Re: [PATCH] mm/migrate: fix BUG_ON in migrate_misplaced_folio() and compact_zone()

[Andrew Morton]

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

> From: Zi Yan <ziy@xxxxxxxxxx>
> 
> 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.