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Date:   Mon, 7 Jun 2021 09:52:16 +0200
From:   Oscar Salvador <osalvador@suse.de>
To:     Michal Hocko <mhocko@suse.com>
Cc:     David Hildenbrand <david@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Pavel Tatashin <pasha.tatashin@soleen.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 1/3] mm,page_alloc: Use {get,put}_online_mems() to get
 stable zone's values
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On Fri, Jun 04, 2021 at 09:41:45AM +0200, Oscar Salvador wrote:
> 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.


Any thoughts on this? :-)

 
-- 
Oscar Salvador
SUSE L3