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SubjectRe: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()
On 1/21/23 09:05, Ira Weiny wrote:
> Helge Deller wrote:
>> On 1/20/23 06:56, Al Viro wrote:
>>> On Fri, Jan 20, 2023 at 05:07:48AM +0000, Al Viro wrote:
>>>> On Fri, Jan 20, 2023 at 04:54:51AM +0000, Matthew Wilcox wrote:
>>>>>> Sure, but... there's also this:
>>>>>> static inline void __kunmap_local(const void *addr)
>>>>>> {
>>>>>> kunmap_flush_on_unmap(addr);
>>>>>> #endif
>>>>>> }
>>>>>> Are you sure that the guts of that thing will be happy with address that is not
>>>>>> page-aligned? I've looked there at some point, got scared of parisc (IIRC)
>>>>>> MMU details and decided not to rely upon that...
>>>>> Ugh, PA-RISC (the only implementor) definitely will flush the wrong
>>>>> addresses. I think we should do this, as having bugs that only manifest
>>>>> on one not-well-tested architecture seems Bad.
>>>>> static inline void __kunmap_local(const void *addr)
>>>>> {
>>>>> - kunmap_flush_on_unmap(addr);
>>>>> + kunmap_flush_on_unmap(PAGE_ALIGN_DOWN(addr));
>>>>> #endif
>>>>> }
>>>> PTR_ALIGN_DOWN(addr, PAGE_SIZE), perhaps?
>>> Anyway, that's a question to parisc folks; I _think_ pdtlb
>>> quietly ignores the lower bits of address, so that part seems
>>> to be safe, but I wouldn't bet upon that.
>> No, on PA2.0 (64bit) CPUs the lower bits of the address of pdtlb
>> encodes the amount of memory (page size) to be flushed, see:
>> (page 7-106)
>> So, the proposed page alignment with e.g. PTR_ALIGN_DON() is needed.
> I'm not sure I completely understand.
> First, arn't PAGE_ALIGN_DOWN(addr) and PTR_ALIGN_DOWN(addr, PAGE_SIZE) the
> same?

Yes, they are.

> align.h
> #define PTR_ALIGN_DOWN(p, a) ((typeof(p))ALIGN_DOWN((unsigned long)(p), (a)))
> mm.h:
> Did parisc redefine it somewhere I'm not seeing?

No, there is nothing special in this regard on parisc.

> Second, if the lower bits encode the amount of memory to be flushed is it
> required to return the original value returned from page_address()?

If the lower bits are zero, then the default page size (4k) is used for the tlb purge.
So, if you simply strip the lower bits (by using PAGE_ALIGN_DOWN() or ALIGN_DOWN())
you are fine.


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