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

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Message-ID: <99978295-6643-0cf2-8760-43e097f20dad@gmx.de>
Date:   Fri, 20 Jan 2023 08:17:19 +0100
From:   Helge Deller <deller@....de>
To:     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,
        Ira Weiny <ira.weiny@...el.com>, linux-parisc@...r.kernel.org
Subject: Re: [PATCH v3 4/4] fs/sysv: Replace kmap() with kmap_local_page()

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.

Helge