Re: [PATCH] mm: page_alloc: fix cma pageblock was stolen in rmqueue fallback

[Mel Gorman]

On Tue, Sep 05, 2023 at 11:37:30AM +0200, Vlastimil Babka wrote:
> On 9/5/23 11:09, Mel Gorman wrote:
> > On Wed, Aug 30, 2023 at 07:13:33PM +0800, Lecopzer Chen wrote:
> >> commit 4b23a68f9536 ("mm/page_alloc: protect PCP lists with a
> >> spinlock") fallback freeing page to free_one_page() if pcp trylock
> >> failed. This make MIGRATE_CMA be able to fallback and be stolen
> >> whole pageblock by MIGRATE_UNMOVABLE in the page allocation.
> >> 
> >> PCP free is fine because free_pcppages_bulk() will always get
> >> migratetype again before freeing the page, thus this only happen when
> >> someone tried to put CMA page in to other MIGRATE_TYPE's freelist.
> >> 
> >> Fixes: 4b23a68f9536 ("mm/page_alloc: protect PCP lists with a spinlock")
> >> Reported-by: Joe Liu <joe.liu@xxxxxxxxxxxx>
> >> Signed-off-by: Lecopzer Chen <lecopzer.chen@xxxxxxxxxxxx>
> >> Cc: Mark-pk Tsai <mark-pk.tsai@xxxxxxxxxxxx>
> >> Cc: Joe Liu <joe.liu@xxxxxxxxxxxx>
> > 
> > Sorry for the long delay and thanks Lecopzer for the patch.
> > 
> > This changelog is difficult to parse but the fix may also me too specific
> > and could be more robust against types other than CMA. It is true that
> > a failed PCP acquire may return a !is_migrate_isolate page to the wrong
> > list but it's more straight-forward to unconditionally lookup the PCP
> > migratetype of the spinlock is not acquired.
> > 
> > How about this? It unconditionally looks up the PCP migratetype after
> > spinlock contention. It's build tested only
> > 
> > --8<--
> > mm: page_alloc: Free pages to correct buddy list after PCP lock contention
> > 
> > Commit 4b23a68f9536 ("mm/page_alloc: protect PCP lists with a spinlock")
> > returns pages to the buddy list on PCP lock contention. However, for
> > migratetypes that are not MIGRATE_PCPTYPES, the migratetype may have
> > been clobbered already for pages that are not being isolated. In
> > practice, this means that CMA pages may be returned to the wrong
> > buddy list. While this might be harmless in some cases as it is
> > MIGRATE_MOVABLE, the pageblock could be reassigned in rmqueue_fallback
> > and prevent a future CMA allocation. Lookup the PCP migratetype
> > against unconditionally if the PCP lock is contended.
> > 
> > [lecopzer.chen@xxxxxxxxxxxx: CMA-specific fix]
> > Fixes: 4b23a68f9536 ("mm/page_alloc: protect PCP lists with a spinlock")
> 
> I think we should Cc: stable for the sake of 6.1 LTS?
> 

Yep.

> > Reported-by: Joe Liu <joe.liu@xxxxxxxxxxxx>
> > Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> 
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
> 

Thanks.

-- 
Mel Gorman
SUSE Labs