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

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Message-ID: <20200923092205.GA6719@quack2.suse.cz>
Date:   Wed, 23 Sep 2020 11:22:05 +0200
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     Jan Kara <jack@...e.cz>, Peter Xu <peterx@...hat.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Jann Horn <jannh@...gle.com>, Oleg Nesterov <oleg@...hat.com>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Christoph Hellwig <hch@....de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Andrea Arcangeli <aarcange@...hat.com>
Subject: Re: [PATCH 5/5] mm/thp: Split huge pmds/puds if they're pinned when
 fork()

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@...e.com>
SUSE Labs, CR