Linux-kselftest Archive on lore.kernel.org
 help / color / Atom feed
From: Jason Gunthorpe <jgg@ziepe.ca>
To: Ralph Campbell <rcampbell@nvidia.com>
Cc: linux-rdma@vger.kernel.org, linux-mm@kvack.org,
	nouveau@lists.freedesktop.org, kvm-ppc@vger.kernel.org,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	Jerome Glisse <jglisse@redhat.com>,
	John Hubbard <jhubbard@nvidia.com>,
	Christoph Hellwig <hch@lst.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Shuah Khan <shuah@kernel.org>, Ben Skeggs <bskeggs@redhat.com>,
	Bharata B Rao <bharata@linux.ibm.com>
Subject: Re: [PATCH v4 6/6] mm/migrate: remove range invalidation in migrate_vma_pages()
Date: Fri, 31 Jul 2020 16:15:43 -0300
Message-ID: <20200731191543.GJ24045@ziepe.ca> (raw)
In-Reply-To: <7f947311-0034-9148-1dca-fb9b9a10abc4@nvidia.com>

On Tue, Jul 28, 2020 at 03:04:07PM -0700, Ralph Campbell wrote:
> 
> On 7/28/20 12:19 PM, Jason Gunthorpe wrote:
> > On Thu, Jul 23, 2020 at 03:30:04PM -0700, Ralph Campbell wrote:
> > > When migrating the special zero page, migrate_vma_pages() calls
> > > mmu_notifier_invalidate_range_start() before replacing the zero page
> > > PFN in the CPU page tables. This is unnecessary since the range was
> > > invalidated in migrate_vma_setup() and the page table entry is checked
> > > to be sure it hasn't changed between migrate_vma_setup() and
> > > migrate_vma_pages(). Therefore, remove the redundant invalidation.
> > 
> > I don't follow this logic, the purpose of the invalidation is also to
> > clear out anything that may be mirroring this VA, and "the page hasn't
> > changed" doesn't seem to rule out that case?
> > 
> > I'm also not sure I follow where the zero page came from?
> 
> The zero page comes from an anonymous private VMA that is read-only
> and the user level CPU process tries to read the page data (or any
> other read page fault).
> 
> > Jason
> > 
> 
> The overall migration process is:
> 
> mmap_read_lock()
> 
> migrate_vma_setup()
>       // invalidates range, locks/isolates pages, puts migration entry in page table
> 
> <driver allocates destination pages and copies source to dest>
> 
> migrate_vma_pages()
>       // moves source struct page info to destination struct page info.
>       // clears migration flag for pages that can't be migrated.
> 
> <driver updates device page tables for pages still migrating, rollback pages not migrating>
> 
> migrate_vma_finalize()
>       // replaces migration page table entry with destination page PFN.
> 
> mmap_read_unlock()
> 
> Since the address range is invalidated in the migrate_vma_setup() stage,
> and the page is isolated from the LRU cache, locked, unmapped, and the page table
> holds a migration entry (so the page can't be faulted and the CPU page table set
> valid again), and there are no extra page references (pins), the page
> "should not be modified".

That is the physical page though, it doesn't prove nobody else is
reading the PTE.
 
> For pte_none()/is_zero_pfn() entries, migrate_vma_setup() leaves the
> pte_none()/is_zero_pfn() entry in place but does still call
> mmu_notifier_invalidate_range_start() for the whole range being migrated.

Ok..

> In the migrate_vma_pages() step, the pte page table is locked and the
> pte entry checked to be sure it is still pte_none/is_zero_pfn(). If not,
> the new page isn't inserted. If it is still none/zero, the new device private
> struct page is inserted into the page table, replacing the pte_none()/is_zero_pfn()
> page table entry. The secondary MMUs were already invalidated in the migrate_vma_setup()
> step and a pte_none() or zero page can't be modified so the only invalidation needed
> is the CPU TLB(s) for clearing the special zero page PTE entry.

No, the secondary MMU was invalidated but the invalidation start/end
range was exited. That means a secondary MMU is immeidately able to
reload the zero page into its MMU cache.

When this code replaces the PTE that has a zero page it also has to
invalidate again so that secondary MMU's are guaranteed to pick up the
new PTE value.

So, I still don't understand how this is safe?

Jason

  reply index

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-07-23 22:29 [PATCH v4 0/6] mm/migrate: avoid device private invalidations Ralph Campbell
2020-07-23 22:29 ` [PATCH v4 1/6] nouveau: fix storing invalid ptes Ralph Campbell
2020-07-23 22:30 ` [PATCH v4 2/6] mm/migrate: add a flags parameter to migrate_vma Ralph Campbell
2020-07-23 22:30 ` [PATCH v4 3/6] mm/notifier: add migration invalidation type Ralph Campbell
2020-07-28 19:15   ` Jason Gunthorpe
2020-07-28 20:57     ` Ralph Campbell
2020-07-23 22:30 ` [PATCH v4 4/6] nouveau/svm: use the new migration invalidation Ralph Campbell
2020-07-23 22:30 ` [PATCH v4 5/6] mm/hmm/test: " Ralph Campbell
2020-07-23 22:30 ` [PATCH v4 6/6] mm/migrate: remove range invalidation in migrate_vma_pages() Ralph Campbell
2020-07-28 19:19   ` Jason Gunthorpe
2020-07-28 22:04     ` Ralph Campbell
2020-07-31 19:15       ` Jason Gunthorpe [this message]
2020-07-31 19:31         ` Ralph Campbell
2020-07-28 19:22 ` [PATCH v4 0/6] mm/migrate: avoid device private invalidations Jason Gunthorpe

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200731191543.GJ24045@ziepe.ca \
    --to=jgg@ziepe.ca \
    --cc=akpm@linux-foundation.org \
    --cc=bharata@linux.ibm.com \
    --cc=bskeggs@redhat.com \
    --cc=hch@lst.de \
    --cc=jglisse@redhat.com \
    --cc=jhubbard@nvidia.com \
    --cc=kvm-ppc@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-kselftest@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=linux-rdma@vger.kernel.org \
    --cc=nouveau@lists.freedesktop.org \
    --cc=rcampbell@nvidia.com \
    --cc=shuah@kernel.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Linux-kselftest Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/linux-kselftest/0 linux-kselftest/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 linux-kselftest linux-kselftest/ https://lore.kernel.org/linux-kselftest \
		linux-kselftest@vger.kernel.org
	public-inbox-index linux-kselftest

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kselftest


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git