Re: [RFC 1/2] Protect larger order pages from breaking up

[Michal Hocko <mhocko@kernel.org>]

On Mon 19-02-18 10:19:35, Mel Gorman wrote:
[...]
> Access to the pool is unprotected so you might create a reserve for jumbo
> frames only to have them consumed by something else entirely. It's not
> clear if that is even fixable as GFP flags are too coarse.
> 
> It is not covered in the changelog why MIGRATE_HIGHATOMIC was not
> sufficient for jumbo frames which are generally expected to be allocated
> from atomic context. If there is a problem there then maybe
> MIGRATE_HIGHATOMIC should be made more strict instead of a hack like
> this. It'll be very difficult, if not impossible, for this to be tuned
> properly.
> 
> Finally, while I accept that fragmentation over time is a problem for
> unmovable allocations (fragmentation protection was originally designed
> for THP/hugetlbfs), this is papering over the problem. If greater
> protections are needed then the right approach is to be more strict about
> fallbacks. Specifically, unmovable allocations should migrate all movable
> pages out of migrate_unmovable pageblocks before falling back and that
> can be controlled by policy due to the overhead of migration. For atomic
> allocations, allow fallback but use kcompact or a workqueue to migrate
> movable pages out of migrate_unmovable pageblocks to limit fallbacks in
> the future.

Completely agreed!

> I'm not a fan of this patch.

Yes, I think the approach is just wrong. It will just hit all sorts of
weird corner cases and won't work reliable for those who care.
-- 
Michal Hocko
SUSE Labs

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