From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id C7955C27C5F for ; Mon, 10 Jun 2024 20:06:08 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 2AA5E6B0095; Mon, 10 Jun 2024 16:06:08 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 2327E6B0096; Mon, 10 Jun 2024 16:06:08 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 0AC1D6B0098; Mon, 10 Jun 2024 16:06:08 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0017.hostedemail.com [216.40.44.17]) by kanga.kvack.org (Postfix) with ESMTP id DA2C76B0095 for ; Mon, 10 Jun 2024 16:06:07 -0400 (EDT) Received: from smtpin23.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay05.hostedemail.com (Postfix) with ESMTP id 7E6E4411C1 for ; Mon, 10 Jun 2024 20:06:07 +0000 (UTC) X-FDA: 82216060374.23.1F6FBAA Received: from mail-ed1-f48.google.com (mail-ed1-f48.google.com [209.85.208.48]) by imf01.hostedemail.com (Postfix) with ESMTP id A033740004 for ; Mon, 10 Jun 2024 20:06:05 +0000 (UTC) Authentication-Results: imf01.hostedemail.com; dkim=pass header.d=google.com header.s=20230601 header.b=mVETR85p; spf=pass (imf01.hostedemail.com: domain of yosryahmed@google.com designates 209.85.208.48 as permitted sender) smtp.mailfrom=yosryahmed@google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1718049965; 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=EOwI6DcCCI12dNBNkZD00pCUv7/h5/UDISzx/oKHb3g=; b=XkAcpPKVV6puJlC3okTyfKq6UtlNVUE/I6n4tb6NLhXJ0tgUGGJpxh11FhEHSXOWXBMZRA CpzmZM741Vf91MLYXZ0u2YVmKp3DLOP52QUp32K01l9pZJIScHsHHnSZ2Yg0zjK/ligIRX erkhrsr8AVNOtu/xEF25sJglsiO11pA= ARC-Authentication-Results: i=1; imf01.hostedemail.com; dkim=pass header.d=google.com header.s=20230601 header.b=mVETR85p; spf=pass (imf01.hostedemail.com: domain of yosryahmed@google.com designates 209.85.208.48 as permitted sender) smtp.mailfrom=yosryahmed@google.com; dmarc=pass (policy=reject) header.from=google.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1718049965; a=rsa-sha256; cv=none; b=FL0MhlrbWQrqIYeSp11hml/+iRqseTPwdUAuKI5zB0BZLmPHCAuiMTOZpUmwr8qNulZ2w2 +E1GAb5dwH1A/HZFiiRMYakxokC2t7faO13RHlIQNzn83YNaygE62EPWFduNwqdrHs4TWb OewN0n0tY4Nf1LJRdCQw/8ezWLKfHCQ= Received: by mail-ed1-f48.google.com with SMTP id 4fb4d7f45d1cf-57a30dbdb7fso6978621a12.3 for ; Mon, 10 Jun 2024 13:06:05 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1718049964; x=1718654764; darn=kvack.org; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:from:to:cc:subject:date :message-id:reply-to; bh=EOwI6DcCCI12dNBNkZD00pCUv7/h5/UDISzx/oKHb3g=; b=mVETR85p0i/XHa02shlU6GQAuC1BTsT62tMzquKRO/M7mJNxTmxU/8ble0pdZ+66+E Kmjx/yX218bGG7Bba+BMOH1qfG4c7gRAq+beDxbFy8vKRIQZn52oQ2lLysIBrrFfm68U TWulLZjsLOn95Ab2fnrZ6NMzHeF/fuFpusNT8JLeFul/vuIjc98+Mti+KXNYwHcd0eWx BWSPS3s7pXRZ/1yx2sBOenV/iIy87baX7mspy7N9WJ6S29rP2wn9amM6dBxXv5cOsCIg 3MqUBJM52qL2tNzVeRFbc9pq26/8DDoegZ6GU0HdfLvnUHckzzfJW2JcdeUc5Wmi7Rmq e4cA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1718049964; x=1718654764; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=EOwI6DcCCI12dNBNkZD00pCUv7/h5/UDISzx/oKHb3g=; b=nkqnPpc5F7BYuL3gc15VxPPRMaiAIIJAGVrIlMqHuH/3Exta+w2RueYPlTGnguGwo0 NZILa4GHSXNdaonYysBXtixpr9prnEDLT6ZvgKOA0WttxbTwxqkhDsMVguAZc3JQuf4o fWM35kMSri5jQwtnnqYqfLDfl4LG7pzakwsoZ4iBoir8/xB+8+0/gN+hIgl5On9Xv8G/ jMu0BZnKk2pJL7SaYiHm2ZFzfevyk7LcKrBAOrZ3tL1RjqhNRTtFya1DRxzmp/M3OCRh vsO5Uz+v3aunfx6SDIUpre6I8Pa+ZofwMpWUMY0LKfOgj7nB4EAlaKor+Ujwze6xP3fe YJJA== X-Forwarded-Encrypted: i=1; AJvYcCXbPL89zygp4o0y0DscEOT16KBXvfupCfK+yEG6qe0JqBQx+bSOVB3bqLP6gCY1T3X3XPvgl5Ius46Jd3KYPo5RXT4= X-Gm-Message-State: AOJu0YybQg1JWeAhxzWAt/ACyeicZQmc+G6n+QYhrpzSc9UliXW16Eg3 mC4gszNplOXC18K9WCvRoRUfKD9zS8CQJbOfl6lsDP8wB+MWkUcG/su3OAAoPF7yyoYsBQCeCmL 9Z1hOW5AnpV4pCDz20JCxRHjrmHq4uhs/JD88 X-Google-Smtp-Source: AGHT+IE03edLaD5o4NEBvXfLFP+kMiyLAjRNIwjqBu9SUSMn4dd8ukKhZoCOUCXo52b4yzosP9Ppa4fhfYlxBrh0Mcs= X-Received: by 2002:a17:906:3b1a:b0:a6f:3763:3cbe with SMTP id a640c23a62f3a-a6f37633da7mr6496966b.67.1718049963596; Mon, 10 Jun 2024 13:06:03 -0700 (PDT) MIME-Version: 1.0 References: <20240610143037.812955-1-usamaarif642@gmail.com> In-Reply-To: From: Yosry Ahmed Date: Mon, 10 Jun 2024 13:05:25 -0700 Message-ID: Subject: Re: [PATCH] mm: Do not start/end writeback for pages stored in zswap To: Shakeel Butt Cc: Usama Arif , akpm@linux-foundation.org, willy@infradead.org, hannes@cmpxchg.org, nphamcs@gmail.com, chengming.zhou@linux.dev, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@meta.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: A033740004 X-Stat-Signature: mqintqgj5hopnm1y3xhrz3a77z3hjhdk X-HE-Tag: 1718049965-216958 X-HE-Meta: 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 C87KxU58 NYmYWSmZwdW6SMYQZjtRoS4b7RiNK/HXPZrqiiSGRHvCzVbyfPT6Z92sKr9iRWZeA+L15+83Cs1uvHWBXmLsybJBhx/ddYhR/tL5RHlpqmRUK/4j+6IE1cmkT84gDUOPsdp91qA2/IRKJiHj4QS/hS3wBNHgx78FMD0Ij7Is020DnzeBq+oueXKuFaBTFAzcnEZuzZXghzpcwztppJ8vwVwjoqFl1vQ8cf1OT X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Jun 10, 2024 at 12:08=E2=80=AFPM Shakeel Butt wrote: > > On Mon, Jun 10, 2024 at 10:31:36AM GMT, Yosry Ahmed wrote: > > On Mon, Jun 10, 2024 at 7:31=E2=80=AFAM Usama Arif wrote: > > > > > > start/end writeback combination incorrectly increments NR_WRITTEN > > > counter, eventhough the pages aren't written to disk. Pages successfu= lly > > > stored in zswap should just unlock folio and return from writepage. > > > > > > Signed-off-by: Usama Arif > > > --- > > > mm/page_io.c | 2 -- > > > 1 file changed, 2 deletions(-) > > > > > > diff --git a/mm/page_io.c b/mm/page_io.c > > > index a360857cf75d..501784d79977 100644 > > > --- a/mm/page_io.c > > > +++ b/mm/page_io.c > > > @@ -196,9 +196,7 @@ int swap_writepage(struct page *page, struct writ= eback_control *wbc) > > > return ret; > > > } > > > if (zswap_store(folio)) { > > > - folio_start_writeback(folio); > > > folio_unlock(folio); > > > - folio_end_writeback(folio); > > > > Removing these calls will have several effects, I am not really sure it= 's safe. > > > > 1. As you note in the commit log, NR_WRITTEN stats (and apparently > > others) will no longer be updated. While this may make sense, it's a > > user-visible change. I am not sure if anyone relies on this. > > > > I couldn't imagine how this stat can be useful for the zswap case and I > don't see much risk in changing this stat behavior for such cases. It seems like NR_WRITTEN is only used in 'global_dirty_state' trace event. NR_WRITEBACK and NR_ZONE_WRITE_PENDING are state counters, not event counters. They are incremented in folio_start_writeback() and decremented in folio_end_writeback(). They are probably just causing noise. I think for both cases it's probably fine and not really visible to userspa= ce. > > > 2. folio_end_writeback() calls folio_rotate_reclaimable() after > > writeback completes to put a folio that has been marked with > > PG_reclaim at the tail of the LRU, to be reclaimed first next time. Do > > we get this call through other paths now? > > > > The folio_rotate_reclaimable() only makes sense for async writeback > pages i.e. not for zswap where we synchronously reclaim the page. Looking at pageout(), it seems like we will clear PG_reclaim if the folio is not under writeback, and in shrink_folio_list() if the folio is not dirty or under writeback, we will reclaim right away. I thought zswap being synchronous was an odd case, but apparently there is wider support for synchronous reclaim. Thanks for pointing this out. > > > 3. If I remember correctly, there was some sort of state machine where > > folios go from dirty to writeback to clean. I am not sure what happens > > if we take the writeback phase out of the equation. > > > > Is there really such a state machine? We only trigger writeback if the > page is dirty and we have cleared it. The only thing I can think of is > the behavior of the waiters on PG_locked bit but the window of > PG_writeback is so small that it seems like it does not matter. I remember Matthew talking about it during LSF/MM this year when he was discussing page flags, but maybe I am misremembering.