On 28.05.21 12:09, Mel Gorman wrote:
On Fri, May 28, 2021 at 11:52:53AM +0200, David Hildenbrand wrote:
"Disable pcplists so that page isolation cannot race with freeing
in a way that pages from isolated pageblock are left on pcplists."
Guess we'd then want to move the draining before start_isolate_page_range()
in alloc_contig_range().
Or instead of draining, validate the PFN range in alloc_contig_range
is within the same zone and if so, call zone_pcp_disable() before
start_isolate_page_range and enable after __alloc_contig_migrate_range.
We require the caller to only pass a range within a single zone, so that
should be fine.
The only ugly thing about zone_pcp_disable() is
mutex_lock(&pcp_batch_high_lock) which would serialize all
alloc_contig_range() and even with offline_pages().
True so it would have to be accessed if that is bad or not. If racing
against offline_pages, memory is potentially being offlined in the
target zone which may cause allocation failure. If racing with other
alloc_contig_range calls, the two callers are potentially racing to
isolate and allocate the same range. The arguement could be made that
alloc_contig_ranges should be serialised within one zone to improve the
allocation success rate at the potential cost of allocation latency.
We have 3 main users of alloc_contig_range():
1. CMA
CMA synchronizes allocation within a CMA area via the allocation bitmap.
So parallel CMA is perfectly possible and avoids races by design.
2. alloc_contig_pages() / Gigantic pages
Gigantic page allocation could race with virtio-mem. CMA does not apply.
Possible but unlikely to matter in practice. virtio-mem will retry later
again.
3. virito-mem
A virtio-mem device only operates on its assigned memory region, so we
cannot have alloc_contig_range() from different devices racing, even
when within a single zone. It could only race with gigantic pages as CMA
does not apply.
So serializing would mostly harm parallel CMA (possible and likely) and
parallel virtio-mem operation (e.g., unplug memory of two virtio-mem
devices -- unlikely but possible).
Memory offlining racing with CMA is not an issue (impossible).
virtio-mem synchronizes with memory offlining via memory notifiers,
there is only a tiny little race window that usually doesn't matter as
virtio-mem is expected to usually triggers offlining itself, and not
user space rancomly. Memory offlining can race with dynamic gigantic
page allocation, wich is highly unreliable already.
I wonder if we could optimize locking in zone_pcp_disable() instead.
--
Thanks,
David / dhildenb
