Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()

From: Oleg Nesterov <oleg@redhat.com>
To: Peter Xu <peterx@redhat.com>
Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org,
	Jason Gunthorpe <jgg@ziepe.ca>,
	Andrew Morton <akpm@linux-foundation.org>,
	Jan Kara <jack@suse.cz>, Michal Hocko <mhocko@suse.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Kirill Shutemov <kirill@shutemov.name>,
	Hugh Dickins <hughd@google.com>, Christoph Hellwig <hch@lst.de>,
	Andrea Arcangeli <aarcange@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jann Horn <jannh@google.com>
Subject: Re: [PATCH 3/5] mm: Rework return value for copy_one_pte()
Date: Tue, 22 Sep 2020 17:48:46 +0200	[thread overview]
Message-ID: <20200922154845.GE11679@redhat.com> (raw)
In-Reply-To: <20200922153612.GF19098@xz-x1>

On 09/22, Peter Xu wrote:
>
> On Tue, Sep 22, 2020 at 12:18:16PM +0200, Oleg Nesterov wrote:
> > On 09/22, Oleg Nesterov wrote:
> > >
> > > On 09/21, Peter Xu wrote:
> > > >
> > > > @@ -866,13 +877,18 @@ static int copy_pte_range(struct mm_struct *dst_mm, struct mm_struct *src_mm,
> > > >  	pte_unmap_unlock(orig_dst_pte, dst_ptl);
> > > >  	cond_resched();
> > > >
> > > > -	if (entry.val) {
> > > > -		if (add_swap_count_continuation(entry, GFP_KERNEL) < 0)
> > > > +	switch (copy_ret) {
> > > > +	case COPY_MM_SWAP_CONT:
> > > > +		if (add_swap_count_continuation(data.entry, GFP_KERNEL) < 0)
> > > >  			return -ENOMEM;
> > > > -		progress = 0;
> > > > +		break;
> > >
> > > Note that you didn't clear copy_ret, it is still COPY_MM_SWAP_CONT,
> > >
> > > > +	default:
> > > > +		break;
> > > >  	}
> > > > +
> > > >  	if (addr != end)
> > > >  		goto again;
> > >
> > > After that the main loop can stop again because of need_resched(), and
> > > in this case add_swap_count_continuation(data.entry) will be called again?
> >
> > No, this is not possible, copy_one_pte() should be called at least once,
> > progress = 0 before restart. Sorry for noise.
>
> Oh wait, I think you're right... when we get a COPY_MM_SWAP_CONT, goto "again",
> then if there're 32 pte_none() ptes _plus_ an need_resched(), then we might
> reach again at the same add_swap_count_continuation() with the same swp entry.

Yes, please see my reply to 4/5 ;)

> However since I didn't change this logic in this patch, it probably means this
> bug is also in the original code before this series...  I'm thinking maybe I
> should prepare a standalone patch to clear the swp_entry_t and cc stable.

Well, if copy_one_pte(src_pte) hits a swap entry and returns entry.val != 0, then
pte_none(*src_pte) is not possible after restart? This means that copy_one_pte()
will be called at least once.

So _think_ that the current code is fine, but I can be easily wrong and I agree
this doesn't look clean.

Oleg.