在 2022/7/4 21:15, Will Deacon 写道:
On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote:
在 2022/7/4 19:14, Will Deacon 写道:
On Mon, Jul 04, 2022 at 06:58:20PM +0800, guanghui.fgh wrote:
在 2022/7/4 18:35, Will Deacon 写道:
On Sat, Jul 02, 2022 at 11:57:53PM +0800, Guanghui Feng wrote:
The arm64 can build 2M/1G block/sectiion mapping. When using DMA/DMA32 zone
(enable crashkernel, disable rodata full, disable kfence), the mem_map will
use non block/section mapping(for crashkernel requires to shrink the region
in page granularity). But it will degrade performance when doing larging
continuous mem access in kernel(memcpy/memmove, etc).
Hmm. It seems a bit silly to me that we take special care to unmap the
crashkernel from the linear map even when can_set_direct_map() is false, as
we won't be protecting the main kernel at all!
Why don't we just leave the crashkernel mapped if !can_set_direct_map()
and then this problem just goes away?
This question had been asked lask week.
Sorry, I didn't spot that. Please could you link me to the conversation, as
I'm still unable to find it in my inbox?
Please access this link:
https://lore.kernel.org/linux-arm-kernel/075b0a8e-cb7e-70f6-b45a-54cd31886794@xxxxxxxxxxxxxxxxx/T/
Sorry, but I read through the thread and I still can't find where the
possibility of leaving the crashkernel mapped was discussed.
1.Quoted messages from arch/arm64/mm/init.c
"Memory reservation for crash kernel either done early or deferred
depending on DMA memory zones configs (ZONE_DMA) --
In absence of ZONE_DMA configs arm64_dma_phys_limit initialized
here instead of max_zone_phys(). This lets early reservation of
crash kernel memory which has a dependency on arm64_dma_phys_limit.
Reserving memory early for crash kernel allows linear creation of block
mappings (greater than page-granularity) for all the memory bank rangs.
In this scheme a comparatively quicker boot is observed.
If ZONE_DMA configs are defined, crash kernel memory reservation
is delayed until DMA zone memory range size initialization performed in
zone_sizes_init(). The defer is necessary to steer clear of DMA zone
memory range to avoid overlap allocation.
[[[
So crash kernel memory boundaries are not known when mapping all bank memory
ranges, which otherwise means not possible to exclude crash kernel range
from creating block mappings so page-granularity mappings are created for
the entire memory range.
]]]"
Namely, the init order: memblock init--->linear mem mapping(4k mapping for
crashkernel, requirinig page-granularity changing))--->zone dma
limit--->reserve crashkernel.
So when enable ZONE DMA and using crashkernel, the mem mapping using 4k
mapping.
Yes, I understand that is how things work today but I'm saying that we may
as well leave the crashkernel mapped (at block granularity) if
!can_set_direct_map() and then I think your patch becomes a lot simpler.
But Page-granularity mapppings are necessary for crash kernel memory range
for shrinking its size via /sys/kernel/kexec_crash_size interfac(Quoted from
arch/arm64/mm/init.c).
So this patch split block/section mapping to 4k page-granularity mapping for
crashkernel mem.
Why? I don't see why the mapping granularity is relevant at all if we
always leave the whole thing mapped.
Will
There is another reason.
When loading crashkernel finish, the do_kexec_load will use
arch_kexec_protect_crashkres to invalid all the pagetable for
crashkernel mem(protect crashkernel mem from access).
arch_kexec_protect_crashkres--->set_memory_valid--->...--->apply_to_pmd_range
In the apply_to_pmd_range, there is a judement： BUG_ON(pud_huge(*pud)).
And if the crashkernel use block/section mapping, there will be some error.
Namely, it's need to use non block/section mapping for crashkernel mem
before shringking.
Thanks.
