From: Peter Xu <peterx@redhat.com>
To: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
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>,
Jan Kara <jack@suse.cz>, Kirill Tkhai <ktkhai@virtuozzo.com>,
Andrea Arcangeli <aarcange@redhat.com>,
Christoph Hellwig <hch@lst.de>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 1/4] mm: Trial do_wp_page() simplification
Date: Tue, 15 Sep 2020 17:33:30 -0400 [thread overview]
Message-ID: <20200915213330.GE2949@xz-x1> (raw)
In-Reply-To: <20200915193838.GN1221970@ziepe.ca>
On Tue, Sep 15, 2020 at 04:38:38PM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 15, 2020 at 03:13:46PM -0400, Peter Xu wrote:
> > On Tue, Sep 15, 2020 at 03:29:33PM -0300, Jason Gunthorpe wrote:
> > > On Tue, Sep 15, 2020 at 01:05:53PM -0300, Jason Gunthorpe wrote:
> > > > On Tue, Sep 15, 2020 at 10:50:40AM -0400, Peter Xu wrote:
> > > > > On Mon, Sep 14, 2020 at 08:28:51PM -0300, Jason Gunthorpe wrote:
> > > > > > Yes, this stuff does pin_user_pages_fast() and MADV_DONTFORK
> > > > > > together. It sets FOLL_FORCE and FOLL_WRITE to get an exclusive copy
> > > > > > of the page and MADV_DONTFORK was needed to ensure that a future fork
> > > > > > doesn't establish a COW that would break the DMA by moving the
> > > > > > physical page over to the fork. DMA should stay with the process that
> > > > > > called pin_user_pages_fast() (Is MADV_DONTFORK still needed with
> > > > > > recent years work to GUP/etc? It is a pretty terrible ancient thing)
> > > > >
> > > > > ... Now I'm more confused on what has happened.
> > > >
> > > > I'm going to try to confirm that the MADV_DONTFORK is actually being
> > > > done by userspace properly, more later.
> > >
> > > It turns out the test is broken and does not call MADV_DONTFORK when
> > > doing forks - it is an opt-in it didn't do.
> > >
> > > It looks to me like this patch makes it much more likely that the COW
> > > break after page pinning will end up moving the pinned physical page
> > > to the fork while before it was not very common. Does that make sense?
> >
> > My understanding is that the fix should not matter much with current failing
> > test case, as long as it's with FOLL_FORCE & FOLL_WRITE. However what I'm not
> > sure is what if the RDMA/DMA buffers are designed for pure read from userspace.
>
> No, they are write. Always FOLL_WRITE.
>
> > E.g. for vfio I'm looking at vaddr_get_pfn() where I believe such pure read
> > buffers will be a GUP with FOLL_PIN and !FOLL_WRITE which will finally pass to
> > pin_user_pages_remote(). So what I'm worrying is something like this:
>
> I think the !(prot & IOMMU_WRITE) case is probably very rare for
> VFIO. I'm also not sure it will work reliably, in RDMA we had this as
> a more common case and long ago found bugs. The COW had to be broken
> for the pin anyhow.
If I'm not wrong.. QEMU/KVM (assuming there's one vIOMMU in the guest) will try
to do VFIO maps in this read-only way if the IOVA mapped in the guest points to
read only buffers (say, allocated with PCI_DMA_FROMDEVICE).
>
> > 1. Proc A gets a private anon page X for DMA, mapcount==refcount==1.
> >
> > 2. Proc A fork()s and gives birth to proc B, page X will now have
> > mapcount==refcount==2, write-protected. proc B quits. Page X goes back
> > to mapcount==refcount==1 (note! without WRITE bits set in the PTE).
>
> > 3. pin_user_pages(write=false) for page X. Since it's with !FORCE & !WRITE,
> > no COW needed. Refcount==2 after that.
> >
> > 4. Pass these pages to device. We either setup IOMMU page table or just use
> > the PFNs, which is not important imho - the most important thing is the
> > device will DMA into page X no matter what.
> >
> > 5. Some thread of proc A writes to page X, trigger COW since it's
> > write-protected with mapcount==1 && refcount==2. The HVA that pointing to
> > page X will be changed to point to another page Y after the COW.
> >
> > 6. Device DMA happens, data resides on X. Proc A can never get the data,
> > though, because it's looking at page Y now.
>
> RDMA doesn't ever use !WRITE
>
> I'm guessing #5 is the issue, just with a different ordering. If the
> #3 pin_user_pages() preceeds the #2 fork, don't we get to the same #5?
Right, but only if without MADV_DONTFORK? When without MADV_DONTFORK I'll
probably still see that as an userspace bug instead of a kernel one when the
userspace decided to fork() after step #3.
>
> > If this is a problem, we may still need the fix patch (maybe not as urgent as
> > before at least). But I'd like to double confirm, just in case I miss some
> > obvious facts above.
>
> I'm worred that the sudden need to have MAD_DONTFORK is going to be a
> turn into a huge regression. It already blew up our first level of
> synthetic test cases. I'm worried what we will see when the
> application suite is run in a few months :\
For my own preference I'll consider changing kernel behavior if the impact is
still under control (the performance report of 30%+ boost is also attractive
after the simplify-cow patch). The other way is to maintain the old reuse
logic forever, that'll be another kind of burden. Seems no easy way on either
side...
>
> > > Given that the tests are wrong it seems like broken userspace,
> > > however, it also worked reliably for a fairly long time.
> >
> > IMHO it worked because the page to do RDMA has mapcount==1, so it was reused
> > previously just as-is even after the fork without MADV_DONTFORK and after the
> > child quits.
>
> That would match the results we see.. So this patch changes things so
> it is not re-used as-is, but replaced with Y?
Yes. The patch lets "replaced with Y" (cow) happen earlier at step #3. Then
with MADV_DONTFORK, reuse should not happen any more.
Thanks,
--
Peter Xu
next prev parent reply other threads:[~2020-09-15 21:38 UTC|newest]
Thread overview: 98+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-21 23:49 [PATCH 0/4] mm: Simplfy cow handling Peter Xu
2020-08-21 23:49 ` [PATCH 1/4] mm: Trial do_wp_page() simplification Peter Xu
2020-08-24 8:36 ` Kirill Tkhai
2020-08-24 14:30 ` Jan Kara
2020-08-24 15:37 ` Kirill Tkhai
2020-08-24 18:22 ` Linus Torvalds
2020-09-01 7:01 ` Hugh Dickins
2020-09-14 14:38 ` Jason Gunthorpe
2020-09-14 17:32 ` Linus Torvalds
2020-09-14 18:34 ` Peter Xu
2020-09-14 21:15 ` Peter Xu
2020-09-14 22:55 ` Jason Gunthorpe
2020-09-14 22:59 ` Linus Torvalds
2020-09-14 23:28 ` Jason Gunthorpe
2020-09-15 0:19 ` Linus Torvalds
2020-09-15 14:50 ` Peter Xu
2020-09-15 15:17 ` Peter Xu
2020-09-15 16:05 ` Jason Gunthorpe
2020-09-15 18:29 ` Jason Gunthorpe
2020-09-15 19:13 ` Peter Xu
2020-09-15 19:38 ` Jason Gunthorpe
2020-09-15 21:33 ` Peter Xu [this message]
2020-09-15 23:22 ` Jason Gunthorpe
2020-09-16 1:50 ` John Hubbard
2020-09-16 17:48 ` Jason Gunthorpe
2020-09-16 18:46 ` Peter Xu
2020-09-17 11:25 ` Jason Gunthorpe
2020-09-17 18:11 ` Linus Torvalds
2020-09-17 19:38 ` Jason Gunthorpe
2020-09-17 19:51 ` Linus Torvalds
2020-09-18 16:40 ` Peter Xu
2020-09-18 17:16 ` Linus Torvalds
2020-09-18 19:57 ` Peter Xu
2020-09-18 17:32 ` Jason Gunthorpe
2020-09-18 20:40 ` Peter Xu
2020-09-18 20:59 ` Linus Torvalds
2020-09-19 0:28 ` Jason Gunthorpe
2020-09-18 21:06 ` John Hubbard
2020-09-19 0:01 ` Jason Gunthorpe
2020-09-21 8:35 ` Jan Kara
2020-09-21 12:03 ` Jason Gunthorpe
2022-02-16 16:59 ` Oded Gabbay
2022-02-16 17:24 ` Oded Gabbay
2022-02-16 19:04 ` Linus Torvalds
2022-02-16 19:20 ` Oded Gabbay
2022-02-16 19:24 ` David Hildenbrand
2020-09-21 13:42 ` Michal Hocko
2020-09-21 14:18 ` Peter Xu
2020-09-21 14:28 ` Michal Hocko
2020-09-21 14:38 ` Tejun Heo
2020-09-21 14:43 ` Christian Brauner
2020-09-21 14:55 ` Michal Hocko
2020-09-21 15:04 ` Christian Brauner
2020-09-21 16:06 ` Michal Hocko
2020-09-23 7:53 ` Michal Hocko
2020-09-21 14:41 ` Christian Brauner
2020-09-21 14:57 ` Michal Hocko
2020-09-21 16:31 ` Peter Xu
2020-09-17 18:14 ` Peter Xu
2020-09-17 18:26 ` Linus Torvalds
2020-09-17 19:03 ` Peter Xu
2020-09-17 19:42 ` Linus Torvalds
2020-09-17 19:55 ` John Hubbard
2020-09-17 20:06 ` Jason Gunthorpe
2020-09-17 20:19 ` John Hubbard
2020-09-17 20:25 ` Jason Gunthorpe
2020-09-17 20:35 ` Linus Torvalds
2020-09-17 21:40 ` Peter Xu
2020-09-17 22:09 ` Jason Gunthorpe
2020-09-17 22:25 ` Linus Torvalds
2020-09-17 22:48 ` Ira Weiny
2020-09-18 9:36 ` Jan Kara
2020-09-18 9:44 ` Jan Kara
2020-09-18 16:19 ` Jason Gunthorpe
2020-09-15 10:23 ` Leon Romanovsky
2020-09-15 15:56 ` Jason Gunthorpe
2020-09-15 15:03 ` Oleg Nesterov
2020-09-15 16:18 ` Peter Xu
2020-08-21 23:49 ` [PATCH 2/4] mm/ksm: Remove reuse_ksm_page() Peter Xu
2020-08-21 23:49 ` [PATCH 3/4] mm/gup: Remove enfornced COW mechanism Peter Xu
2020-09-14 14:27 ` Oleg Nesterov
2020-09-14 17:59 ` Peter Xu
2020-09-14 19:03 ` Linus Torvalds
2020-08-21 23:49 ` [PATCH 4/4] mm: Add PGREUSE counter Peter Xu
2020-08-22 16:14 ` Linus Torvalds
2020-08-24 0:24 ` Peter Xu
2020-08-22 16:05 ` [PATCH 0/4] mm: Simplfy cow handling Linus Torvalds
2020-08-23 23:58 ` Peter Xu
2020-08-24 8:38 ` Kirill Tkhai
2020-08-27 14:15 ` Peter Xu
2021-02-02 14:40 [PATCH 1/4] mm: Trial do_wp_page() simplification Gal Pressman
2021-02-02 16:31 ` Peter Xu
2021-02-02 16:44 ` Jason Gunthorpe
2021-02-02 17:05 ` Peter Xu
2021-02-02 17:13 ` Jason Gunthorpe
2021-02-03 12:43 ` Gal Pressman
2021-02-03 14:00 ` Jason Gunthorpe
2021-02-03 14:47 ` Gal Pressman
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