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Date: Wed, 12 Jun 2024 09:00:40 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hugh Dickins <hughd@google.com>
Cc: "Huang, Ying" <ying.huang@intel.com>, Zi Yan <ziy@nvidia.com>, Baolin
 Wang <baolin.wang@linux.alibaba.com>, David Hildenbrand <david@redhat.com>,
 linux-kernel@vger.kernel.org, linux-mm@kvack.org
Subject: Re: [PATCH hotfix] mm/migrate: fix kernel BUG at
 mm/compaction.c:2761!
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On Wed, 12 Jun 2024 00:11:44 -0700 (PDT) Hugh Dickins <hughd@google.com> wrote:

> > It appears better to add
> > 
> >         stats->nr_thp_failed++;
> > 
> > too.  Otherwise, if migrate_pages_batch() is called via migrate_pages(,
> > MIGRATE_ASYNC, ), nr_thp_failed will not increase.  But if
> > migrate_pages_batch() is called via migrate_pages(, MIGRATE_SYNC*, ),
> > nr_thp_failed will increase in migrate_pages_sync() via
> > 
> >         stats->nr_thp_failed += astats.nr_thp_split;
> > 
> > That is, they are not consistent.  The issue exists since commit
> > 7262f208ca68 ("mm/migrate: split source folio if it is on deferred split
> > list").
> 
> Sorry, I'll have to let you take over and send your own patch instead
> of mine - thanks. Those stats, and any attempt at consistency there,
> is way beyond me! I thought consistency was impossible, unless all
> the numbers were changed to order-0 counts.

I'll add this patch as-is for now.