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[198.145.29.99]) by mx.google.com with ESMTPS id a17si4188382pfa.45.2019.07.01.16.32.12 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 01 Jul 2019 16:32:12 -0700 (PDT) Received-SPF: pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) client-ip=198.145.29.99; Authentication-Results: mx.google.com; dkim=pass header.i=@kernel.org header.s=default header.b=yulAQMWr; spf=pass (google.com: domain of akpm@linux-foundation.org designates 198.145.29.99 as permitted sender) smtp.mailfrom=akpm@linux-foundation.org Received: from localhost.localdomain (c-73-223-200-170.hsd1.ca.comcast.net [73.223.200.170]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id E146821479; Mon, 1 Jul 2019 23:32:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1562023932; bh=dBXpbIE1R4CzpTJv8obwrKm6+ibARscFWpAQm9lSQZA=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=yulAQMWrcw1P+CopEJYaogBfFneJfl2za6FZd5pFA6D5io9m+xQnEm+yQaDWjdT/Y 9DFHXlk5mdBoyq4T6dTSKA58o6T74Hd/T8dTqYp/w0HOnpZqf6i5nqYGJuQ9bA1XgI Mo0ZF2H9lZFRsD+XZV8fRh2gemTtyGNq9stOi+ew= Date: Mon, 1 Jul 2019 16:32:11 -0700 From: Andrew Morton To: Henry Burns Cc: Vitaly Wool , Vitaly Vul , Mike Rapoport , Xidong Wang , Shakeel Butt , Jonathan Adams , linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm/z3fold.c: Lock z3fold page before __SetPageMovable() Message-Id: <20190701163211.e9e0f2cf5332c06640e3019d@linux-foundation.org> In-Reply-To: <20190701212303.168581-1-henryburns@google.com> References: <20190701212303.168581-1-henryburns@google.com> X-Mailer: Sylpheed 3.5.1 (GTK+ 2.24.31; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit 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 Mon, 1 Jul 2019 14:23:03 -0700 Henry Burns wrote: > __SetPageMovable() expects it's page to be locked, but z3fold.c doesn't > lock the page. So this triggers the VM_BUG_ON_PAGE(!PageLocked(page), page) in __SetPageMovable(), yes? > Following zsmalloc.c's example we call trylock_page() and > unlock_page(). Also makes z3fold_page_migrate() assert that newpage is > passed in locked, as documentation. > > ... > > --- a/mm/z3fold.c > +++ b/mm/z3fold.c > @@ -918,7 +918,9 @@ static int z3fold_alloc(struct z3fold_pool *pool, size_t size, gfp_t gfp, > set_bit(PAGE_HEADLESS, &page->private); > goto headless; > } > + WARN_ON(!trylock_page(page)); If this warn triggers then someone else has locked the page. > __SetPageMovable(page, pool->inode->i_mapping); > + unlock_page(page); and we proceed to undo their lock. So that other code path will then perform an unlock of an unlocked page. Etcetera. It would be much much better to do a plain old lock_page() here. If that results in a deadlock then let's find out why and fix it without trylock hacks.