Re: [PATCH 2/5] Swapless V2: Add migration swap entries

[Lee Schermerhorn]

On Thu, 2006-04-13 at 22:25 -0700, Andrew Morton wrote:
> Christoph Lameter <clameter@xxxxxxx> wrote:
> >
> > On Thu, 13 Apr 2006, Andrew Morton wrote:
> > 
> > > Andrew Morton <akpm@xxxxxxxx> wrote:
> > > >
> > > > Perhaps it would be better to go to
> > > >  sleep on some global queue, poke that queue each time a page migration
> > > >  completes?
> > > 
> > > Or take mmap_sem for writing in do_migrate_pages()?  That takes the whole
> > > pagefault path out of the picture.
> > 
> > We would have to take that for each task mapping the page. Very expensive 
> > operation.
> 
> So...  why does do_migrate_pages() take mmap_sem at all?
> 
> And the code we're talking about here deals with anonymous pages, which are
> not shared betweem mm's.

I think that anon pages are shared, copy-on-write, between parent and
child after a fork().  If no exec() and no task writes the page, the
sharing can become quite extensive.  I encountered this testing the
migrate-on-fault patches.  With MPOL_MF_MOVE, these shared anon pages
don't get migrated at all [sometimes this is what you want, sometimes
not...], but with '_MOVE_ALL the shared anon pages DO get migrated, so
you can have races between a faulting task and the migrating task.

Lee

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