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

[Matthew Wilcox]

On Sun, May 28, 2023 at 04:49:52PM -0700, John Hubbard wrote:
> On 5/26/23 20:18, Matthew Wilcox wrote:
> > On Fri, May 26, 2023 at 07:11:05PM -0700, John Hubbard wrote:
> > > > So any user with 1024 processes can fragment physical memory? :/
> > > > 
> > > > Sorry, I'd like to minimize the usage of folio_maybe_dma_pinned().
> > > 
> > > I was actually thinking that we should minimize any more cases of
> > > fragile mapcount and refcount comparison, which then leads to
> > > Matthew's approach here!
> > 
> > I was wondering if we shouldn't make folio_maybe_dma_pinned() a little
> > more accurate.  eg:
> > 
> >          if (folio_test_large(folio))
> >                  return atomic_read(&folio->_pincount) > 0;
> > 	return (unsigned)(folio_ref_count(folio) - folio_mapcount(folio)) >=
> > 			GUP_PIN_COUNTING_BIAS;
> 
> I'm trying to figure out what might be wrong with that, but it seems
> OK. We must have talked about this earlier, but I recall vaguely that
> there was not a lot of concern about the case of a page being mapped
> > 1024 times. Because pinned or not, it's likely to be effectively
> locked into memory due to LRU effects. As mentioned here, too.

That was my point of view, but David convinced me that a hostile process
can effectively lock its own memory into place.

> Anyway, sure.
> 
> A detail:
> 
> The unsigned cast, I'm not sure that helps or solves anything, right?
> That is, other than bugs, is it possible to get refcount < mapcount?
> 
> And if it's only due to bugs, then the casting, again, isn't likely to
> going to mitigate the fallout from whatever mess the bug caused.

I wasn't thinking too hard about the cast.  If the caller has the folio
lock, I don't think it's possible for refcount < mapcount.  This caller
has a refcount, but doesn't hold the lock, so it is possible for them
to read mapcount first, then have both mapcount and refcount decremented
and see refcount < mapcount.

I don't think it matters too much.  We don't hold the folio lock, so
it might transition from pinned to unpinned as much as a refcount might
be decremented or a mapcount incremented.  What's important is that a
hostile process can't prevent memory from being moved indefinitely.

David, have I missed something else?