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

On Sat, Mar 28, 2020 at 12:47:32AM +0800, Wang, Li wrote:

在 2020/3/27 22:29, Mark Rutland 写道:

quoted

On Thu, Mar 26, 2020 at 09:36:25AM -0700, Li Wang wrote:

quoted

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.

quoted

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?

about 2 Process:

Process A:

the memory is not declared by map function, it is just a global variable.

Then it is exactly as I described previously, and Process A has it
mapped with a Normal Write-Back Cacheable mappping.

Process B requests a mapping of that memory via /dev/mem. It passes the
O_SYNC flag, and to ensure that accesses go to "the underlying hardware"
the kernel makes this mapping Normal Non-Cacheable (which means it
should not look in a cache, or be allocated into one).

The two mappings are not coherent because process A uses the cache, but
process B does not. This is the expected behaviour, consistent with the
semantic of O_SYNC. If you need the two to be coherent, they must both
use the same attributes.

Process B can be coherent with process A if it does *not* pass O_SYNC,
which would give it a Normal Write-Back Cacheable mapping that was
coherent with process A.

if you agree that O_SYNC flag means "is transferred to the underlying
hardware",

the arm64 does not do that:

when use O_SYNC flag under arm64 arch, it adds write cache feature,

As above, this is not the case. O_SYNC causes the kernel to use a
non-cacheable mapping, where it would normally create a cacheable
mapping. i.e. O_SYNC *removes* cacheability.

It just happens that process A is using a cacheable mapping, which is
the case regardless of what process B does.

Thanks,
Mark.

_______________________________________________
linux-arm-kernel mailing list
linux-arm-kernel@lists.infradead.org
http://lists.infradead.org/mailman/listinfo/linux-arm-kernel