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

[John Hubbard]

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.

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.

thanks,
--
John Hubbard
NVIDIA