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[34.85.230.48]) by smtp.gmail.com with ESMTPSA id g19-20020a37e213000000b006b949afa980sm2847751qki.56.2022.09.28.06.48.46 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Sep 2022 06:48:46 -0700 (PDT) Date: Wed, 28 Sep 2022 13:48:46 +0000 From: Joel Fernandes To: Hyeonggon Yoo <42.hyeyoo@gmail.com> Cc: Hugh Dickins , Matthew Wilcox , Vlastimil Babka , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org Subject: Re: amusing SLUB compaction bug when CC_OPTIMIZE_FOR_SIZE Message-ID: References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1664379816; a=rsa-sha256; cv=none; b=h6idHKJLjyLA6BIK77W5XMMW7Ycwr5Dwmh9zb1xhD2IBdWBuh16PQ3IBnvGxFbMsSjm2XQ Ew9nVySt5/CRQR+fOu2xgDLUbptDxK4T3wu4iqh8hiKQvawRlfqAnk6NWGNaOqkXU9Oo0H ZQQoy03qRvBIqkBwYCVwqRbEjFhN+rc= ARC-Authentication-Results: i=1; imf22.hostedemail.com; dkim=pass header.d=joelfernandes.org header.s=google header.b=AWBShZZE; spf=pass (imf22.hostedemail.com: domain of joel@joelfernandes.org designates 209.85.222.177 as permitted sender) smtp.mailfrom=joel@joelfernandes.org; dmarc=none ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1664379816; 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: in-reply-to:in-reply-to:references:references:dkim-signature; bh=5+THzLJ8qEOkkmZ45Nfy4NnndEMBDTzKKsy7ol7yOEg=; b=V1wNLVpVOowwKLMd0vFtNhDItxMb1JLAdQ3Vb5tLXBOe94ry6O6qGW6+WUREWL9vCg9PXE tn1mcq79k/ECCcj6gpZPt1hNqiZYwTzRIkgGWCG5zmQaVLl8HceInsy7+kmpJe/BXobeoT sK5UT62gCXrFkHdYM5Hi4LLDbsL7qTY= X-Stat-Signature: jt4tpywuamcp56cbxq7ib4ccpoieocqx X-Rspamd-Queue-Id: C8D45C0386 X-Rspam-User: Authentication-Results: imf22.hostedemail.com; dkim=pass header.d=joelfernandes.org header.s=google header.b=AWBShZZE; spf=pass (imf22.hostedemail.com: domain of joel@joelfernandes.org designates 209.85.222.177 as permitted sender) smtp.mailfrom=joel@joelfernandes.org; dmarc=none X-Rspamd-Server: rspam01 X-HE-Tag: 1664379816-712520 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: On Wed, Sep 28, 2022 at 02:49:02PM +0900, Hyeonggon Yoo wrote: > On Tue, Sep 27, 2022 at 10:16:35PM -0700, Hugh Dickins wrote: > > It's a bug in linux-next, but taking me too long to identify which > > commit is "to blame", so let me throw it over to you without more > > delay: I think __PageMovable() now needs to check !PageSlab(). > > > > I had made a small experimental change somewhere, rebuilt and rebooted, > > was not surprised to crash once swapping and compaction came in, > > but was surprised to find the crash in isolate_movable_page(), > > called by compaction's isolate_migratepages_block(). > > > > page->mapping was ffffffff811303aa, which qualifies as __PageMovable(), > > which expects struct movable_operations at page->mapping minus low bits. > > But ffffffff811303aa was the address of SLUB's rcu_free_slab(): I have > > CONFIG_CC_OPTIMIZE_FOR_SIZE=y, so function addresses may have low bits set. > > > > Over to you! Thanks, > > Hugh > > Wow, didn't expect this. > Thank you for report! > > That should be due to commit 65505d1f2338e7 > ("mm/sl[au]b: rearrange struct slab fields to allow larger rcu_head") > as now rcu_head can use some bits that shares with mapping. > > Hmm IMO we have two choices... > > 1. simply drop the commit as it's only for debugging (RCU folks may not like [1]) Yeah definitely don't like this option as patches are out that depend on this (not yet merged though). :-) > 2. make __PageMovable() to use true page flag, with approach [2]) What are the drawbacks of making it a true flag? thanks, - Joel > [1] https://lore.kernel.org/all/85afd876-d8bb-0804-b2c5-48ed3055e702@joelfernandes.org/ > [2] https://lore.kernel.org/linux-mm/20220919125708.276864-1-42.hyeyoo@gmail.com/ > > Thanks! > > -- > Thanks, > Hyeonggon