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Date:   Sat, 21 Jan 2023 11:57:12 +0100
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Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()
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To:     Ira Weiny <ira.weiny@intel.com>, Al Viro <viro@zeniv.linux.org.uk>,
        Matthew Wilcox <willy@infradead.org>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        linux-parisc@vger.kernel.org
References: <20230119153232.29750-1-fmdefrancesco@gmail.com>
 <20230119153232.29750-5-fmdefrancesco@gmail.com> <Y8oWsiNWSXlDNn5i@ZenIV>
 <Y8oYXEjunDDAzSbe@casper.infradead.org> <Y8ocXbztTPbxu3bq@ZenIV>
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 <Y8os8QR1pRXyu4N8@ZenIV> <99978295-6643-0cf2-8760-43e097f20dad@gmx.de>
 <63cb9ce63db7e_c68ad29473@iweiny-mobl.notmuch>
From:   Helge Deller <deller@gmx.de>
In-Reply-To: <63cb9ce63db7e_c68ad29473@iweiny-mobl.notmuch>
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Precedence: bulk

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)
>>>>>> {
>>>>>> #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
>>>>>>           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 paris=
c (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 mani=
fest
>>>>> on one not-well-tested architecture seems Bad.
>>>>>
>>>>>    static inline void __kunmap_local(const void *addr)
>>>>>    {
>>>>>    #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
>>>>> -       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:
>> http://ftp.parisc-linux.org/docs/arch/parisc2.0.pdf (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) t=
he
> same?

Yes, they are.

> align.h
> #define PTR_ALIGN_DOWN(p, a)    ((typeof(p))ALIGN_DOWN((unsigned long)(p=
), (a)))
>
> mm.h:
> #define PAGE_ALIGN_DOWN(addr) ALIGN_DOWN(addr, PAGE_SIZE)
>
> 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 i=
t
> required to return the original value returned from page_address()?

No.
If the lower bits are zero, then the default page size (4k) is used for th=
e tlb purge.
So, if you simply strip the lower bits (by using PAGE_ALIGN_DOWN() or ALIG=
N_DOWN())
you are fine.

Helge