Re: [PATCH v15 0/7] mm / virtio: Provide support for free page reporting

[Alexander Duyck <alexander.h.duyck@linux.intel.com>]

On Fri, 2019-12-13 at 11:08 +0000, Mel Gorman wrote:
> On Fri, Dec 13, 2019 at 11:00:42AM +0100, David Hildenbrand wrote:
> > > A brief history on the background of free page reporting can be found at:
> > > https://lore.kernel.org/lkml/29f43d5796feed0dec8e8bb98b187d9dac03b900.camel@linux.intel.com/
> > > 
> > > Changes from v13:
> > > https://lore.kernel.org/lkml/20191105215940.15144.65968.stgit@localhost.localdomain/
> > > Rewrote core reporting functionality
> > >   Merged patches 3 & 4
> > >   Dropped boundary list and related code
> > >   Folded get_reported_page into page_reporting_fill
> > >   Folded page_reporting_fill into page_reporting_cycle
> > > Pulled reporting functionality out of free_reported_page
> > >   Renamed it to __free_isolated_page
> > >   Moved page reporting specific bits to page_reporting_drain
> > > Renamed phdev to prdev since we aren't "hinting" we are "reporting"
> > > Added documentation to describe the usage of unused page reporting
> > > Updated cover page and patch descriptions to avoid mention of boundary
> > > 
> > > Changes from v14:
> > > https://lore.kernel.org/lkml/20191119214454.24996.66289.stgit@localhost.localdomain/
> > > Renamed "unused page reporting" to "free page reporting"
> > >   Updated code, kconfig, and patch descriptions
> > > Split out patch for __free_isolated_page
> > >   Renamed function to __putback_isolated_page
> > > Rewrote core reporting functionality
> > >   Added logic to reschedule worker in 2 seconds instead of run to completion
> > >   Removed reported_pages statistics
> > >   Removed REPORTING_REQUESTED bit used in zone flags
> > >   Replaced page_reporting_dev_info refcount with state variable
> > >   Removed scatterlist from page_reporting_dev_info
> > >   Removed capacity from page reporting device
> > >   Added dynamic scatterlist allocation/free at start/end of reporting process
> > >   Updated __free_one_page so that reported pages are not always added to tail
> > >   Added logic to handle error from report function
> > > Updated virtio-balloon patch that adds support for page reporting
> > >   Updated patch description to try and highlight differences in approaches
> > >   Updated logic to reflect that we cannot limit the scatterlist from device
> > 
> > Last time Mel said
> > 
> > "Ok, I'm ok with how this hooks into the allocator as the overhead is
> > minimal. However, the patch itself still includes a number of
> > optimisations instead of being a bare-boned implementation of the
> > feature with optimisations layered on top."
> > 
> 
> I didn't get the chance to take a close look as I'm trying to clear as
> much as possible from my table on the run-up to Christmas so I don't come
> back to a disaster inbox. I also noted that the Acks for earlier patches
> were not included so I was uncertain if doing a full review would still
> be a good use of time when time was tight.

Sorry about that. I will go back through and make sure to collect the Acks
on the earlier patches. I guess I had overlooked them while focusing on
rewriting the core functionality.

> That said, some optimisations are still included but much reduced. For
> example, list rotations are still there but it's very straight-forward.

I will go ahead and split the rotations out into a separate patch for v16.
I can probably do that and pull the budget bit I had added out and put it
together as a "work conserving/limiting" optimization for the patch set.

> The refcount is gone which is good and replaced by a state, which could be
> be better documented, but is more straight forward and the zone->lock is
> back protecting the free lists primarily and not zone metadata or prdev
> metadata (at least not obviously). I didn't put in the time to see if
> the atomic_set in page_reporting_process() is ok or whether state could
> be lost but I *think* it's ok because it should be called from just one
> workqueue request and they shouldn't be stacked. A comment there explaining
> why atomic_set is definitely correct would be helpful.

I will go though and add some more documentation about the state.

> I'm inclined to decide that yes, this version is potentially ok as a
> bare minimum but didn't put in the time to be 100% sure.

Sounds good. I will go through and address the concerns you brought up,
and probably post a v16 by the end of next week.

Thanks for the feedback.

- Alex