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

Catalin Marinas
2020-03-26T16:55:25Z

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);
>
> 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.
>
> 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.
>
> 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

Other architectures may have transparent caches and don't require
different attributes.

> Signed-off-by: Li Wang <li.wang@windriver.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: linux-arm-kernel@lists.infradead.org
> Cc: linux-kernel@vger.kernel.org
> ---
> 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);

A better solution is for user space not to pass O_SYNC when opening
/dev/mem. We've had this ABI for a long time (arch/arm/ and several
other architectures do the same), why change it now?

--
Catalin