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Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest fr
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest free pages from the dirty bitmap |
Date: |
Wed, 14 Mar 2018 22:38:19 +0200 |
On Wed, Mar 14, 2018 at 07:42:59PM +0000, Dr. David Alan Gilbert wrote:
> * Michael S. Tsirkin (address@hidden) wrote:
> > On Wed, Mar 14, 2018 at 06:11:37PM +0000, Dr. David Alan Gilbert wrote:
> > > > + used_len = block->used_length - offset;
> > > > + addr += used_len;
> > > > + }
> > > > +
> > > > + start = offset >> TARGET_PAGE_BITS;
> > > > + npages = used_len >> TARGET_PAGE_BITS;
> > > > + ram_state->migration_dirty_pages -=
> > > > + bitmap_count_one_with_offset(block->bmap, start,
> > > > npages);
> > > > + bitmap_clear(block->bmap, start, npages);
> > >
> > > If this is happening while the migration is running, this isn't safe -
> > > the migration code could clear a bit at about the same point this
> > > happens, so that the count returned by bitmap_count_one_with_offset
> > > wouldn't match the word that was cleared by bitmap_clear.
> > >
> > > The only way I can see to fix it is to run over the range using
> > > bitmap_test_and_clear_atomic, using the return value to decrement
> > > the number of dirty pages.
> > > But you also need to be careful with the update of the
> > > migration_dirty_pages value itself, because that's also being read
> > > by the migration thread.
> > >
> > > Dave
> >
> > I see that there's migration_bitmap_sync but it does not seem to be
>
> Do you mean bitmap_mutex?
Yes. Sorry.
> > taken on all paths. E.g. migration_bitmap_clear_dirty and
> > migration_bitmap_find_dirty are called without that lock sometimes.
> > Thoughts?
>
> Hmm, that doesn't seem to protect much at all! It looks like it was
> originally added to handle hotplug causing the bitmaps to be resized;
> that extension code was removed in 66103a5 so that lock can probably go.
>
> I don't see how the lock would help us though; the migration thread is
> scanning it most of the time so would have to have the lock held
> most of the time.
>
> Dave
>
> > --
> > MST
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK
[Qemu-devel] [PATCH v4 1/4] bitmap: bitmap_count_one_with_offset, Wei Wang, 2018/03/07
[Qemu-devel] [PATCH v4 4/4] migration: use the free page hint feature from balloon, Wei Wang, 2018/03/07