VOP_WRITE & read-only file system

[Kostik Belousov]

On Thu, May 28, 2009 at 08:29:43PM +0300, Jaakko Heinonen wrote:
> 
> Hi,
> 
> On 2009-05-27, Kostik Belousov wrote:
> > > 1) mmap(2)
> > >   - mmap(2) a file
> > >   - close(2) the file handle
> > >   - remount file-system as read-only
> > >   - modify mapped memory
> 
> > Yes, the issue is there. Real cause for the problem is that mmap() does
> > not increment v_writecnt of the vnode that backs shared writable mappings.
> > 
> > I have a patch that fixes this, see
> > http://people.freebsd.org/~kib/misc/vm_map_delete.3.patch
> 
> The patch seems to have a problem with forced remount or unmount. After
> a forced remount to read-only munmap(2) crashes in _vm_map_unlock()
> due to NULL vp.
> 
> #12 0xc08f0cb3 in _vn_lock (vp=0x0, flags=525312, 
>     file=0xc0c55e8f "/home/jaakko/src/head/sys/vm/vm_map.c", line=494)
>     at vnode_if.h:830
> #13 0xc0a8cb4f in _vm_map_unlock (map=0xc634f000, process_freelist=1, 
>     file=0xc0c56902 "/home/jaakko/src/head/sys/vm/vm_mmap.c", line=597)
>     at /home/jaakko/src/head/sys/vm/vm_map.c:494
> #14 0xc0a91d98 in munmap (td=0xc5a996c0, uap=0xf480ecf8)
>     at /home/jaakko/src/head/sys/vm/vm_mmap.c:597
> #15 0xc0b637b3 in syscall (frame=0xf480ed38)
>     at /home/jaakko/src/head/sys/i386/i386/trap.c:1073
> #16 0xc0b46a10 in Xint0x80_syscall ()
>     at /home/jaakko/src/head/sys/i386/i386/exception.s:261

I expect that
http://people.freebsd.org/~kib/misc/vm_map_delete.4.patch
fix the issue.
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