Re: [PATCH] mm/cma: make detection of highmem_start more robust

[Pratyush Yadav <ptyadav@amazon.de>]

Hi Mike,

On Mon, May 19 2025, Mike Rapoport wrote:

> From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
>
> Pratyush Yadav reports the following crash:
>
>     ------------[ cut here ]------------
>     kernel BUG at arch/x86/mm/physaddr.c:23!
>     ception 0x06 IP 10:ffffffff812ebbf8 error 0 cr2 0xffff88903ffff000
>     CPU: 0 UID: 0 PID: 0 Comm: swapper Not tainted 6.15.0-rc6+ #231 PREEMPT(undef)
>     Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
>     RIP: 0010:__phys_addr+0x58/0x60
>     Code: 01 48 89 c2 48 d3 ea 48 85 d2 75 05 e9 91 52 cf 00 0f 0b 48 3d ff ff ff 1f 77 0f 48 8b 05 20 54 55 01 48 01 d0 e9 78 52 cf 00 <0f> 0b 90 0f 1f 44 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90
>     RSP: 0000:ffffffff82803dd8 EFLAGS: 00010006 ORIG_RAX: 0000000000000000
>     RAX: 000000007fffffff RBX: 00000000ffffffff RCX: 0000000000000000
>     RDX: 000000007fffffff RSI: 0000000280000000 RDI: ffffffffffffffff
>     RBP: ffffffff82803e68 R08: 0000000000000000 R09: 0000000000000000
>     R10: ffffffff83153180 R11: ffffffff82803e48 R12: ffffffff83c9aed0
>     R13: 0000000000000000 R14: 0000001040000000 R15: 0000000000000000
>     FS:  0000000000000000(0000) GS:0000000000000000(0000) knlGS:0000000000000000
>     CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>     CR2: ffff88903ffff000 CR3: 0000000002838000 CR4: 00000000000000b0
>     Call Trace:
>      <TASK>
>      ? __cma_declare_contiguous_nid+0x6e/0x340
>      ? cma_declare_contiguous_nid+0x33/0x70
>      ? dma_contiguous_reserve_area+0x2f/0x70
>      ? setup_arch+0x6f1/0x870
>      ? start_kernel+0x52/0x4b0
>      ? x86_64_start_reservations+0x29/0x30
>      ? x86_64_start_kernel+0x7c/0x80
>      ? common_startup_64+0x13e/0x141
>
>   The reason is that __cma_declare_contiguous_nid() does:
>
>           highmem_start = __pa(high_memory - 1) + 1;
>
>   If dma_contiguous_reserve_area() (or any other CMA declaration) is
>   called before free_area_init(), high_memory is uninitialized. Without
>   CONFIG_DEBUG_VIRTUAL, it will likely work but use the wrong value for
>   highmem_start.
>
> The issue occurs because commit e120d1bc12da ("arch, mm: set high_memory in
> free_area_init()") moved initialization of high_memory after the call to
> dma_contiguous_reserve() -> __cma_declare_contiguous_nid() on several
> architectures.
>
> In the case CONFIG_HIGHMEM is enabled, some architectures that actually
> support HIGHMEM (arm, powerpc and x86) have initialization of high_memory
> before a possible call to __cma_declare_contiguous_nid() and some
> initialized high_memory late anyway (arc, csky, microblase, mips, sparc,
> xtensa) even before the commit e120d1bc12da so they are fine with using
> uninitialized value of high_memory.

I don't know if they are fine or they haven't realized this is a bug
yet. Either way, this patch fixes the crash for me on x86_64, restores
how it should behave, and doesn't seem to make anything worse, so:

Tested-by: Pratyush Yadav <ptyadav@amazon.de>

Thanks for fixing this!

>
> And in the case CONFIG_HIGHMEM is disabled high_memory essentially becomes
> the first address after memory end, so instead of relying on high_memory to
> calculate highmem_start use memblock_end_of_DRAM() and eliminate the
> dependency of CMA area creation on high_memory in majority of
> configurations.
>
> Reported-by: Pratyush Yadav <ptyadav@amazon.de>
> Tested-by: Alexandre Ghiti <alexghiti@rivosinc.com>
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> ---
>  mm/cma.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/mm/cma.c b/mm/cma.c
> index 15632939f20a..c04be488b099 100644
> --- a/mm/cma.c
> +++ b/mm/cma.c
> @@ -608,7 +608,10 @@ static int __init __cma_declare_contiguous_nid(phys_addr_t *basep,
>  	 * complain. Find the boundary by adding one to the last valid
>  	 * address.
>  	 */
> -	highmem_start = __pa(high_memory - 1) + 1;
> +	if (IS_ENABLED(CONFIG_HIGHMEM))
> +		highmem_start = __pa(high_memory - 1) + 1;
> +	else
> +		highmem_start = memblock_end_of_DRAM();
>  	pr_debug("%s(size %pa, base %pa, limit %pa alignment %pa)\n",
>  		__func__, &size, &base, &limit, &alignment);

-- 
Regards,
Pratyush Yadav



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