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

[Will Deacon]

On Mon, Feb 26, 2018 at 06:57:20PM +0800, Hanjun Guo wrote:
> 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 reconstructionïnot only the memory leak problems.  If BBM flow
> >> is not followedïthe speculative prefetch of tlb will made false tlb entries
> >> cached in MMU, the false address will be gotï 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

No, that actually breaks with the use of block mappings for the kernel
text. Anyway, see:

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=15122ee2c515a253b0c66a3e618bc7ebe35105eb

Will