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SubjectRe: [PATCH 0/4] get_user_pages*() and RDMA: first steps
On Sat 29-09-18 04:46:09, Jerome Glisse wrote:
> On Fri, Sep 28, 2018 at 07:28:16PM -0700, John Hubbard wrote:
> > Actually, the latest direction on that discussion was toward periodically
> > writing back, even while under RDMA, via bounce buffers:
> >
> >
> >
> > I still think that's viable. Of course, there are other things besides
> > writeback (see below) that might also lead to waiting.
> Write back under bounce buffer is fine, when looking back at links you
> provided the solution that was discuss was blocking in page_mkclean()
> which is horrible in my point of view.

Yeah, after looking into it for some time, we figured that waiting for page
pins in page_mkclean() isn't really going to fly due to deadlocks. So we
came up with the bounce buffers idea which should solve that nicely.

> > > With the solution put forward here you can potentialy wait _forever_ for
> > > the driver that holds a pin to drop it. This was the point i was trying to
> > > get accross during LSF/MM.
> >
> > I agree that just blocking indefinitely is generally unacceptable for kernel
> > code, but we can probably avoid it for many cases (bounce buffers), and
> > if we think it is really appropriate (file system unmounting, maybe?) then
> > maybe tolerate it in some rare cases.
> >
> > >You can not fix broken hardware that decided to
> > > use GUP to do a feature they can't reliably do because their hardware is
> > > not capable to behave.
> > >
> > > Because code is easier here is what i was meaning:
> > >
> > >
> > >
> > >
> >
> > While that may work sometimes, I don't think it is reliable enough to trust for
> > identifying pages that have been gup-pinned. There's just too much overloading of
> > other mechanisms going on there, and if we pile on top with this constraint of "if you
> > have +3 refcounts, and this particular combination of page counts and mapcounts, then
> > you're definitely a long-term pinned page", I think users will find a lot of corner
> > cases for us that break that assumption.
> So the mapcount == refcount (modulo extra reference for mapping and
> private) should holds, here are the case when it does not:
> - page being migrated
> - page being isolated from LRU
> - mempolicy changes against the page
> - page cache lookup
> - some file system activities
> - i likely miss couples here i am doing that from memory
> What matter is that all of the above are transitory, the extra reference
> only last for as long as it takes for the action to finish (migration,
> mempolicy change, ...).
> So skipping those false positive page while reclaiming likely make sense,
> the blocking free buffer maybe not.

Well, as John wrote, these page refcount are fragile (and actually
filesystem dependent as some filesystems hold page reference from their
page->private data and some don't). So I think we really need a new
reliable mechanism for tracking page references from GUP. And John works
towards that.

Jan Kara <>

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