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
