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

[Jason Gunthorpe]

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 :|

Jason