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

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Message-ID: <e03dba4e-fd1d-a32c-c99c-fc3fa51419c4@nvidia.com>
Date:   Mon, 2 Nov 2020 18:20:45 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        Jason Gunthorpe <jgg@...dia.com>
CC:     Peter Xu <peterx@...hat.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>,
        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 11/2/20 4:41 PM, Ahmed S. Darwish wrote:
> On Mon, Nov 02, 2020 at 08:25:32PM -0400, Jason Gunthorpe wrote:
>> On Tue, Nov 03, 2020 at 01:17:12AM +0100, Ahmed S. Darwish wrote:
>>
>>> Please stick with the official exported API: raw_write_seqcount_begin().
>>
>> How did you know this was 'offical exported API' ??
>>
> 
> All the official exported seqlock.h APIs are marked with verbose
> kernel-doc annotations on top. The rest are internal...
> 

OK, but no one here was able to deduce that, probably because there is not
enough consistency throughout the kernel to be able to assume such things--even
though your seqlock project is internally consistent. It's just not *quite*
enough communication.

I think if we added the following it would be very nice:

a) Short comments to the "unofficial and internal" routines, identifying them as
such, and

b) Short comments to the "official API for general use", also identifying
those as such.

c) A comment about what makes "raw" actually raw, for seqlock.


Since I'm proposing new work, I'll also offer to help, perhaps by putting together
a small patch to get it kicked off, if you approve of the idea.

thanks,
-- 
John Hubbard
NVIDIA