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

From: Oscar Salvador <osalvador@suse.de>
To: David Hildenbrand <david@redhat.com>
Cc: Michal Hocko <mhocko@suse.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
Date: Mon, 7 Jun 2021 12:23:25 +0200	[thread overview]
Message-ID: <20210607102318.GA12683@linux> (raw)
In-Reply-To: <85984701-55ae-dfa5-2a8d-f637051b612d@redhat.com>

On Mon, Jun 07, 2021 at 10:49:01AM +0200, David Hildenbrand wrote:
> I'd like to point out that I think the seqlock is not in place to
> synchronize with actual growing/shrinking but to get consistent zone ranges
> -- like using atomics, but we have two inter-dependent values here.

I guess so, at least that's what it should do.
But the way it is placed right now is misleading.

If we really want to get consistent zone ranges, we should start using
zone's seqlock where it matters and that is pretty much all those
places that use zone_spans_pfn().
Otherwise there is no way you can be sure the pfn you're checking is
within the limits. Moreover, as Michal pointed out early, if we really
want to go down that road the locking should be made in the caller
evolving the operation, otheriwse things might change once the lock
is dropped and you're working with a wrong assumption.

I can see arguments for both riping it out and doing it right (but none for
the way it is right now).
For riping it out, one could say that those races might not be fatal,
as usually the pfn you're working with (the one you want to check falls
within a certain range) you know is valid, so the worst can happen is
you get false positives/negatives and that might or might not be detected
further down. How bad are false positive/negatives I guess it depends on the
situation, but we already do that right now.
The zone_spans_pfn() from page_outside_zone_boundaries() is the only one using
locking right now, so well, if we survided this long without locks in other places
using zone_spans_pfn() makes one wonder if it is that bad.

On the other hand, one could argue that for correctness sake, we should be holding
zone's seqlock whenever checking for zone_spans_pfn() to avoid any inconsistency.

-- 
Oscar Salvador
SUSE L3