Re: 答复: [RFC patch] ioremap: don't set up huge I/O mappings when p4d/pud/pmd is zero

[Hanjun Guo]

On 2018/2/21 19:57, Will Deacon wrote:
> [sorry, trying to deal with top-posting here]
> 
> On Wed, Feb 21, 2018 at 07:36:34AM +0000, Wangxuefeng (E) wrote:
>>      The old flow of reuse the 4k page as 2M page does not follow the BBM flow
>> for page table reconstruction，not only the memory leak problems.  If BBM flow
>> is not followed，the speculative prefetch of tlb will made false tlb entries
>> cached in MMU, the false address will be got， panic will happen.
> 
> If I understand Toshi's suggestion correctly, he's saying that the PMD can
> be cleared when unmapping the last PTE (like try_to_free_pte_page). In this
> case, there's no issue with the TLB because this is exactly BBM -- the PMD
> is cleared and TLB invalidation is issued before the PTE table is freed. A
> subsequent 2M map request will see an empty PMD and put down a block
> mapping.
> 
> The downside is that freeing becomes more expensive as the last level table
> becomes more sparsely populated and you need to ensure you don't have any
> concurrent maps going on for the same table when you're unmapping. I also
> can't see a neat way to fit this into the current vunmap code. Perhaps we
> need an iounmap_page_range.
> 
> In the meantime, the code in lib/ioremap.c looks totally broken so I think
> we should deselect CONFIG_HAVE_ARCH_HUGE_VMAP on arm64 until it's fixed.

Simply do something below at now (before the broken code is fixed)?

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index b2b95f7..a86148c 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -84,7 +84,6 @@ config ARM64
        select HAVE_ALIGNED_STRUCT_PAGE if SLUB
        select HAVE_ARCH_AUDITSYSCALL
        select HAVE_ARCH_BITREVERSE
-   select HAVE_ARCH_HUGE_VMAP
        select HAVE_ARCH_JUMP_LABEL
        select HAVE_ARCH_KASAN if !(ARM64_16K_PAGES && ARM64_VA_BITS_48)
        select HAVE_ARCH_KGDB

Thanks
Hanjun