Date: Sun, 31 Jul 2005 21:32:12 -0400 From: Garrett Wollman <wollman@csail.mit.edu> To: Giorgos Keramidas <keramida@freebsd.org> Cc: freebsd-current@freebsd.org Subject: Re: mmap bug? Message-ID: <17133.31644.343077.241748@khavrinen.csail.mit.edu> In-Reply-To: <20050731224601.GA2024@gothmog.gr> References: <20050731141801.GA49300@gothmog.gr> <84dead7205073108564f71f1ab@mail.gmail.com> <20050731160853.GC49839@gothmog.gr> <84dead720507311115290f0140@mail.gmail.com> <20050731201858.GC1052@gothmog.gr> <17133.21281.703540.383144@khavrinen.csail.mit.edu> <20050731224601.GA2024@gothmog.gr>
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<<On Mon, 1 Aug 2005 01:46:01 +0300, Giorgos Keramidas <keramida@freebsd.org> said: > Is the fact that the data written to the mapped region visible after the > close (by mapping the same region) expected too? No, that's a bug. (It's not a particularly surprising one, although I'm no VM expert and I have no idea how difficult it would be to fix.) The problem is that the VM system has no efficient way of telling that you modified the part of the page that's after the end of the file, so when you close the file, the dirty but only partially valid page is cached. Because you did a MAP_SHARED, mmap() can't use an anonymous page -- it has to use the actual page from the file. If the page isn't in cache, it will correctly zero-fill the parts beyond the end of the file, but it doesn't appear to do this for cached pages on 0->1 refcount transitions. -GAWollman
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