linux-kernel - Re: [PATCH v3 2/2] mm: prevent gup_fast from racing with COW during fork

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Date:   Fri, 6 Nov 2020 18:17:16 +0100
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     linux-kernel@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>,
        Christoph Hellwig <hch@....de>,
        Hugh Dickins <hughd@...gle.com>, Jan Kara <jack@...e.cz>,
        Jann Horn <jannh@...gle.com>,
        John Hubbard <jhubbard@...dia.com>,
        Kirill Shutemov <kirill@...temov.name>,
        Kirill Tkhai <ktkhai@...tuozzo.com>,
        Leon Romanovsky <leonro@...dia.com>,
        Linux-MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...e.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v3 2/2] mm: prevent gup_fast from racing with COW during
 fork

Hi Jason,

On Fri, Nov 06, 2020 at 11:55:14AM -0400, Jason Gunthorpe wrote:
...
> +	if (gup_flags & FOLL_PIN) {
> +		seq = raw_read_seqcount(&current->mm->write_protect_seq);
> +		if (seq & 1)
> +			return 0;
> +	}
> +
...
> +	if (gup_flags & FOLL_PIN) {
> +		if (read_seqcount_t_retry(&current->mm->write_protect_seq,
> +					  seq)) {
> +			unpin_user_pages(pages, nr_pinned);
> +			return 0;
> +		}
> +	}

>From seqlock.h:

    /**
     * raw_read_seqcount() - read the raw seqcount_t counter value
     * ...
     * Return: count to be passed to read_seqcount_retry()
     */
    #define raw_read_seqcount(s)

Please avoid using the internal API (read_seqcount_*t*_retry) and just
use read_seqcount_retry() as the documentation suggests.

(I guess you just missed changing that last one... I'm in process of
 changing all these "*_seqcount_t_*" stuff to "do_*" as we talked on the
 v2 thread. Hopefully there will be no more confusion after that.)

Kind regards,

--
Ahmed S. Darwish
Linutronix GmbH