Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get stable zone's values

[Oscar Salvador]

On Thu, Jun 03, 2021 at 02:45:13PM +0200, Michal Hocko wrote:
> I believe we need to define the purpose of the locking first. The

If you ask me, this locking would be meant to make sure zone's zone_start_pfn
or spanned_pages do not change under us, in case we __need__ the value to be
stable.

> existing locking doesn't serve much purpose, does it? The state might

Well, half-way. Currently, the locking is taken in write mode whenever
the zone is expanded or shrinked, and in read mode when called from
bad_range()->page_outside_zone_boundaries() (only on VM_DEBUG).

But as you pointed out, such state might change right after the locking is
released and all the work would be for nothing.
So indeed, the __whole__ operation should be envolved by the lock in the caller
The way that stands right now is not optimal.

> change right after the lock is released and the caller cannot really
> rely on the result. So aside of the current implementation, I would
> argue that any locking has to be be done on the caller layer.
> 
> But the primary question is whether anybody actually cares about
> potential races in the first place.

I have been checking move_freepages_block() and alloc_contig_pages(), which
are two of the functions that call zone_spans_pfn().

move_freepages_block() uses it in a way to align the given pfn to pageblock
top and bottom, and then check that aligned pfns are still within the same zone.
>From a memory-hotplug perspective that's ok as we know that we are offlining
PAGES_PER_SECTION (which implies whole pageblocks).

alloc_contig_pages() (used by the hugetlb gigantic allocator) runs through a
node's zonelist and checks whether zone->zone_start_pfn + nr_pages stays within
the same zone.
IMHO, the race with zone_spans_last_pfn() vs mem-hotplug would not be that bad,
as it will be caught afters by e.g: __alloc_contig_pages when pages cannot be
isolated because they are offline etc.

So, I would say we do not really need the lock, but I might be missing something.
But if we chose to care about this, then the locking should be done right, not
half-way as it is right now.


-- 
Oscar Salvador
SUSE L3