142-778c-4efd-9675-3c68b33e7e3d@kernel.org> Date: Wed, 24 Jun 2026 16:58:57 +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: bypass swap readahead for zswap To: Alexandre Ghiti , akpm@linux-foundation.org, hannes@cmpxchg.org, yosry@kernel.org, nphamcs@gmail.com Cc: chengming.zhou@linux.dev, ljs@kernel.org, liam@infradead.org, vbabka@kernel.org, rppt@kernel.org, surenb@google.com, mhocko@suse.com, kasong@tencent.com, chrisl@kernel.org, baohua@kernel.org, usama.arif@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20260624075700.751467-1-alex@ghiti.fr> 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 6/24/26 09:55, Alexandre Ghiti wrote: > Commit 0bcac06f27d7 ("mm, swap: skip swapcache for swapin of synchronous > device") made SWP_SYNCHRONOUS_IO devices (e.g. zram) skip swap readahead. > > zswap is the same kind of in-memory, synchronous backend as zram, not a > swap device flagged SWP_SYNCHRONOUS_IO so it still goes through > swapin_readahead(). > > Here are the results from bypassing readahead for zswap too: it was > measured with a kernel build (make -j16) in a memcg, zswap=zstd, shrinker > off, on Sapphire Rapids and 3 iterations. > > 768M memcg (sustained swap thrash): > metric mm-new + bypass delta > build time (s) 405.0 341.7 -15.6% > zswap-in (GB) 79.5 53.0 -33% > zswap-out (GB) 144.8 115.6 -20% > swap readahead (pages) 6.79M 0.45M -93% > swap_ra hit (%) 72.1 89.9 +18pp > > 1G memcg (light pressure, build not memory-bound): > metric mm-new + bypass delta > build time (s) 177.7 176.0 ~same (no regression) > zswap-in (GB) 10.2 7.5 -26% > zswap-out (GB) 27.7 25.1 -9% > swap readahead (pages) 1.07M 0.08M -93% > swap_ra hit (%) 68.6 87.2 +19pp > > The gain is from no longer prefetching pages that are pointless for an > in-memory backend: readahead inflates anon residency and thrashes the > page cache (file pages get evicted and re-read), lengthens each fault by > synchronously (de)compressing a cluster of neighbours, and adds > compression traffic when those extra pages are reclaimed. > > Bypassing swap readahead for zswap therefore makes sense. > > Signed-off-by: Alexandre Ghiti > --- [...] > #endif /* _LINUX_ZSWAP_H */ > diff --git a/mm/memory.c b/mm/memory.c > index ff338c2abe92..5aa1ea9eb48a 100644 > --- a/mm/memory.c > +++ b/mm/memory.c > @@ -4827,8 +4827,9 @@ vm_fault_t do_swap_page(struct vm_fault *vmf) > if (folio) > swap_update_readahead(folio, vma, vmf->address); > if (!folio) { > - /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices */ > - if (data_race(si->flags & SWP_SYNCHRONOUS_IO)) > + /* Swapin bypasses readahead for SWP_SYNCHRONOUS_IO devices and zswap */ > + if (data_race(si->flags & SWP_SYNCHRONOUS_IO) || > + zswap_present_test(entry)) This should really be abstracted into a reasonably-named helper that can live in swap code. -- Cheers, David