Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation

[guanghui.fgh]

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?
> > >
> > > Will
> >
> > 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/

> > 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.
>
> Will

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.

Thanks.