Re: [PATCH 6/6] mm/gup: migrate pinned pages out of movable zone

[Jason Gunthorpe]

On Fri, Dec 04, 2020 at 11:24:56AM -0500, Pavel Tatashin wrote:
> On Thu, Dec 3, 2020 at 2:36 PM Jason Gunthorpe <jgg@xxxxxxxx> wrote:
> >
> > On Thu, Dec 03, 2020 at 02:15:36PM -0500, Pavel Tatashin wrote:
> >
> > > I studied some more, and I think this is not a race:
> > > list_add_tail(&head->lru, &cma_page_list) is called only when
> > > isolate_lru_page(head) succeeds.
> > > isolate_lru_page(head) succeeds only when PageLRU(head) is true.
> > > However, in this function we also clear LRU flag before returning
> > > success.
> > > This means, that if we race with another thread, the other thread
> > > won't get to unprotected list_add_tail(&head->lru, &cma_page_list)
> > > until head is is back on LRU list.
> >
> > Oh interesting, I totally didn't see how that LRU stuff is
> > working. So.. this creates a ridiculously expensive spin lock? Not
> > broken, but yikes :|
> 
> Not really a spin lock, the second thread won't be able to isolate
> this page, and will skip migration of this page.

It looks like the intent is that it will call gup again, then goto
check_again, and once again try to isolate the LRU. ie it loops.

If it gets to a point where all the CMA pages fail to isolate then it
simply exits with success as the cma_page_list will be empty.

Is this a bug? It seems like a bug, the invariant here is to not
return with a CMA page, so why do we have a path that does return with
a CMA page?

Jason