Re: [PATCH] arm64: mmu: no write cache for O_SYNC flag

[Mark Rutland]

On Thu, Mar 26, 2020 at 09:36:25AM -0700, Li Wang wrote:
> reproduce steps:
> 1.
> disable CONFIG_STRICT_DEVMEM in linux kernel
> 2.
> Process A gets a Physical Address of global variable by
> "/proc/self/pagemap".
> 3.
> Process B writes a value to the same Physical Address by mmap():
> fd=open("/dev/mem",O_SYNC);
> Virtual Address=mmap(fd);

Is this just to demonstrate the behaviour, or is this meant to be
indicative of a real use-case? I'm struggling to see the latter.

> problem symptom:
> after Process B write a value to the Physical Address,
> Process A of the value of global variable does not change.
> They both W/R the same Physical Address.

If Process A is not using the same attributes as process B, there is no
guarantee of coherency. How did process A map this memory?

> technical reason:
> Process B writing the Physical Address is by the Virtual Address,
> and the Virtual Address comes from "/dev/mem" and mmap().
> In arm64 arch, the Virtual Address has write cache.
> So, maybe the value is not written into Physical Address.

I don't think that's true. I think what's happening here is:

* Process A has a Normal WBWA Cacheable mapping.
* Process B as a Normal Non-cacheable mapping.
* Process B's write does not snoop any caches, and goes straight to
  memory.
* Process A reads a value from cache, which does not include process B's
  write.

That's a natural result of using mismatched attributes, and is
consistent with the O_SYNC flag meaning that the write "is transferred
to the underlying hardware".

> 
> fix reason:
> giving write cache flag in arm64 is in phys_mem_access_prot():
> =====
> arch/arm64/mm/mmu.c
> phys_mem_access_prot()
> {
>   if (!pfn_valid(pfn))
>     return pgprot_noncached(vma_prot);
>   else if (file->f_flags & O_SYNC)
>     return pgprot_writecombine(vma_prot);
>   return vma_prot;
> }
> ====
> the other arch and the share function drivers/char/mem.c of phys_mem_access_prot()
> does not add write cache flag.
> So, removing the flag to fix the issue

This will change behaviour that other software may be relying upon, and
as above I do not believe this actually solves the problem you describe.

Thanks,
Mark.

> 
> Signed-off-by: Li Wang <li.wang@xxxxxxxxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> ---
>  arch/arm64/mm/mmu.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 128f70852bf3..d7083965ca17 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -81,8 +81,6 @@ pgprot_t phys_mem_access_prot(struct file *file, unsigned long pfn,
>  {
>  	if (!pfn_valid(pfn))
>  		return pgprot_noncached(vma_prot);
> -	else if (file->f_flags & O_SYNC)
> -		return pgprot_writecombine(vma_prot);
>  	return vma_prot;
>  }
>  EXPORT_SYMBOL(phys_mem_access_prot);
> -- 
> 2.24.1
>