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

[Matthew Wilcox <willy@infradead.org>]

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().