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

From: Matthew Wilcox
Date: Fri May 26 2023 - 12:44:54 EST

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