linux-kernel - Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local

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Message-ID: <c08af0ee-fc12-7a3b-541d-677c3e562f56@gmx.de>
Date:   Sat, 21 Jan 2023 11:57:12 +0100
From:   Helge Deller <deller@....de>
To:     Ira Weiny <ira.weiny@...el.com>, Al Viro <viro@...iv.linux.org.uk>,
        Matthew Wilcox <willy@...radead.org>
Cc:     "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-parisc@...r.kernel.org
Subject: Re: [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)
>>>>>> {
>>>>>> #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 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)
>>>>>    {
>>>>>    #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) the
> 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 it
> 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 the tlb purge.
So, if you simply strip the lower bits (by using PAGE_ALIGN_DOWN() or ALIGN_DOWN())
you are fine.

Helge