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

Wang, Li
2020-03-26T17:34:17Z

在 2020/3/27 0:55, Catalin Marinas 写道:
> 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?

1.

no pass O_SYNC in user space is not a good idea.

in fact, the codes come from 'devmem' command of busybox:

=====

busybox-1.24.1/miscutils$ vim devmem.c

fd = xopen("/dev/mem", O_SYNC);

=====

the codes are used for a long time.

2.

according to info of open man about "O_SYNC":

=====

http://man7.org/linux/man-pages/man2/open.2.html

the output data and associated file metadata have been transferred to
the underlying hardware

=====

I think "O_SYNC" means no cache.

3.

/dev/mem of driver offers 2 ways to operate  physical memory.

one is mmap, the other is read/write.

when use read/write way, it operates uncached memory:

=====

kernel-source/drivers/char/mem.c

write_mem(){

/* it must also be accessed uncached */

}

=====

4.

arm64 arch is different with other arch about phys_mem_access_prot().

you can see no any other arch add cache flag in the function.

only arm and arm64 add write cache for O_SYNC flag.

x86/mm/pat.c

phys_mem_access_prot(){

return vma_prot;

}

powerpc/mm/mem.c

phys_mem_access_prot(){
if (ppc_md.phys_mem_access_prot)
return ppc_md.phys_mem_access_prot(file, pfn, size,
vma_prot);
if (!page_is_ram(pfn))
vma_prot = pgprot_noncached(vma_prot);
return vma_prot;
}

drivers/char/mem.c

phys_mem_access_prot()
{
#ifdef pgprot_noncached
phys_addr_t offset = pfn << PAGE_SHIFT;

if (uncached_access(file, offset))
return pgprot_noncached(vma_prot);
#endif
return vma_prot;
}

Thanks,

LiWang.