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[2607:7c80:54:e::133]) by mx.google.com with ESMTPS id x32si5476572pld.190.2019.06.15.07.12.18 for (version=TLS1_3 cipher=AEAD-AES256-GCM-SHA384 bits=256/256); Sat, 15 Jun 2019 07:12:18 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of batv+78a6abdb7ec5759febfc+5774+infradead.org+hch@bombadil.srs.infradead.org designates 2607:7c80:54:e::133 as permitted sender) client-ip=2607:7c80:54:e::133; Authentication-Results: mx.google.com; dkim=pass header.i=@infradead.org header.s=bombadil.20170209 header.b=dNgRTy1V; spf=pass (google.com: best guess record for domain of batv+78a6abdb7ec5759febfc+5774+infradead.org+hch@bombadil.srs.infradead.org designates 2607:7c80:54:e::133 as permitted sender) smtp.mailfrom=BATV+78a6abdb7ec5759febfc+5774+infradead.org+hch@bombadil.srs.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=Xeia7Y2H20qxvC8T957Brc8A65sMgIu+4oNZeZROWY8=; b=dNgRTy1VE+B9PBH6hwZ144/wJ hPJ6HGzKtH5KFgLTt3v3DzpcFqBEl0IyH4qot6+xc/fHoQLRWtTYKcvLodHmCCN46/wHpw9o1LC+d nsupPfh3zYNfmm6O8UQibHe4FMPlCLRsrs6+iyIHs0QZrxD56KndDvsC847sZcv6SIrm6GQgUprWL dLf+9pS7RKXZVIxFmz9AdlM5H70IdanFowU889RfB3jQkG9MqEaLpup3QiULxuk0eU94Wk5oziV5R nPOTz5pupFoa5pc3bHOanYkNsZ9BGWIx7d1wmp4/iHlZ+xYvhlfjsKxuQQnPCPdSq4GwKA3+eNie/ GNiDvfjrg==; Received: from hch by bombadil.infradead.org with local (Exim 4.92 #3 (Red Hat Linux)) id 1hc9Pf-000272-29; Sat, 15 Jun 2019 14:12:11 +0000 Date: Sat, 15 Jun 2019 07:12:11 -0700 From: Christoph Hellwig To: Jason Gunthorpe Cc: Jerome Glisse , Ralph Campbell , John Hubbard , Felix.Kuehling@amd.com, linux-rdma@vger.kernel.org, linux-mm@kvack.org, Andrea Arcangeli , dri-devel@lists.freedesktop.org, amd-gfx@lists.freedesktop.org, Ben Skeggs , Jason Gunthorpe , Ira Weiny , Philip Yang Subject: Re: [PATCH v3 hmm 04/12] mm/hmm: Simplify hmm_get_or_create and make it reliable Message-ID: <20190615141211.GD17724@infradead.org> References: <20190614004450.20252-1-jgg@ziepe.ca> <20190614004450.20252-5-jgg@ziepe.ca> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190614004450.20252-5-jgg@ziepe.ca> User-Agent: Mutt/1.11.4 (2019-03-13) X-SRS-Rewrite: SMTP reverse-path rewritten from by bombadil.infradead.org. See http://www.infradead.org/rpr.html 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: > + spin_lock(&mm->page_table_lock); > + if (mm->hmm) { > + if (kref_get_unless_zero(&mm->hmm->kref)) { > + spin_unlock(&mm->page_table_lock); > + return mm->hmm; > + } > + } > + spin_unlock(&mm->page_table_lock); This could become: spin_lock(&mm->page_table_lock); hmm = mm->hmm if (hmm && kref_get_unless_zero(&hmm->kref)) goto out_unlock; spin_unlock(&mm->page_table_lock); as the last two lines of the function already drop the page_table_lock and then return hmm. Or drop the "hmm = mm->hmm" asignment above and return mm->hmm as that should be always identical to hmm at the end to save another line. > + /* > + * The mm->hmm pointer is kept valid while notifier ops can be running > + * so they don't have to deal with a NULL mm->hmm value > + */ The comment confuses me. How does the page_table_lock relate to possibly running notifiers, as I can't find that we take page_table_lock? Or is it just about the fact that we only clear mm->hmm in the free callback, and not in hmm_free?