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From: Jason Gunthorpe <jgg@nvidia.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.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>,
	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 Xu <peterx@redhat.com>
Subject: Re: [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork
Date: Tue, 27 Oct 2020 08:32:28 -0300	[thread overview]
Message-ID: <20201027113228.GF1523783@nvidia.com> (raw)
In-Reply-To: <CAHk-=whEL2B-PSGzhQxyC3VR1XpUee6nKw6YiJG6w7hRL1f9Lg@mail.gmail.com>

On Mon, Oct 26, 2020 at 05:14:35PM -0700, Linus Torvalds wrote:
> On Mon, Oct 26, 2020 at 4:50 PM Jason Gunthorpe <jgg@nvidia.com> wrote:
> >
> > Right, it isn't a "seqcount" because the read side doesn't spin.
> 
> Not all readers spin for seqcount either.
> 
> Sure, it's the *common* case, but look at name resolution - it will
> basically fail on a seqcount failure in RCU mode, and go to the "slow
> and safe" version:
> 
>                 if (unlikely(read_seqcount_retry(&d->d_seq, nd->seq)))
>                         return -ECHILD;
> 
> Exactly like gup_fast vs fork, in fact.
> 
> So no, you don't need to retry for seqcount failures, doing
> 
>      seq = seqcount_begin(..);
>      ... optimistic fast case ...
>      if (!read_seqcount_retry(.., seq))
>           return 0; // success
>        .. do slow case ...
> 
> is valid and happens.

Okay, I see d_seq using one of the raw_write versions, so I'll make
this look like that

Thanks,
Jason


  reply	other threads:[~2020-10-27 11:32 UTC|newest]

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-24  0:19 [PATCH 0/2] Add a 'seqcount' between gup_fast and copy_page_range Jason Gunthorpe
2020-10-24  0:19 ` [PATCH 1/2] mm: reorganize internal_get_user_pages_fast() Jason Gunthorpe
2020-10-24  4:44   ` John Hubbard
2020-10-26 23:59     ` Jason Gunthorpe
2020-10-27  9:33     ` Jan Kara
2020-10-27  9:55       ` Christoph Hellwig
2020-10-28  6:00         ` John Hubbard
2020-10-27 13:15       ` Jason Gunthorpe
2020-10-28  6:00         ` John Hubbard
2020-10-28  6:05           ` John Hubbard
2020-10-24  0:19 ` [PATCH 2/2] mm: prevent gup_fast from racing with COW during fork Jason Gunthorpe
2020-10-24  5:19   ` John Hubbard
2020-10-24  5:31     ` John Hubbard
2020-10-26 23:49       ` Jason Gunthorpe
2020-10-27  0:14         ` Linus Torvalds
2020-10-27 11:32           ` Jason Gunthorpe [this message]
2020-10-27  0:35         ` John Hubbard
2020-10-27  7:32   ` John Hubbard
2020-11-02  3:25   ` [mm] e498078ae9: will-it-scale.per_thread_ops -1.4% regression kernel test robot
2020-10-24  5:14 ` [PATCH 0/2] Add a 'seqcount' between gup_fast and copy_page_range John Hubbard

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