Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when fork()

Jan Kara
2020-09-23T09:22:10Z

On Tue 22-09-20 13:01:13, John Hubbard wrote:
> On 9/22/20 3:33 AM, Jan Kara wrote:
> > On Mon 21-09-20 23:41:16, John Hubbard wrote:
> > > On 9/21/20 2:20 PM, Peter Xu wrote:
> > > ...
> > > > + if (unlikely(READ_ONCE(src_mm->has_pinned) &&
> > > > + page_maybe_dma_pinned(src_page))) {
> > >
> > > This condition would make a good static inline function. It's used in 3
> > > places, and the condition is quite special and worth documenting, and
> > > having a separate function helps with that, because the function name
> > > adds to the story. I'd suggest approximately:
> > >
> > > page_likely_dma_pinned()
> > >
> > > for the name.
> >
> > Well, but we should also capture that this really only works for anonymous
> > pages. For file pages mm->has_pinned does not work because the page may be
> > still pinned by completely unrelated process as Jann already properly
> > pointed out earlier in the thread. So maybe anon_page_likely_pinned()?
> > Possibly also assert PageAnon(page) in it if we want to be paranoid...
> >
> > Honza
>
> The file-backed case doesn't really change anything, though:
> page_maybe_dma_pinned() is already a "fuzzy yes" in the same sense: you
> can get a false positive. Just like here, with an mm->has_pinned that
> could be a false positive for a process.
>
> And for that reason, I'm also not sure an "assert PageAnon(page)" is
> desirable. That assertion would prevent file-backed callers from being
> able to call a function that provides a fuzzy answer, but I don't see
> why you'd want or need to do that. The goal here is to make the fuzzy
> answer a little bit more definite, but it's not "broken" just because
> the result is still fuzzy, right?
>
> Apologies if I'm missing a huge point here... :)

But the problem is that if you apply mm->has_pinned check on file pages,
you can get false negatives now. And that's not acceptable...

Honza

--
Jan Kara <jack@suse.com>
SUSE Labs, CR