Messages in this thread | | | Date | Tue, 5 Jul 2022 10:44:43 +0800 | Subject | Re: [PATCH v4] arm64: mm: fix linear mem mapping access performance degradation | From | "guanghui.fgh" <> |
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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.
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