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d=hostedemail.com; t=1684855783; a=rsa-sha256; cv=none; b=YtDnsc5VjAkhyqWoi/85VlKr5ONicwEPQsZdFUycRdoWFs+QSCKb8z1sAIVvPLlu6wNDZL 7JGbuv6kzucJHH6YKxySfAun3rauDsMIGdx8JzS6BWMeouPcjj00ZqYqVl5Ov+IvwfzOR1 Mm1ArwJIlydgGQbdo9RZ6Y6q4in5dAg= ARC-Authentication-Results: i=1; imf15.hostedemail.com; dkim=none; spf=none (imf15.hostedemail.com: domain of hch@lst.de has no SPF policy when checking 213.95.11.211) smtp.mailfrom=hch@lst.de; dmarc=none Received: by verein.lst.de (Postfix, from userid 2407) id 4CBB06732D; Tue, 23 May 2023 17:29:39 +0200 (CEST) Date: Tue, 23 May 2023 17:29:39 +0200 From: Christoph Hellwig To: Thomas Gleixner Cc: linux-mm@kvack.org, Andrew Morton , Christoph Hellwig , Uladzislau Rezki , Lorenzo Stoakes , Peter Zijlstra , Baoquan He Subject: Re: [patch 4/6] mm/vmalloc: Check free space in vmap_block lockless Message-ID: <20230523152939.GD12341@lst.de> References: <20230523135902.517032811@linutronix.de> <20230523140002.747157575@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230523140002.747157575@linutronix.de> User-Agent: Mutt/1.5.17 (2007-11-01) X-Rspamd-Queue-Id: D6745A0017 X-Rspam-User: X-Stat-Signature: oncqch9u717gy8j41iqmq9xxco956ksw X-Rspamd-Server: rspam03 X-HE-Tag: 1684855782-398680 X-HE-Meta: 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 OhYhcDAU RK0odKvO9M8NfP9H6R6gXla6qO1DnltCVKFyhEfQMPTw7u+G410L3EDEI5fhw/kvthUxxy6GPCL1PnwCWcFax1b8/6QxUithatSKuTT3tjGDviabaGmTFzyIFEuhSh2ONx10P 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 Tue, May 23, 2023 at 04:02:14PM +0200, Thomas Gleixner wrote: > + if (READ_ONCE(vb->free) < (1UL << order)) > + continue; > + > spin_lock(&vb->lock); > if (vb->free < (1UL << order)) { > spin_unlock(&vb->lock); > @@ -2174,7 +2177,7 @@ static void *vb_alloc(unsigned long size > > pages_off = VMAP_BBMAP_BITS - vb->free; > vaddr = vmap_block_vaddr(vb->va->va_start, pages_off); > - vb->free -= 1UL << order; > + WRITE_ONCE(vb->free, vb->free - (1UL << order)); Maybe just a matter of preference, but wouldn't an atomic_t be better here? We'd have another locked instruction in the alloc path, but I always find the READ_ONCE/WRITE_ONCE usage a bit fragile that I'd rather reserve them to well documented hot path code.