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

[Matthew Wilcox]

On Fri, May 26, 2023 at 06:46:15PM +0200, David Hildenbrand wrote:
> 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.

But according to our documentation, FOLL_LONGTERM implies FOLL_PIN.
Anyway, right now, the code skips any pages which are merely FOLL_GET,
so we'll skip fewer pages if we do only skip the FOLL_PIN ones,
regardless if we'd prefer to only skip the FOLL_LONGTERM ones.

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

True, but maybe we should be skipping any page with that many mappings,
given how disruptive it is to the rest of the system to unmap a page
from >1024 processes.