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Re: [PATCH v1 0/3] migration: Fixes to the 'background-snapshot' code
From: |
Peter Xu |
Subject: |
Re: [PATCH v1 0/3] migration: Fixes to the 'background-snapshot' code |
Date: |
Wed, 24 Mar 2021 11:41:58 -0400 |
On Wed, Mar 24, 2021 at 11:09:27AM +0300, Andrey Gruzdev wrote:
> > I'm also looking into introducing UFFD_FEATURE_WP_UNALLOCATED so as to
> > wr-protect page holes too for a uffd-wp region when the feature bit is set.
> > With that feature we should be able to avoid pre-fault as what we do in the
> > last patch of this series. However even if that can work out, we'll still
> > need
> > this for old kernel anyways.
>
> I'm curious this new feature is based on adding wr-protection at the level of
> VMAs,
> so we won't miss write faults for missing pages?
I think we can do it with multiple ways.
The most efficient one would be wr-protect the range during uffd-wp
registration, so as you said it'll be per-vma attribute. However that'll
change the general semantics of uffd-wp as normally we need registration and
explicit wr-protect. Then it'll still be pte-based for faulted in pages (the
ones we wr-protected during registration will still be), however for the rest
it'll become vma-based. It's indeed a bit confusing.
The other way is we can fault in zero page during UFFDIO_WRITEPROTECT. However
that's less efficient, since it's close to pre-fault on read but it's just
slightly more cleaner than doing it in userspace. When I rethink about this it
may not worth it to do in kernel if userspace can achieve things similar.
So let's stick with current solution; that idea may need more thoughts..
Thanks,
--
Peter Xu
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, (continued)
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Peter Xu, 2021/03/23
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Andrey Gruzdev, 2021/03/23
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Peter Xu, 2021/03/23
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Andrey Gruzdev, 2021/03/24
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Dr. David Alan Gilbert, 2021/03/24
- Re: [PATCH v1 1/3] migration: Fix missing qemu_fflush() on buffer file in bg_migration_thread, Peter Xu, 2021/03/24
[PATCH v1 3/3] migration: Pre-fault memory before starting background snasphot, Andrey Gruzdev, 2021/03/19
[PATCH v1 2/3] migration: Inhibit virtio-balloon for the duration of background snapshot, Andrey Gruzdev, 2021/03/19
Re: [PATCH v1 0/3] migration: Fixes to the 'background-snapshot' code, Peter Xu, 2021/03/23