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

[Pavel Tatashin]

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.

>
> Jason