linux-kernel - Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork

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Date:   Tue, 3 Nov 2020 01:33:46 +0100
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Peter Xu <peterx@...hat.com>
Cc:     Jason Gunthorpe <jgg@...dia.com>, linux-kernel@...r.kernel.org,
        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>,
        Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during
 fork

On Sat, Oct 31, 2020 at 11:26:05AM -0400, Peter Xu wrote:

...

> Shall we document this explicitly (if this patch still needs a repost)?

Yes, this patch series needs a v3 :)

> Seems not straightforward since that seems not the usual way to use seqcount,
> not sure whether I'm the only one that feels this way, though.

Yes, this usage is correct but not common. I've proposed a more explicit
comment above the write section code, in my reply to patch #2.

...

> The other thing is, considering this use of seqcount seems to be quite special
> as explained below, I'm just not sure whether this would confuse lockdep or
> kcsan, etc., if we decide to use write_seqcount_t_begin().
>

Lockdep won't be confused as it's not used in the raw_*() variant of the
seqcount APIs.

AFAIK KCSAN also has some margin to protect itself from this: see
seqlock.h KCSAN_SEQLOCK_REGION_MAX.

Thanks,

> Peter Xu

Ahmed Darwish
Linutronix GmbH