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

[Will Deacon <will@kernel.org>]

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?

> 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