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

[Lecopzer Chen]

Hi Mel,

Of course, because we only test for CMA and only make sure that
CMA part is wrong.
You patch is definitly better.
Please go ahead with your patch, thank you.

> 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")
> 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);
>  }