[RFC 1/2] cma: Fix CMA_MIN_ALIGNMENT_BYTES during early_init

[Ritesh Harjani (IBM)]

David Hildenbrand <david at redhat.com> writes:

> On 08.10.24 15:27, Ritesh Harjani (IBM) wrote:
>> During early init CMA_MIN_ALIGNMENT_BYTES can be PAGE_SIZE,
>> since pageblock_order is still zero and it gets initialized
>> later during paging_init() e.g.
>> paging_init() -> free_area_init() -> set_pageblock_order().
>> 
>> One such use case is -
>> early_setup() -> early_init_devtree() -> fadump_reserve_mem()
>> 
>> This causes CMA memory alignment check to be bypassed in
>> cma_init_reserved_mem(). Then later cma_activate_area() can hit
>> a VM_BUG_ON_PAGE(pfn & ((1 << order) - 1)) if the reserved memory
>> area was not pageblock_order aligned.
>> 
>> Instead of fixing it locally for fadump case on PowerPC, I believe
>> this should be fixed for CMA_MIN_ALIGNMENT_BYTES.
>
> I think we should add a way to catch the usage of 
> CMA_MIN_ALIGNMENT_BYTES before it actually has meaning (before 
> pageblock_order was set)

Maybe by enforcing that the pageblock_order should not be zero where we
do the alignment check then?

i.e. in cma_init_reserved_mem() 

diff --git a/mm/cma.c b/mm/cma.c
index 3e9724716bad..36d753e7a0bf 100644
--- a/mm/cma.c
+++ b/mm/cma.c
@@ -182,6 +182,15 @@ int __init cma_init_reserved_mem(phys_addr_t base, phys_addr_t size,
        if (!size || !memblock_is_region_reserved(base, size))
                return -EINVAL;

+       /*
+        * CMA uses CMA_MIN_ALIGNMENT_BYTES as alignment requirement which
+        * needs pageblock_order to be initialized. Let's enforce it.
+        */
+       if (!pageblock_order) {
+               pr_err("pageblock_order not yet initialized. Called during early boot?\n");
+               return -EINVAL;
+       }
+
        /* ensure minimal alignment required by mm core */
        if (!IS_ALIGNED(base | size, CMA_MIN_ALIGNMENT_BYTES))
                return -EINVAL;


> and fix the PowerPC usage by reshuffling the 
> code accordingly.

Ok. I will submit a v2 with the above patch incldued.

Thanks for the review!
-ritesh