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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Lorenzo Stoakes <lstoakes@gmail.com>
Cc:     Jason Gunthorpe <jgg@nvidia.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
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        Peter Zijlstra <peterz@infradead.org>,
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References: <cover.1681508038.git.lstoakes@gmail.com>
        <5a4cf1ebf1c6cdfabbf2f5209facb0180dd20006.1681508038.git.lstoakes@gmail.com>
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        <9be77e7e-4531-4e1c-9e0d-4edbb5ad3bd5@lucifer.local>
        <ZD1GrBezHrJTo6x2@nvidia.com>
        <241f0c22-f3d6-436e-a0d8-be04e281ed2f@lucifer.local>
Date:   Mon, 17 Apr 2023 10:07:53 -0500
In-Reply-To: <241f0c22-f3d6-436e-a0d8-be04e281ed2f@lucifer.local> (Lorenzo
        Stoakes's message of "Mon, 17 Apr 2023 14:23:52 +0100")
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Subject: Re: [PATCH 3/7] mm/gup: remove vmas parameter from
 get_user_pages_remote()
Precedence: bulk

Lorenzo Stoakes <lstoakes@gmail.com> writes:

> On Mon, Apr 17, 2023 at 10:16:28AM -0300, Jason Gunthorpe wrote:
>> On Mon, Apr 17, 2023 at 02:13:39PM +0100, Lorenzo Stoakes wrote:
>> > On Mon, Apr 17, 2023 at 10:09:36AM -0300, Jason Gunthorpe wrote:
>> > > On Sat, Apr 15, 2023 at 12:27:31AM +0100, Lorenzo Stoakes wrote:
>> > > > The only instances of get_user_pages_remote() invocations which used the
>> > > > vmas parameter were for a single page which can instead simply look up the
>> > > > VMA directly. In particular:-
>> > > >
>> > > > - __update_ref_ctr() looked up the VMA but did nothing with it so we simply
>> > > >   remove it.
>> > > >
>> > > > - __access_remote_vm() was already using vma_lookup() when the original
>> > > >   lookup failed so by doing the lookup directly this also de-duplicates the
>> > > >   code.
>> > > >
>> > > > This forms part of a broader set of patches intended to eliminate the vmas
>> > > > parameter altogether.
>> > > >
>> > > > Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>
>> > > > ---
>> > > >  arch/arm64/kernel/mte.c   |  5 +++--
>> > > >  arch/s390/kvm/interrupt.c |  2 +-
>> > > >  fs/exec.c                 |  2 +-
>> > > >  include/linux/mm.h        |  2 +-
>> > > >  kernel/events/uprobes.c   | 10 +++++-----
>> > > >  mm/gup.c                  | 12 ++++--------
>> > > >  mm/memory.c               |  9 +++++----
>> > > >  mm/rmap.c                 |  2 +-
>> > > >  security/tomoyo/domain.c  |  2 +-
>> > > >  virt/kvm/async_pf.c       |  3 +--
>> > > >  10 files changed, 23 insertions(+), 26 deletions(-)
>> > > >
>> > > > diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
>> > > > index f5bcb0dc6267..74d8d4007dec 100644
>> > > > --- a/arch/arm64/kernel/mte.c
>> > > > +++ b/arch/arm64/kernel/mte.c
>> > > > @@ -437,8 +437,9 @@ static int __access_remote_tags(struct mm_struct *mm, unsigned long addr,
>> > > >  		struct page *page = NULL;
>> > > >
>> > > >  		ret = get_user_pages_remote(mm, addr, 1, gup_flags, &page,
>> > > > -					    &vma, NULL);
>> > > > -		if (ret <= 0)
>> > > > +					    NULL);
>> > > > +		vma = vma_lookup(mm, addr);
>> > > > +		if (ret <= 0 || !vma)
>> > > >  			break;
>> > >
>> > > Given the slightly tricky error handling, it would make sense to turn
>> > > this pattern into a helper function:
>> > >
>> > > page = get_single_user_page_locked(mm, addr, gup_flags, &vma);
>> > > if (IS_ERR(page))
>> > >   [..]
>> > >
>> > > static inline struct page *get_single_user_page_locked(struct mm_struct *mm,
>> > >    unsigned long addr, int gup_flags, struct vm_area_struct **vma)
>> > > {
>> > > 	struct page *page;
>> > > 	int ret;
>> > >
>> > > 	ret = get_user_pages_remote(*mm, addr, 1, gup_flags, &page, NULL, NULL);
>> > > 	if (ret < 0)
>> > > 	   return ERR_PTR(ret);
>> > > 	if (WARN_ON(ret == 0))
>> > > 	   return ERR_PTR(-EINVAL);
>> > >         *vma = vma_lookup(mm, addr);
>> > > 	if (WARN_ON(!*vma) {
>> > > 	   put_user_page(page);
>> > > 	   return ERR_PTR(-EINVAL);
>> > >         }
>> > > 	return page;
>> > > }
>> > >
>> > > It could be its own patch so this change was just a mechanical removal
>> > > of NULL
>> > >
>> > > Jason
>> > >
>> >
>> > Agreed, I think this would work better as a follow up patch however so as
>> > not to distract too much from the core change.
>>
>> I don't think you should open code sketchy error handling in several
>> places and then clean it up later. Just do it right from the start.
>>
>
> Intent was to do smallest change possible (though through review that grew
> of course), but I see your point, in this instance this is fiddly stuff and
> probably better to abstract it to enforce correct handling.
>
> I'll respin + add something like this.

Could you include in your description why looking up the vma after
getting the page does not introduce a race?

I am probably silly and just looking at this quickly but it does not
seem immediately obvious why the vma and the page should match.

I would not be surprised if you hold the appropriate mutex over the
entire operation but it just isn't apparent from the diff.

I am concerned because it is an easy mistake to refactor something into
two steps and then discover you have introduced a race.

Eric