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

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Date:   Tue, 22 Sep 2020 12:33:15 +0200
From:   Jan Kara <jack@...e.cz>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     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>, Jan Kara <jack@...e.cz>,
        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 Mon 21-09-20 23:41:16, John Hubbard wrote:
> On 9/21/20 2:20 PM, Peter Xu wrote:
> ...
> > diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> > index 7ff29cc3d55c..c40aac0ad87e 100644
> > --- a/mm/huge_memory.c
> > +++ b/mm/huge_memory.c
> > @@ -1074,6 +1074,23 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> >   	src_page = pmd_page(pmd);
> >   	VM_BUG_ON_PAGE(!PageHead(src_page), src_page);
> > +
> > +	/*
> > +	 * If this page is a potentially pinned page, split and retry the fault
> > +	 * with smaller page size.  Normally this should not happen because the
> > +	 * userspace should use MADV_DONTFORK upon pinned regions.  This is a
> > +	 * best effort that the pinned pages won't be replaced by another
> > +	 * random page during the coming copy-on-write.
> > +	 */
> > +	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

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