Re: locking question: do_mmap(), do_munmap()

[Manfred Spraul <manfreds@colorfullife.com>]

Alexander Viro wrote:
> 
> On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:
> 
> > Hi,
> >
> > On Sun, 10 Oct 1999 15:03:45 -0400 (EDT), Alexander Viro
> > <viro@math.psu.edu> said:
> >
> > > Hold on. In swap_out_mm() you have to protect find_vma() (OK, it doesn't
> > > block, but we'll have to take care of mm->mmap_cache) _and_ you'll have to
> > > protect vma from destruction all way down to try_to_swap_out(). And to
> > > vma->swapout(). Which can sleep, so spinlocks are out of question
> > > here.
> >
> > No, spinlocks would be ideal.  The vma swapout codes _have_ to be
> > prepared for the vma to be destroyed as soon as we sleep.  In fact, the
> > entire mm may disappear if the process happens to exit.  Once we know
> > which page to write where, the swapout operation becomes a per-page
> > operation, not per-vma.
> 
> Aha, so you propose to drop it in ->swapout(), right? (after get_file() in
> filemap_write_page()... Ouch. Probably we'ld better lambda-expand the call
> in filemap_swapout() - the thing is called from other places too)...

What about something like a rw-semaphore which protects the vma list:
vma-list modifiers [ie merge_segments(), insert_vm_struct() and
do_munmap()] grab it exclusive, swapper grabs it "shared, starve
exclusive".
All other vma-list readers are protected by mm->mmap_sem.

This should not dead-lock, and no changes are required in
vm_ops->swapout().

--
	Manfred

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