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

[Khalid Aziz]

On 5/25/23 15:31, Matthew Wilcox wrote:
> On Thu, May 25, 2023 at 09:45:34PM +0100, Matthew Wilcox wrote:
> > > > Isn't "long term pinning" the wrong description of the problem?  Long term
> > > > pins suggest to me FOLL_LONGTERM.  I think this is simple short term
> > > > pins that we care about here.
> > >
> > > vfio pins are held for a long time - Steve
> >
> > So this is a third sense of "pinned pages" that is neither what
> > filesystems nor the mm means by pinned pages, but whatever it is that
> > vfio means by pinned pages?  If only "pin" weren't such a desirable
> > word.  Can somebody explain to me in small words what a vfio pin looks
> > like because I've tried reading vfio_iommu_type1_pin_pages() and I
> > don't recognise anything there that looks like pinning in either of
> > the other two senses.
>
> 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?

Thanks,
Khalid