Re: [PATCH] mm: Take a page reference when removing device exclusive entries

From: Alistair Popple <apopple@nvidia.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: John Hubbard <jhubbard@nvidia.com>,
	linux-mm@kvack.org, Ralph Campbell <rcampbell@nvidia.com>,
	nouveau@lists.freedesktop.org, stable@vger.kernel.org
Subject: Re: [PATCH] mm: Take a page reference when removing device exclusive entries
Date: Wed, 29 Mar 2023 12:45:44 +1100	[thread overview]
Message-ID: <87lejgtjbh.fsf@nvidia.com> (raw)
In-Reply-To: <20230328125558.90cf40060e238b24add51d23@linux-foundation.org>

John Hubbard <jhubbard@nvidia.com> writes:

>> warnings such as PAGE_FLAGS_CHECK_AT_FREE due to the page being locked
>> when the refcount drops to zero. Note that during removal of the
>> device exclusive entry the PTE is currently re-checked under the PTL
>> so no futher bad page accesses occur once it is locked.
>
> Maybe change that last sentence to something like this:
>
> "Fix this by taking a page reference before starting to remove a device
> exclusive pte. This is done safely in a lock-free way by first getting a
> reference via get_page_unless_zero(), and then re-checking after
> acquiring the PTL, that the page is the correct one."
>
> ?
>
> ...well, maybe that's not all that much help. But it does at least
> provide the traditional description of what the patch *does*, at
> the end of the commit description. But please treat this as just
> an optional suggestion.

My wording was probably a little awkward. The intent was to point out
the existing code subsequent to taking the page lock was already
correct/safe. I figured the patch itself does a pretty good of
describing the actual fix so am inclined to leave it.

Andrew Morton <akpm@linux-foundation.org> writes:

> On Mon, 27 Mar 2023 23:25:49 -0700 John Hubbard <jhubbard@nvidia.com> wrote:
>
>> On the patch process, I see that this applies to linux-stable's 6.1.y
>> branch. I'd suggest two things:
>> 
>> 1) Normally, what I've seen done is to post against either the current
>> top of tree linux.git, or else against one of the mm-stable branches.
>> And then after it's accepted, create a version for -stable. 
>
> Yup.  I had to jiggle the patch a bit because
> mmu_notifier_range_init_owner()'s arguments have changed.  Once this
> hits mainline, the -stable maintainers will probably ask for a version
> which suits the relevant kernel version(s).

Thanks Andrew. That's my bad, I was developing on top of v6.1 and
neglected to rebase. Happy to provide versions for -stable as required.