Date: Tue, 07 Feb 2006 01:42:46 +0100 From: Raaf <freebsd@luna.afraid.org> To: John-Mark Gurney <gurney_j@resnet.uoregon.edu> Cc: freebsd-hackers@freebsd.org Subject: Re: Freeing mmapped memory Message-ID: <43E7ED06.4030403@luna.afraid.org> In-Reply-To: <20060206225649.GJ69162@funkthat.com> References: <43E7C583.3040904@luna.afraid.org> <20060206225649.GJ69162@funkthat.com>
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John-Mark Gurney wrote: > Raaf wrote this message on Mon, Feb 06, 2006 at 22:54 +0100: >> Hi, i am working on a usb driver that allocates some memory when >> the device is opened using malloc. >> >> Now i want user processes to be able to access this memory using >> mmap and i want to free this memory when it is no longer needed. >> >> The problem is that there seems to be no way of knowing for my >> driver at what time the memory is no longer mapped in a process >> address space so that i can safely free this memory. > > why not at close time? I would imagine that the device won't be closed > until all the mmap's that are backed by the device are unmapped.. it > shouldn't be hard to test... the mapping should hold a reference to > the device until it's munmapped.. > The problem is that it is perfectly legal to access the mapped memory after a close, consider following code: fd = open() mem = mmap() close(fd) process_data(mem) Unfortunately the mapping doesn't seem to hold a reference to the related fileobject, so the close in above code actually ends up in the close function of my driver but the mapping is still there.
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