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

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Date:   Mon, 21 Sep 2020 23:41:16 -0700
From:   John Hubbard <jhubbard@...dia.com>
To:     Peter Xu <peterx@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>
CC:     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 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.

> +		pte_free(dst_mm, pgtable);
> +		spin_unlock(src_ptl);
> +		spin_unlock(dst_ptl);
> +		__split_huge_pmd(vma, src_pmd, addr, false, NULL);
> +		return -EAGAIN;
> +	}


Why wait until we are so deep into this routine to detect this and unwind?
It seems like if you could do a check near the beginning of this routine, and
handle it there, with less unwinding? In fact, after taking only the src_ptl,
the check could be made, right?


thanks,
-- 
John Hubbard
NVIDIA