Re: [PATCH v2] mm/cma: simplify cma_declare_contiguous_multi()

["Oscar Salvador (SUSE)" <osalvador@kernel.org>]

On Wed, Nov 11, 2026 at 12:14:40AM +0200, Mike Rapoport (Microsoft) wrote:
> When cma_declare_contiguous_multi() cannot allocate a CMA area as a
> single range, it falls back to collecting a list of up to CMA_MAX_RANGES
> largest free ranges above 4G, reserves them in memblock and adds them to
> cma->ranges.
> 
> The list and subsequently cma->ranges are supposed to be sorted in the
> order of base addresses of the "allocated" ranges.
> 
> This implementation has two bugs:
> 
> * The allocation may fail even when there is enough free memory because
>   list_insert_sorted() sorts the list in the wrong order.
> 
>   When the list is full, the largest range is evicted rather than the
>   smallest, the final list is not sorted by address, and the smallest
>   ranges are used first.
> 
> * the function may report success while it reserved less memory than
>   requested because sizesum accounts for ranges that were skipped.
> 
> And overall it is a lot of list management and open coded range
> selection for no real gain.
> 
> Replace the current code with a loop that finds the largest free range
> above 4G, reserves as much of it as needed and repeats until the entire
> area is reserved or CMA_MAX_RANGES ranges are used.
> 
> After successful alocation sort the cma->ranges by their base address.
> 
> The new code scans free memory once for every range rather than once
> for all ranges, so it is slightly slower, but it is way simpler and more
> robust.
> 
> Fixes: c009da4258f9 ("mm, cma: support multiple contiguous ranges, if requested")
> Reported-by: Ivy Lopez <skunkolee@gmail.com>
> Closes: https://lore.kernel.org/all/cover.1792032699.git.skunkolee@gmail.com
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>

Reviewed-by: Oscar Salvador (SUSE) <osalvador@kernel.org>

 

-- 
Oscar Salvador
SUSE Labs