Re: [PATCH 1/4] mm: Trial do_wp_page() simplification

From: David Hildenbrand <david@redhat.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
	Oded Gabbay <oded.gabbay@gmail.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>, Jan Kara <jack@suse.cz>,
	John Hubbard <jhubbard@nvidia.com>,
	Leon Romanovsky <leonro@nvidia.com>,
	Linux-MM <linux-mm@kvack.org>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	"Maya B . Gokhale" <gokhale2@llnl.gov>,
	Yang Shi <yang.shi@linux.alibaba.com>,
	Marty Mcfadden <mcfadden8@llnl.gov>,
	Kirill Shutemov <kirill@shutemov.name>,
	Oleg Nesterov <oleg@redhat.com>, Jann Horn <jannh@google.com>,
	Kirill Tkhai <ktkhai@virtuozzo.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Daniel Vetter <daniel.vetter@ffwll.ch>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Peter Xu <peterx@redhat.com>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Wed, 16 Feb 2022 20:24:12 +0100	[thread overview]
Message-ID: <39d17db6-0f8a-0e54-289b-85b9baf1e936@redhat.com> (raw)
In-Reply-To: <CAHk-=wgLk5xWj8v6mQXaWL0zWAf2cis8EpPkQUuEZ7K3LxVQHQ@mail.gmail.com>

On 16.02.22 20:04, Linus Torvalds wrote:
> [ Added David Hildenbrand to the participants. David, see
> 
>     https://bugzilla.kernel.org/show_bug.cgi?id=215616
> 
>   for details ]
> 

Thanks for sharing.

> On Wed, Feb 16, 2022 at 8:59 AM Oded Gabbay <oded.gabbay@gmail.com> wrote:
>>
>> All the details are in the bug, but the bottom line is that somehow,
>> this patch causes corruption when the numa balancing feature is
>> enabled AND we don't use process affinity AND we use GUP to pin pages
>> so our accelerator can DMA to/from system memory.
> 
> Hmm. I thought all the remaining issues were related to THP - and
> David Hildenbrand had a series to fix those up.

What I shared so far recently [1] was part 1 of my COW fixes to fix the
COW security issues -- missed COW. This fixes 1) of [2].

Part 2 is around fixing "wrong COW" for FOLL_PIN a.k.a. memory
corruption. That's essentially what PageAnonExclusive() will be all
about, making sure that we don't lose synchronicity between GUP and mm
due to a wrong COW. This will fix 3) of [2]

Part 3 is converting O_DIRECT to use FOLL_PIN instead of FOLL_GET to
similarly fix "wrong COW" for O_DIRECT. John is working on that. This
will fix 2) of [2].

> 
> The fact that it also shows up with numa balancing is a bit
> unfortunate, because I think that means that that patch series may not
> have caught that case.
> 
> That said - what does "we use GUP to pin pages" mean? Does it actually
> use the pinning logic, or just regular old GUP?

If it uses FOLL_PIN it might be handled by part 2, if it uses O_DIRECT
magic it might be covered by part 3. If neither of both, more work might
be needed to convert it to FOLL_PIN, as with the new COW logic we won't
be able to have the same guarantees for FOLL_GET as we'll have for
FOLL_PIN (which is a difference to our original plans to fix it all [3]).

[1] https://lkml.kernel.org/r/20220126095557.32392-1-david@redhat.com
[2]
https://lore.kernel.org/all/3ae33b08-d9ef-f846-56fb-645e3b9b4c66@redhat.com/
[3] https://lore.kernel.org/all/20211217113049.23850-1-david@redhat.com/T/#u

-- 
Thanks,

David / dhildenb