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Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots

From: Andrey Gruzdev
Subject: Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots
Date: Thu, 11 Feb 2021 19:19:47 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 09.02.2021 22:06, David Hildenbrand wrote:

just stumbled over this, quick question:

I recently played with UFFD_WP and notices that write protection is
only effective on pages/ranges that have already pages populated (IOW:
!pte_none() in the kernel).

In case memory was never populated (or was discarded using e.g.,
madvice(DONTNEED)), write-protection will be skipped silently and you
won't get WP events for applicable pages.

So if someone writes to a yet unpoupulated page ("zero"), you won't
get WP events.

I can spot that you do a single uffd_change_protection() on the whole

How are you handling that scenario, or why don't you have to handle
that scenario?

Hi David,

I really wonder if such a problem exists.. If we are talking about a

I immediately ran into this issue with my simplest test cases. :)

write to an unpopulated page, we should get first page fault on
non-present page and populate it with protection bits from respective vma.
For UFFD_WP vma's  page will be populated non-writable. So we'll get
another page fault on present but read-only page and go to handle_userfault.

See the attached test program. Triggers for me on 5.11.0-rc6+ and 5.9.13-200.fc33

gcc -lpthread uffdio_wp.c -o uffdio_wp
WP did not fire

Uncomment the placement of the zeropage just before registering to make the WP actually trigger. If there is no PTE, there is nothing to protect.

And it makes sense: How should the fault handler know which ranges you wp-ed, if there is no place to store that information (the PTEs!). The VMA cannot tell that story, it only knows that someone registered UFFD_WP to selectively wp some parts.

You might have to register also for MISSING faults and place zero pages.

Looked at the kernel code, agree that we miss WP events for unpopulated pages, UFFD_WP softbit won't be set in this case. But it doesn't make saved snapshot inconsistent or introduce security issues. The only side effect is that we may save updated page instead of zeroed, just increasing snapshot size. However this guest-physical page has never been touched from the point of view of saved vCPU/device state and is not a concern.

Often (at least on desktop Windows guests) only a small part of RAM has ever been allocated by guest. Migration code needs to read each guest-physical page, so we'll have a lot of additional UFFD events, much more MISSING events then WP-faults.

And the main problem is that adding MISSING handler is impossible in current single-threaded snapshot code. We'll get an immediate deadlock on iterative page read.

Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397

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