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[209.85.220.65]) by mx.google.com with SMTPS id r20sor7457170otq.0.2019.02.11.11.58.58 for (Google Transport Security); Mon, 11 Feb 2019 11:58:58 -0800 (PST) Received-SPF: pass (google.com: domain of dan.j.williams@intel.com designates 209.85.220.65 as permitted sender) client-ip=209.85.220.65; Authentication-Results: mx.google.com; dkim=pass header.i=@intel-com.20150623.gappssmtp.com header.s=20150623 header.b=v2SmmsxY; spf=pass (google.com: domain of dan.j.williams@intel.com designates 209.85.220.65 as permitted sender) smtp.mailfrom=dan.j.williams@intel.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=intel.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=aiCrOaqvqc6IXkmSpeBibC6k7Dt5ORUBY+Yth87tg8g=; b=v2SmmsxYbVz7eajzqaw1KE0S7eRG0vZw4fSOQMdWssxdYjWlIBUN/cdSMhrRO2Oitk ce0cr7TXBTH440tREOOROwN6aGzf0N0dK4YcxZbUDZxm8RsViTMLBKq5smoz1WnY93NS oQR4kw6bT14+qy26LgFr1aO0SNt82fha5GlY5Qi3ogUWx/2pP48/typtmKY+rDAX8Lv1 F+/dJB7h3xKZs31nObrffQ/AvUKRhukK8lsADOxWW1EmvCCpE1S283Z4enFVd2DVpUEb Ma0S29ugwJQDvFOs3J8JSPV9d4SwIbvkitbF1yEKwKQi+RsWEbn7PTdayZRFQFB5jyqt Sa0g== X-Google-Smtp-Source: AHgI3IZz5K9TqTpU5mfrAWqX17LGBxJt/PQR8+mBd04rCm+lHgw/p2iUDBeXhmYO4CyNzWTMaYVOQ2s59VGZwx2eYiI= X-Received: by 2002:a9d:7493:: with SMTP id t19mr16664268otk.98.1549915138337; Mon, 11 Feb 2019 11:58:58 -0800 (PST) MIME-Version: 1.0 References: <01000168c8e2de6b-9ab820ed-38ad-469c-b210-60fcff8ea81c-000000@email.amazonses.com> <20190208044302.GA20493@dastard> <20190208111028.GD6353@quack2.suse.cz> <20190211102402.GF19029@quack2.suse.cz> <20190211180654.GB24692@ziepe.ca> <20190211181921.GA5526@iweiny-DESK2.sc.intel.com> <20190211182649.GD24692@ziepe.ca> <20190211184040.GF12668@bombadil.infradead.org> In-Reply-To: <20190211184040.GF12668@bombadil.infradead.org> From: Dan Williams Date: Mon, 11 Feb 2019 11:58:47 -0800 Message-ID: Subject: Re: [LSF/MM TOPIC] Discuss least bad options for resolving longterm-GUP usage by RDMA To: Matthew Wilcox Cc: Jason Gunthorpe , Ira Weiny , Jan Kara , Dave Chinner , Christopher Lameter , Doug Ledford , lsf-pc@lists.linux-foundation.org, linux-rdma , Linux MM , Linux Kernel Mailing List , John Hubbard , Jerome Glisse , Michal Hocko Content-Type: text/plain; charset="UTF-8" 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, Feb 11, 2019 at 10:40 AM Matthew Wilcox wrote: > > On Mon, Feb 11, 2019 at 11:26:49AM -0700, Jason Gunthorpe wrote: > > On Mon, Feb 11, 2019 at 10:19:22AM -0800, Ira Weiny wrote: > > > What if user space then writes to the end of the file with a regular write? > > > Does that write end up at the point they truncated to or off the end of the > > > mmaped area (old length)? > > > > IIRC it depends how the user does the write.. > > > > pwrite() with a given offset will write to that offset, re-extending > > the file if needed > > > > A file opened with O_APPEND and a write done with write() should > > append to the new end > > > > A normal file with a normal write should write to the FD's current > > seek pointer. > > > > I'm not sure what happens if you write via mmap/msync. > > > > RDMA is similar to pwrite() and mmap. > > A pertinent point that you didn't mention is that ftruncate() does not change > the file offset. So there's no user-visible change in behaviour. ...but there is. The blocks you thought you freed, especially if the system was under -ENOSPC pressure, won't actually be free after the successful ftruncate().