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

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Date:   Fri, 13 Dec 2019 11:08:06 +0000
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     David Hildenbrand <david@...hat.com>
Cc:     Alexander Duyck <alexander.duyck@...il.com>, kvm@...r.kernel.org,
        mst@...hat.com, linux-kernel@...r.kernel.org, willy@...radead.org,
        mhocko@...nel.org, linux-mm@...ck.org, akpm@...ux-foundation.org,
        vbabka@...e.cz, yang.zhang.wz@...il.com, nitesh@...hat.com,
        konrad.wilk@...cle.com, pagupta@...hat.com, riel@...riel.com,
        lcapitulino@...hat.com, dave.hansen@...el.com,
        wei.w.wang@...el.com, aarcange@...hat.com, pbonzini@...hat.com,
        dan.j.williams@...el.com, alexander.h.duyck@...ux.intel.com,
        osalvador@...e.de
Subject: Re: [PATCH v15 0/7] mm / virtio: Provide support for free page
 reporting

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.

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