linux-kernel - Re: [PATCH] mm/migrate: fix BUG_ON in migrate_misplaced_folio() and compact

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Date: Mon, 17 Jun 2024 12:25:20 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Zi Yan <ziy@...dia.com>
Cc: Zi Yan <zi.yan@...t.com>, Hugh Dickins <hughd@...gle.com>, "Huang, Ying"
 <ying.huang@...el.com>, linux-mm@...ck.org, oe-lkp@...ts.linux.dev,
 lkp@...el.com, kernel test robot <oliver.sang@...el.com>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Yang Shi
 <shy828301@...il.com>, David Hildenbrand <david@...hat.com>, Yin Fengwei
 <fengwei.yin@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/migrate: fix BUG_ON in migrate_misplaced_folio() and
 compact_zone()

On Mon, 17 Jun 2024 10:39:25 -0400 Zi Yan <zi.yan@...t.com> wrote:

> From: Zi Yan <ziy@...dia.com>
> 
> Both Hugh Dickins and kernel test bot hit an issue that migrate_pages()
> reports success but still leaves unmigrated pages on from list. It
> triggers BUG_ON in migrate_misplaced_folio() and compact_zone().
> 
> Our convention is that if migrate_pages() reports complete success (0),
> then the migratepages list will be empty; but if it reports an error or
> some pages remaining, then its caller must putback_movable_pages().
> 
> There's a new case in which migrate_pages() has been reporting complete
> success, but returning with pages left on the migratepages list: when
> migrate_pages_batch() successfully split a folio on the deferred list,
> but then the "Failure isn't counted" call does not dispose of them all.
> 
> Since that block is expecting the large folio to have been counted as 1
> failure already, and since the return code is later adjusted to success
> whenever the returned list is found empty, the simple way to fix this
> safely is to count splitting the deferred folio as "a failure".
> 
> This patch is based on Hugh's fix and suggestions from Huang, Ying.
> 

I have just sent Hugh's version in to Linus.  So if/when that is
merged, please send along any needed updates.