linux-kernel - Re: [PATCH 4/5] mm: Do early cow for pinned pages during fork() for ptes

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Date:   Tue, 22 Sep 2020 13:48:40 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     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>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Hugh Dickins <hughd@...gle.com>,
        Leon Romanovsky <leonro@...dia.com>, Jan Kara <jack@...e.cz>,
        John Hubbard <jhubbard@...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 4/5] mm: Do early cow for pinned pages during fork() for
 ptes

On 09/21, Peter Xu wrote:
>
> @@ -859,6 +989,25 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>  			    spin_needbreak(src_ptl) || spin_needbreak(dst_ptl))
>  				break;
>  		}
> +
> +		if (unlikely(data.cow_new_page)) {
> +			/*
> +			 * If cow_new_page set, we must be at the 2nd round of
> +			 * a previous COPY_MM_BREAK_COW.  Try to arm the new
> +			 * page now.  Note that in all cases page_break_cow()
> +			 * will properly release the objects in copy_mm_data.
> +			 */
> +			WARN_ON_ONCE(copy_ret != COPY_MM_BREAK_COW);
> +			if (pte_install_copied_page(dst_mm, new, src_pte,
> +						    dst_pte, addr, rss,
> +						    &data)) {
> +				/* We installed the pte successfully; move on */
> +				progress++;
> +				continue;

I'm afraid I misread this patch too ;)

But it seems to me in this case the main loop can really "leak"
COPY_MM_BREAK_COW. Suppose the the next 31 pte's are pte_none() and
need_resched() is true.

No?

Oleg.