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Date:   Tue, 3 Nov 2020 00:58:31 +0100
From:   "Ahmed S. Darwish" <a.darwish@linutronix.de>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     linux-kernel@vger.kernel.org, Peter Xu <peterx@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Christoph Hellwig <hch@lst.de>,
        Hugh Dickins <hughd@google.com>, Jan Kara <jack@suse.cz>,
        Jann Horn <jannh@google.com>,
        John Hubbard <jhubbard@nvidia.com>,
        Kirill Shutemov <kirill@shutemov.name>,
        Kirill Tkhai <ktkhai@virtuozzo.com>,
        Leon Romanovsky <leonro@nvidia.com>,
        Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@suse.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Sebastian Siewior <bigeasy@linutronix.de>
Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during
 fork
Message-ID: <20201102235831.GA52235@lx-t490>
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In-Reply-To: <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
Precedence: bulk

On Fri, Oct 30, 2020 at 11:46:21AM -0300, Jason Gunthorpe wrote:

...

> diff --git a/mm/memory.c b/mm/memory.c
> index c48f8df6e50268..294c2c3c4fe00d 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1171,6 +1171,12 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>  		mmu_notifier_range_init(&range, MMU_NOTIFY_PROTECTION_PAGE,
>  					0, src_vma, src_mm, addr, end);
>  		mmu_notifier_invalidate_range_start(&range);

> +		/*
> +		 * The read side doesn't spin, it goes to the mmap_lock, so the
> +		 * raw version is used to avoid disabling preemption here
> +		 */
> +		mmap_assert_write_locked(src_mm);
> +		raw_write_seqcount_t_begin(&src_mm->write_protect_seq);
>  	}
>

Please, s/raw_write_seqcount_t_begin()/raw_write_seqcount_begin()/g. For
plain seqcount_t, it's the same, while still respecting the seqlock.h
API boundaries.

Let's make the comment also a bit more clear (IMHO, "lockdep" needs to
be mentioned somewhere):

		/*
		 * Disabling preemption is not needed for the write side, as
		 * the read side doesn't spin, but goes to the mmap_lock.
		 *
		 * Use the raw variant of the seqcount_t write API to avoid
		 * lockdep complaining about preemptibility.
		 */
		mmap_assert_write_locked(src_mm);
		raw_write_seqcount_t_begin(&src_mm->write_protect_seq);

>  	ret = 0;
> @@ -1187,8 +1193,10 @@ copy_page_range(struct vm_area_struct *dst_vma, struct vm_area_struct *src_vma)
>  		}
>  	} while (dst_pgd++, src_pgd++, addr = next, addr != end);
>
> -	if (is_cow)
> +	if (is_cow) {
> +		raw_write_seqcount_t_end(&src_mm->write_protect_seq);

ditto.

s/raw_write_seqcount_t_end()/raw_write_seqcount_end()/g

>  		mmu_notifier_invalidate_range_end(&range);
> +	}
>  	return ret;
>  }
>

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH