LKML Archive on lore.kernel.org
 help / color / Atom feed
From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Peter Xu <peterx@redhat.com>,
	linux-kernel@vger.kernel.org,
	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
Date: Tue, 3 Nov 2020 01:17:12 +0100
Message-ID: <20201103001712.GB52235@lx-t490> (raw)
In-Reply-To: <20201030235121.GQ2620339@nvidia.com>

On Fri, Oct 30, 2020 at 08:51:21PM -0300, Jason Gunthorpe wrote:
> On Fri, Oct 30, 2020 at 06:52:50PM -0400, Peter Xu wrote:

...

>
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index c48f8df6e50268..294c2c3c4fe00d 100644
> > > +++ 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);
> >
> > Would raw_write_seqcount_begin() be better here?
>
> Hum..
>
> I felt no because it had the preempt stuff added into it, however it
> would work - __seqcount_lock_preemptible() == false for the seqcount_t
> case (see below)
>
> Looking more closely, maybe the right API to pick is
> write_seqcount_t_begin() and write_seqcount_t_end() ??
>

No, that's not the right API: it is also internal to seqlock.h.

Please stick with the official exported API: raw_write_seqcount_begin().

It should satisfy your needs, and the raw_*() variant is created exactly
for contexts wishing to avoid the lockdep checks (e.g. NMI handlers
cannot invoke lockdep, etc.)

> However, no idea what the intention of the '*_seqcount_t_*' family is
> - it only seems to be used to implement the seqlock..
>

Exactly. '*_seqcount_t_*' is a seqlock.h implementation detail, and it
has _zero_ relevance to what is discussed in this thread actually.

...

> Ahmed explained in commit 8117ab508f the reason the seqcount_t write
> side has preemption disabled is because it can livelock RT kernels if
> the read side is spinning after preempting the write side. eg look at
> how __read_seqcount_begin() is implemented:
>
> 	while ((seq = __seqcount_sequence(s)) & 1)			\
> 		cpu_relax();						\
>
> However, in this patch, we don't spin on the read side.
>
> If the read side collides with a writer it immediately goes to the
> mmap_lock, which is sleeping, and so it will sort itself out properly,
> even if it was preempted.
>

Correct.

Thanks,

--
Ahmed Darwish
Linutronix GmbH

  parent reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <0-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 14:46 ` [PATCH v2 1/2] mm: reorganize internal_get_user_pages_fast() Jason Gunthorpe
2020-10-30 16:29   ` Jan Kara
2020-10-30 21:31   ` John Hubbard
2020-10-30 22:36   ` Peter Xu
2020-11-02 22:19 ` [PATCH v2 0/2] Add a seqcount between gup_fast and copy_page_range() Ahmed S. Darwish
2020-11-02 22:39   ` Linus Torvalds
2020-11-02 23:18     ` Ahmed S. Darwish
     [not found] ` <2-v2-dfe9ecdb6c74+2066-gup_fork_jgg@nvidia.com>
2020-10-30 16:51   ` [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork Jan Kara
     [not found]     ` <20201030170226.GF2620339@nvidia.com>
2020-11-02  8:31       ` Jan Kara
2020-10-30 22:52   ` Peter Xu
     [not found]     ` <20201030235121.GQ2620339@nvidia.com>
2020-10-31 15:26       ` Peter Xu
2020-11-03  0:33         ` Ahmed S. Darwish
2020-11-03  0:17       ` Ahmed S. Darwish [this message]
     [not found]         ` <20201103002532.GL2620339@nvidia.com>
2020-11-03  0:41           ` Ahmed S. Darwish
2020-11-03  2:20             ` John Hubbard
2020-11-03  6:52               ` Ahmed S. Darwish
2020-11-03 17:40                 ` Linus Torvalds
2020-11-04  1:32                   ` Ahmed S. Darwish
2020-11-04  2:01                     ` John Hubbard
2020-11-04  3:17                       ` Ahmed S. Darwish
2020-11-04 18:38                         ` Linus Torvalds
2020-11-04 19:54                           ` Ahmed S. Darwish
2020-11-10 11:53                         ` Peter Zijlstra
2020-11-03 17:03         ` Peter Xu
2020-11-02 23:58   ` Ahmed S. Darwish

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20201103001712.GB52235@lx-t490 \
    --to=a.darwish@linutronix.de \
    --cc=aarcange@redhat.com \
    --cc=akpm@linux-foundation.org \
    --cc=aneesh.kumar@linux.ibm.com \
    --cc=bigeasy@linutronix.de \
    --cc=hch@lst.de \
    --cc=hughd@google.com \
    --cc=jack@suse.cz \
    --cc=jannh@google.com \
    --cc=jgg@nvidia.com \
    --cc=jhubbard@nvidia.com \
    --cc=kirill@shutemov.name \
    --cc=ktkhai@virtuozzo.com \
    --cc=leonro@nvidia.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=mhocko@suse.com \
    --cc=mingo@redhat.com \
    --cc=oleg@redhat.com \
    --cc=peterx@redhat.com \
    --cc=peterz@infradead.org \
    --cc=tglx@linutronix.de \
    --cc=torvalds@linux-foundation.org \
    --cc=will@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lkml.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lkml.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lkml.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lkml.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lkml.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lkml.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lkml.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lkml.kernel.org/lkml/7 lkml/git/7.git
	git clone --mirror https://lkml.kernel.org/lkml/8 lkml/git/8.git
	git clone --mirror https://lkml.kernel.org/lkml/9 lkml/git/9.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lkml.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git