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

From: Jan Kara <jack@suse.cz>
To: Jason Gunthorpe <jgg@nvidia.com>
Cc: Jan Kara <jack@suse.cz>,
	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>,
	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>
Subject: Re: [PATCH v2 2/2] mm: prevent gup_fast from racing with COW during fork
Date: Mon, 2 Nov 2020 09:31:22 +0100	[thread overview]
Message-ID: <20201102083122.GA23988@quack2.suse.cz> (raw)
In-Reply-To: <20201030170226.GF2620339@nvidia.com>

On Fri 30-10-20 14:02:26, Jason Gunthorpe wrote:
> On Fri, Oct 30, 2020 at 05:51:05PM +0100, Jan Kara wrote:
> > > @@ -446,6 +447,12 @@ struct mm_struct {
> > >  		 */
> > >  		atomic_t has_pinned;
> > >  
> > > +		/**
> > > +		 * @write_protect_seq: Odd when any thread is write protecting
> > > +		 * pages in this mm, for instance during fork().
> > > +		 */
> > > +		seqcount_t write_protect_seq;
> > > +
> > 
> > So this comment isn't quite true. We can be writeprotecting pages due to
> > many other reasons and not touch write_protect_seq. E.g. for shared
> > mappings or due to explicit mprotect() calls. So the write_protect_seq
> > protection has to be about something more than pure write protection. One
> > requirement certainly is that the VMA has to be is_cow_mapping(). What
> > about mprotect(2) calls? I guess the application would have only itself to
> > blame so we don't care?
> 
> Yes, that sounds right, How about
> 
> /**
>  * @write_protect_seq: Locked when any thread is write protecting
>  * pages for COW in this mm, for instance during page table copying
           ^^^ maybe I'd write a bit more explicitly "... write protecting
pages mapped by this mm to enforce later COW, ..."

>  * for fork().
>  */
> 
> mprotect and shared mappings cause faults on write access not COW?

Correct.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR