On 21.04.23 19:14, Matthew Wilcox wrote:
On Fri, Apr 21, 2023 at 05:11:56PM +0100, Mel Gorman wrote:
It was considered once upon a time and comes up every so often as variants
of a "sticky" pageblock pageblock bit that prevents mixing. The risks was
ending up in a context where memory within a suitable pageblock cannot
be freed and all of the available MOVABLE pageblocks have at least one
pinned page that cannot migrate from the allocating context. It can also
potentially hit a case where the majority of memory is UNMOVABLE pageblocks,
each of which has a single pagetable page that cannot be freed without an
OOM kill. Variants of issues like this would manifestas an OOM kill with
plenty of memory free bug or excessive CPu usage on reclaim or compaction.
It doesn't kill the idea of the series at all but it puts a lot of emphasis
in splitting the series by low-risk and high-risk. Maybe to the extent where
the absolute protection against mixing can be broken in OOM situations,
kernel command line or sysctl.
Has a variant been previously considered where MOVABLE allocations are
allowed to come from UNMOVABLE blocks? After all, MOVABLE allocations
are generally, well, movable. So an UNMOVABLE allocation could try to
migrate pages from a MIXED pageblock in order to turn the MIXED pageblock
back into an UNMOVABLE pageblock.
I might be completely off, but my understanding was that movable
allocations can be happily placed into unmovable blocks if required already?
IIRC, it's primarily the zone fallback rules that prevent e.g., ZONE_DMA
to get filled immediately with movable data in your example. I might eb
wrong, though.
I guess what you mean is serving movable allocations much earlier from
these other zones.
Having memory hotunplug in mind ( as always ;) ), I'd expect that such
fragmentation must be allowed to happen to guarantee that memory (esp.
ZONE_MOVABLE) can be properly evacuated even if there are not sufficient
MOVABLE pageblocks around to hold all that (movable) data.
--
Thanks,
David / dhildenb
