Re: [PATCH V2 1/2] mm: hotplug: implement non-movable version of get_user_pages() called get_user_pages_non_movable()

[chen tang <imtangchen@gmail.com>]

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Hi Benjamin,

Thank you for the explaination. But would you please give me more info
about aio ?
See below.

2013/5/14 Benjamin LaHaise <bcrl@kvack.org>

> On Tue, May 14, 2013 at 09:24:58AM +0800, Tang Chen wrote:
> > Hi Mel, Benjamin, Jeff,
> >
> > On 05/13/2013 11:01 PM, Benjamin LaHaise wrote:
> > >On Mon, May 13, 2013 at 10:54:03AM -0400, Jeff Moyer wrote:
> > >>How do you propose to move the ring pages?
> > >
> > >It's the same problem as doing a TLB shootdown: flush the old pages from
> > >userspace's mapping, copy any existing data to the new pages, then
> > >repopulate the page tables.  It will likely require the addition of
> > >address_space_operations for the mapping, but that's not too hard to do.
> > >
> >
> > I think we add migrate_unpin() callback to decrease page->count if
> > necessary,
> > and migrate the page to a new page, and add migrate_pin() callback to pin
> > the new page again.
>
> You can't just decrease the page count for this to work.  The pages are
> pinned because aio_complete() can occur at any time and needs to have a
> place to write the completion events.  When changing pages, aio has to
> take the appropriate lock when changing one page for another.
>

I saw in aio_complete(), it holds kioctx->ctx_lock. Can we hold this lock
when
we migrate aio ring pages ?


>
> > The migrate procedure will work just as before. We use callbacks to
> > decrease
> > the page->count before migration starts, and increase it when the
> migration
> > is done.
> >
> > And migrate_pin() and migrate_unpin() callbacks will be added to
> > struct address_space_operations.
>
> I think the existing migratepage operation in address_space_operations can
> be used.  Does it get called when hot unplug occurs?  That is: is testing
> with the migrate_pages syscall similar enough to the memory removal case?
>

For anonymous pages, they don't have address_space, so they don't have
address_space_operations. And aio ring pages are anonymous pages, right ?

In move_to_new_page(), kernel will decide which function to call.

if (!mapping)
rc = migrate_page(mapping, newpage, page, mode);
else if (mapping->a_ops->migratepage)
rc = mapping->a_ops->migratepage(mapping,
newpage, page, mode);
else
rc = fallback_migrate_page(mapping, newpage, page, mode);

And for aio ring pages, it always call migrate_page(), right ?

Thanks. :)


>
>                 -ben
>
> > Is that right ?
> >
> > If so, I'll be working on it.
> >
> > Thanks. :)
>
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