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>,
Matthew Wilcox <willy@infradead.org>,
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Catalin Marinas <catalin.marinas@arm.com>,
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James Morris <jmorris@namei.org>,
"Serge E . Hallyn" <serge@hallyn.com>,
Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [PATCH 3/7] mm/gup: remove vmas parameter from get_user_pages_remote()
Date: Mon, 17 Apr 2023 10:07:53 -0500 [thread overview]
Message-ID: <87cz427diu.fsf@email.froward.int.ebiederm.org> (raw)
In-Reply-To: <241f0c22-f3d6-436e-a0d8-be04e281ed2f@lucifer.local> (Lorenzo Stoakes's message of "Mon, 17 Apr 2023 14:23:52 +0100")
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
next prev parent reply other threads:[~2023-04-17 15:08 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-04-14 23:25 [PATCH 0/7] remove the vmas parameter from GUP APIs Lorenzo Stoakes
2023-04-14 23:27 ` [PATCH 1/7] mm/gup: remove unused vmas parameter from get_user_pages() Lorenzo Stoakes
2023-04-15 5:27 ` Greg Kroah-Hartman
2023-04-17 13:01 ` Jason Gunthorpe
2023-04-14 23:27 ` [PATCH 2/7] mm/gup: remove unused vmas parameter from pin_user_pages_remote() Lorenzo Stoakes
2023-04-17 13:02 ` Jason Gunthorpe
2023-04-14 23:27 ` [PATCH 3/7] mm/gup: remove vmas parameter from get_user_pages_remote() Lorenzo Stoakes
2023-04-15 0:25 ` Tetsuo Handa
2023-04-15 8:11 ` Lorenzo Stoakes
2023-04-17 13:09 ` Jason Gunthorpe
2023-04-17 13:13 ` Lorenzo Stoakes
2023-04-17 13:16 ` Jason Gunthorpe
2023-04-17 13:23 ` Lorenzo Stoakes
2023-04-17 15:07 ` Eric W. Biederman [this message]
2023-04-17 15:14 ` Lorenzo Stoakes
2023-04-14 23:27 ` [PATCH 4/7] mm/gup: introduce the FOLL_SAME_FILE GUP flag Lorenzo Stoakes
2023-04-17 13:14 ` Jason Gunthorpe
2023-04-17 13:25 ` Lorenzo Stoakes
2023-04-17 13:27 ` Jason Gunthorpe
2023-04-14 23:27 ` [PATCH 5/7] io_uring: rsrc: use FOLL_SAME_FILE on pin_user_pages() Lorenzo Stoakes
2023-04-17 12:56 ` Jason Gunthorpe
2023-04-17 13:19 ` Lorenzo Stoakes
2023-04-17 13:26 ` Jason Gunthorpe
2023-04-17 14:00 ` Lorenzo Stoakes
2023-04-17 14:15 ` Jason Gunthorpe
2023-04-17 15:20 ` Lorenzo Stoakes
2023-04-17 19:00 ` Lorenzo Stoakes
2023-04-17 19:24 ` Jason Gunthorpe
2023-04-17 19:45 ` Lorenzo Stoakes
2023-04-18 16:25 ` Pavel Begunkov
2023-04-18 16:35 ` Pavel Begunkov
2023-04-18 16:36 ` Jason Gunthorpe
2023-04-18 17:25 ` Pavel Begunkov
2023-04-18 18:19 ` Jason Gunthorpe
2023-04-14 23:27 ` [PATCH 6/7] mm/gup: remove vmas parameter from pin_user_pages() Lorenzo Stoakes
2023-04-14 23:27 ` [PATCH 7/7] mm/gup: remove vmas array from internal GUP functions Lorenzo Stoakes
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