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

Alexander Viro
1999-10-11T21:40:52Z

On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:

> Hi,
>
> On Mon, 11 Oct 1999 12:05:23 -0400 (EDT), Alexander Viro
> <viro@math.psu.edu> said:
>
> > On Mon, 11 Oct 1999, Stephen C. Tweedie wrote:
> >> 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)...
>
> Right now it is the big kernel lock which is used for this, and the
> scheduler drops it anyway for us. If anyone wants to replace that lock
> with another spinlock, then yes, the swapout method would have to drop
> it before doing anything which could block. And that is ugly: having
> spinlocks unbalanced over function calls is a maintenance nightmare.

Agreed, but the big lock does not (and IMHO should not) cover the vma list
modifications.