在 2022/7/5 0:38, Will Deacon 写道:
On Mon, Jul 04, 2022 at 10:34:07PM +0800, guanghui.fgh wrote:
Thanks.
在 2022/7/4 22:23, Will Deacon 写道:
On Mon, Jul 04, 2022 at 10:11:27PM +0800, guanghui.fgh wrote:
在 2022/7/4 21:15, Will Deacon 写道:
On Mon, Jul 04, 2022 at 08:05:59PM +0800, guanghui.fgh wrote:
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.
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.
Well, yes, but we can change arch_kexec_[un]protect_crashkres() not to do
that if we're leaving the thing mapped, no?
I think we should use arch_kexec_[un]protect_crashkres for crashkernel mem.
Because when invalid crashkernel mem pagetable, there is no chance to rd/wr
the crashkernel mem by mistake.
If we don't use arch_kexec_[un]protect_crashkres to invalid crashkernel mem
pagetable, there maybe some write operations to these mem by mistake which
may cause crashkernel boot error and vmcore saving error.
I don't really buy this line of reasoning. The entire main kernel is
writable, so why do we care about protecting the crashkernel so much? The
_code_ to launch the crash kernel is writable! If you care about preventing
writes to memory which should not be writable, then you should use
rodata=full.
Will
Thanks.
1.I think the normal kernel mem can be writeable or readonly
2.But the normal kernel should't access(read/write) crashkernel mem
after the crashkernel is loaded to the mem.(despite the normal kernel
write/read access protect)
So invalid pagetable for crashkernel mem is needed.
With this method, it can guarantee the usability of the crashkernel when
occuring emergency and generating vmcore.
