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Re: tmpfs loses unlinked files

From: Richard Braun
Subject: Re: tmpfs loses unlinked files
Date: Sat, 28 Feb 2015 13:05:07 +0100
User-agent: Mutt/1.5.21 (2010-09-15)

On Sat, Feb 28, 2015 at 12:22:47PM +0100, Samuel Thibault wrote:
> For instance the X server crashes on fvwm startup, or mupdf crashes,
> when /run is served by a tmpfs.
> What happens is that they create an shm (shmget, thus a file in
> /run/shm/, backed by tmpfs), then map it (shmat, i.e. open/mmap/close of
> the file), then remove access to it (shmctl(IPC_RMID), i.e. unlink the
> file).
> The issue is that even if the file is still mapped, unlinking it from
> tmpfs makes tmpfs mach_port_deallocate the memobj, which apparently
> drops the object completely, and subsequent accesses to the mmap thus
> fail with SIGBUS.  Would there be a way for tmpfs to know about the
> mappings of the pager object, to avoid shutting it down?  Or a way to
> keep the object alive even if tmpfs has deallocated its port to it?
> I don't know memory management enough to answer it.

I'd say that glibc (in mmap) should add a reference to the file. On
munmap or process termination, all references are dropped.

Richard Braun

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