Re: [HMM-v25 13/19] mm/migrate: new migrate mode MIGRATE_SYNC_NO_COPY

[Jerome Glisse]

On Thu, Aug 17, 2017 at 02:12:45PM -0700, Andrew Morton wrote:
> On Wed, 16 Aug 2017 20:05:42 -0400 J__r__me Glisse <jglisse@xxxxxxxxxx> wrote:
> 
> > Introduce a new migration mode that allow to offload the copy to
> > a device DMA engine. This changes the workflow of migration and
> > not all address_space migratepage callback can support this. So
> > it needs to be tested in those cases.
> 
> Can you please expand on this?  What additional testing must be
> performed before we are able to merge this into mainline?
> 

No additional testing needed. I disable MIGRATE_SYNC_NO_COPY in all
problematic migratepage() callback and i added comment in those to
explain why (part of this patch). The commit message is unclear it
should say that any callback that wish to support this new mode need
to be aware of the difference in the migration flow from other mode.

Some of this callback do extra locking while copying (aio, zsmalloc,
balloon, ...) and for DMA to be effective you want to copy multiple
pages in one DMA operations. But in the problematic case you can not
easily hold the extra lock accross multiple call to this callback.

Usual flow is:

For each page {
 1 - lock page
 2 - call migratepage() callback
 3 - (extra locking in some migratepage() callback)
 4 - migrate page state (freeze refcount, update page cache, buffer
     head, ...)
 5 - copy page
 6 - (unlock any extra lock of migratepage() callback)
 7 - return from migratepage() callback
 8 - unlock page
}

The new mode MIGRATE_SYNC_NO_COPY:
 1 - lock multiple pages
For each page {
 2 - call migratepage() callback
 3 - abort in all problematic migratepage() callback
 4 - migrate page state (freeze refcount, update page cache, buffer
     head, ...)
} // finished all calls to migratepage() callback
 5 - DMA copy multiple pages
 6 - unlock all the pages

To support MIGRATE_SYNC_NO_COPY in the problematic case we would
need a new callback migratepages() (for instance) that deals with
multiple pages in one transaction.

Because the problematic cases are not important for current usage
i did not wanted to complexify this patchset even more for no good
reasons.

I hope this clarify, the commit message and comment in migrate.h
can probably use this extra description i just gave.

Cheers,
Jérôme