Re: [PATCH v4] mm, compaction: Skip all non-migratable pages during scan

[David Hildenbrand]

On 26.05.23 18:44, Matthew Wilcox wrote:
> On Fri, May 26, 2023 at 09:44:34AM -0600, Khalid Aziz wrote:
> > > Oh, I think I found it!  pin_user_pages_remote() is called by
> > > vaddr_get_pfns().  If these are the pages you're concerned about,
> > > then the efficient way to do what you want is simply to call
> > > folio_maybe_dma_pinned().  Far more efficient than the current mess
> > > of total_mapcount().
> >
> > vfio pinned pages triggered this change. Wouldn't checking refcounts against
> > mapcount provide a more generalized way of detecting non-migratable pages?
>
> Well, you changed the comment to say that we were concerned about
> long-term pins.  If we are, than folio_maybe_dma_pinned() is how to test
> for long-term pins.  If we want to skip pages which are short-term pinned,
> then we need to not change the comment, and keep using mapcount/refcount
> differences.

folio_maybe_dma_pinned() is all about FOLL_PIN, not FOLL_LONGTERM.

folio_maybe_dma_pinned() would skip migrating any page that has more 
than 1024 references. (shared libraries?)

--
Thanks,

David / dhildenb