Re: [PATCH 0/9] disable pcplists during memory offline

David Hildenbrand
2020-09-22T17:16:11Z

On 22.09.20 16:37, Vlastimil Babka wrote:
> As per the discussions [1] [2] this is an attempt to implement David's
> suggestion that page isolation should disable pcplists to avoid races with page
> freeing in progress. This is done without extra checks in fast paths, as
> explained in Patch 9. The repeated draining done by [2] is then no longer
> needed. Previous version (RFC) is at [3].
>
> The RFC tried to hide pcplists disabling/enabling into page isolation, but it
> wasn't completely possible, as memory offline does not unisolation. Michal
> suggested an explicit API in [4] so that's the current implementation and it
> seems indeed nicer.
>
> Once we accept that page isolation users need to do explicit actions around it
> depending on the needed guarantees, we can also IMHO accept that the current
> pcplist draining can be also done by the callers, which is more effective.
> After all, there are only two users of page isolation. So patch 7 does
> effectively the same thing as Pavel proposed in [5], and patches 8-9 implement
> stronger guarantees only for memory offline. If CMA decides to opt-in to the
> stronger guarantee, it's easy to do so.
>
> Patches 1-6 are preparatory cleanups for pcplist disabling.
>
> Patchset was briefly tested in QEMU so that memory online/offline works, but
> I haven't done a stress test that would prove the race fixed by [2] is
> eliminated.
>
> Note that patch 9 could be avoided if we instead adjusted page freeing in shown
> in [6], but I believe the current implementation of disabling pcplists is not
> too much complex, so I would prefer this instead of adding new checks and
> longer irq-disabled section into page freeing hotpaths.

Haven't looked into the details (yet), but I assume we can add some flag
to alloc_contig_range(), to also disable+flush+enable. (or let the
caller do it, for example on a bunch of bulk allocations - TBD).

Result of patch #9 looks quite clean.

--
Thanks,

David / dhildenb