From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Jerome Glisse <j.glisse@gmail.com>,
Oleg Nesterov <oleg@redhat.com>, Hugh Dickins <hughd@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrew Morton <akpm@linux-foundation.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>,
Alex Williamson <alex.williamson@redhat.com>,
kirill.shutemov@linux.intel.com, linux-kernel@vger.kernel.org,
"linux-mm@kvack.org" <linux-mm@kvack.org>
Subject: GUP guarantees wrt to userspace mappings redesign
Date: Mon, 2 May 2016 15:14:02 +0300 [thread overview]
Message-ID: <20160502121402.GB23305@node.shutemov.name> (raw)
In-Reply-To: <20160502111513.GA4079@gmail.com>
On Mon, May 02, 2016 at 01:15:13PM +0200, Jerome Glisse wrote:
> On Mon, May 02, 2016 at 01:41:19PM +0300, Kirill A. Shutemov wrote:
> > Other thing I would like to discuss is if there's a problem on vfio side.
> > To me it looks like vfio expects guarantee from get_user_pages() which it
> > doesn't provide: obtaining pin on the page doesn't guarantee that the page
> > is going to remain mapped into userspace until the pin is gone.
> >
> > Even with THP COW regressing fixed, vfio would stay fragile: any
> > MADV_DONTNEED/fork()/mremap()/whatever what would make vfio expectation
> > broken.
> >
>
> Well i don't think it is fair/accurate assessment of get_user_pages(), page
> must remain mapped to same virtual address until pin is gone. I am ignoring
> mremap() as it is a scient decision from userspace and while virtual address
> change in that case, the pined page behind should move with the mapping.
> Same of MADV_DONTNEED. I agree that get_user_pages() is broken after fork()
> but this have been the case since dawn of time, so it is something expected.
>
> If not vfio, then direct-io, have been expecting this kind of behavior for
> long time, so i see this as part of get_user_pages() guarantee.
>
> Concerning vfio, not providing this guarantee will break countless number of
> workload. Thing like qemu/kvm allocate anonymous memory and hand it over to
> the guest kernel which presents it as memory. Now a device driver inside the
> guest kernel need to get bus mapping for a given (guest) page, which from
> host point of view means a mapping from anonymous page to bus mapping but
> for guest to keep accessing the same page the anonymous mapping (ie a
> specific virtual address on the host side) must keep pointing to the same
> page. This have been the case with get_user_pages() until now, so whether
> we like it or not we must keep that guarantee.
>
> This kind of workload knows that they can't do mremap()/fork()/... and keep
> that guarantee but they at expect existing guarantee and i don't think we
> can break that.
Quick look around:
- I don't see any check page_count() around __replace_page() in uprobes,
so it can easily replace pinned page.
- KSM has the page_count() check, there's still race wrt GUP_fast: it can
take the pin between the check and establishing new pte entry.
- khugepaged: the same story as with KSM.
I don't see how we can deliver on the guarantee, especially with lockless
GUP_fast.
Or am I missing something important?
--
Kirill A. Shutemov
next prev parent reply other threads:[~2016-05-02 12:14 UTC|newest]
Thread overview: 36+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-28 16:20 [BUG] vfio device assignment regression with THP ref counting redesign Alex Williamson
2016-04-28 18:17 ` Kirill A. Shutemov
2016-04-28 18:58 ` Alex Williamson
2016-04-28 23:21 ` Andrea Arcangeli
2016-04-29 0:44 ` Alex Williamson
2016-04-29 0:51 ` Kirill A. Shutemov
2016-04-29 2:45 ` Alex Williamson
2016-04-29 7:06 ` Kirill A. Shutemov
2016-04-29 15:12 ` Alex Williamson
2016-04-29 16:34 ` Andrea Arcangeli
2016-04-29 22:34 ` Alex Williamson
2016-05-02 10:41 ` Kirill A. Shutemov
2016-05-02 11:15 ` Jerome Glisse
2016-05-02 12:14 ` Kirill A. Shutemov [this message]
2016-05-02 13:39 ` GUP guarantees wrt to userspace mappings redesign Jerome Glisse
2016-05-02 15:00 ` GUP guarantees wrt to userspace mappings Kirill A. Shutemov
2016-05-02 15:22 ` Jerome Glisse
2016-05-02 16:12 ` Kirill A. Shutemov
2016-05-02 19:14 ` Andrea Arcangeli
2016-05-02 19:11 ` Andrea Arcangeli
2016-05-02 19:02 ` Andrea Arcangeli
2016-05-02 14:15 ` GUP guarantees wrt to userspace mappings redesign Oleg Nesterov
2016-05-02 16:21 ` Kirill A. Shutemov
2016-05-02 16:22 ` Oleg Nesterov
2016-05-02 18:03 ` Kirill A. Shutemov
2016-05-02 17:41 ` Oleg Nesterov
2016-05-02 18:56 ` Andrea Arcangeli
2016-05-02 15:23 ` [BUG] vfio device assignment regression with THP ref counting redesign Andrea Arcangeli
2016-05-02 16:00 ` Kirill A. Shutemov
2016-05-02 18:03 ` Andrea Arcangeli
2016-05-05 1:19 ` Alex Williamson
2016-05-05 14:39 ` Andrea Arcangeli
2016-05-05 15:09 ` Andrea Arcangeli
2016-05-05 15:11 ` Kirill A. Shutemov
2016-05-05 15:24 ` Andrea Arcangeli
2016-05-06 7:29 ` Kirill A. Shutemov
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