Re: [PATCH v2] mm/hugetlb: allow crashkernel to reuse hugetlb CMA memory

[Kiryl Shutsemau <kas@kernel.org>]

On Fri, Nov 06, 2026 at 08:51:42AM -0800, Usama Arif wrote:
> crashkernel=size,cma reserves a separate CMA pool. On systems that
> already reserve several gigabytes with hugetlb_cma= (which is common in
> the Meta fleet), the crash kernel cannot use that existing memory as
> part of its working set. With hugetlb_cma=6G, requesting another 2G
> with crashkernel=2G,cma brings the total CMA reservation to 8G, none of
> which can serve unmovable kernel allocations.
> 
> Use existing HugeTLB CMA memory first for crashkernel=size,cma and
> reserve dedicated CMA areas only for the remainder. With hugetlb_cma=6G,
> a 2G request reuses 2G without reserving more CMA. With hugetlb_cma=2G,
> a 4G request reuses 2G and reserves another 2G.
> 
> The full HugeTLB CMA pool remains available to HugeTLB and movable
> allocations during normal operation. After a crash, the capture kernel
> can use the selected ranges as working memory. The conventional
> reservation is still required to hold the kernel image and initrd.
> 
> The ,cma option already opts into excluding userspace memory from the
> vmcore. This includes allocated HugeTLB pages in the selected ranges,
> which the capture kernel can overwrite. The existing dump completeness
> and pending DMA risks apply to these ranges as well.
> 
> Have architecture code record the requested CMA size and perform the
> reservation in mm_core_init_early(), immediately after
> hugetlb_cma_reserve(). The HugeTLB CMA areas are then available for reuse
> while memblock can still reserve any remainder.
> 
> Use the existing four range slots for both reused and dedicated memory.
> Limit reused memory to the page-aligned request, account for dedicated
> CMA alignment, and report any shortfall. Ignore malformed optional sizes
> and reject requests that would overflow page rounding.
> 
> Suggested-by: Breno Leitao <leitao@debian.org>
> Signed-off-by: Usama Arif <usama.arif@linux.dev>

> ---
> v1 -> v2:
> - Use existing HugeTLB CMA memory first for crashkernel=size,cma and
>   reserve dedicated CMA areas only for the remainder. (Kiryl)

Suggested-by: Kiryl Shutsemau (Meta) <kas@kernel.org>
Acked-by: Kiryl Shutsemau (Meta) <kas@kernel.org>

-- 
  Kiryl Shutsemau / Kirill A. Shutemov