Re: [PATCH] mm/page_alloc: fix memalloc_nocma_{save/restore} APIs

[Vlastimil Babka]

On 7/21/20 5:28 AM, js1304@xxxxxxxxx wrote:
> From: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
> 
> Currently, memalloc_nocma_{save/restore} API that prevents CMA area
> in page allocation is implemented by using current_gfp_context(). However,
> there are two problems of this implementation.
> 
> First, this doesn't work for allocation fastpath. In the fastpath,
> original gfp_mask is used since current_gfp_context() is introduced in
> order to control reclaim and it is on slowpath. So, CMA area can be
> allocated through the allocation fastpath even if
> memalloc_nocma_{save/restore} APIs are used. Currently, there is just
> one user for these APIs and it has a fallback method to prevent actual
> problem.
> Second, clearing __GFP_MOVABLE in current_gfp_context() has a side effect
> to exclude the memory on the ZONE_MOVABLE for allocation target.
> 
> To fix these problems, this patch changes the implementation to exclude
> CMA area in page allocation. Main point of this change is using the
> alloc_flags. alloc_flags is mainly used to control allocation so it fits
> for excluding CMA area in allocation.

Moreover, the ALLOC_CMA flag already exists for exactly this purpose.

> Fixes: d7fefcc8de91 (mm/cma: add PF flag to force non cma alloc)
> Cc: <stable@xxxxxxxxxxxxxxx>
> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>

Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>

Thanks!