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

[Johannes Weiner]

On Tue, Sep 05, 2023 at 10:09:22AM +0100, Mel Gorman wrote:
> 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")
> Reported-by: Joe Liu <joe.liu@xxxxxxxxxxxx>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> ---
>  mm/page_alloc.c | 8 +++++++-
>  1 file changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 452459836b71..4053c377fee8 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -2428,7 +2428,13 @@ void free_unref_page(struct page *page, unsigned int order)
>  		free_unref_page_commit(zone, pcp, page, migratetype, order);
>  		pcp_spin_unlock(pcp);
>  	} else {
> -		free_one_page(zone, page, pfn, order, migratetype, FPI_NONE);
> +		/*
> +		 * The page migratetype may have been clobbered for types
> +		 * (type >= MIGRATE_PCPTYPES && !is_migrate_isolate) so
> +		 * must be rechecked.
> +		 */
> +		free_one_page(zone, page, pfn, order,
> +			      get_pcppage_migratetype(page), FPI_NONE);
>  	}
>  	pcp_trylock_finish(UP_flags);
>  }
> 

I had sent a (similar) fix for this here:

https://lore.kernel.org/lkml/20230821183733.106619-4-hannes@xxxxxxxxxxx/

The context wasn't CMA, but HIGHATOMIC pages going to the movable
freelist. But the class of bug is the same: the migratetype tweaking
really only applies to the pcplist, not the buddy slowpath; I added a
local pcpmigratetype to make it more clear, and hopefully prevent bugs
of this nature down the line.

I'm just preparing v2 of the above series. Do you want me to break
this change out and send it separately?