linux-kernel - Re: [PATCH] mm/highmem: Align-down to page the address for kunmap_flush_on

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Date:   Thu, 26 Jan 2023 21:37:08 +0100
From:   Helge Deller <deller@....de>
To:     Ira Weiny <ira.weiny@...el.com>,
        "Fabio M. De Francesco" <fmdefrancesco@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Alexander Potapenko <glider@...gle.com>,
        Andrey Konovalov <andreyknvl@...il.com>,
        Tony Luck <tony.luck@...el.com>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        David Sterba <dsterba@...e.com>,
        Kees Cook <keescook@...omium.org>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH] mm/highmem: Align-down to page the address for
 kunmap_flush_on_unmap()

On 1/26/23 20:50, Ira Weiny wrote:
> Fabio M. De Francesco wrote:
>
> FWIW I think I would simplify the subject
> [PATCH] mm/highmem: Fix kunmap_local() on flush on unmap architectures
>
> Or something like that.
>
>> If ARCH_HAS_FLUSH_ON_KUNMAP is defined (PA-RISC case), __kunmap_local()
>> calls kunmap_flush_on_unmap(). The latter currently flushes the wrong
>> address (as confirmed by Matthew Wilcox and Helge Deller). Al Viro
>> proposed to call kunmap_flush_on_unmap() on an aligned-down to page
>> address in order to fix this issue. Consensus has been reached on this
>> solution.
>>
>> Therefore, if ARCH_HAS_FLUSH_ON_KUNMAP is defined, call
>> kunmap_flush_on_unmap() on an aligned-down to page address computed with
>> the PTR_ALIGN_DOWN() macro.
>>
>> Cc: Ira Weiny <ira.weiny@...el.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> Suggested-by: Al Viro <viro@...iv.linux.org.uk>
>> Confirmed-by: Helge Deller <deller@....de>
>> Confirmed-by: Matthew Wilcox <willy@...radead.org>
>> Fixes: f3ba3c710ac5 ("mm/highmem: Provide kmap_local*")
>> Signed-off-by: Fabio M. De Francesco <fmdefrancesco@...il.com>
>> ---
>>
>> I have (at least) two problems with this patch...
>>
>> 1) checkpatch.pl complains about the use of the non-standard
>> "Confirmed-by" tags. I don't know how else I can give credit to Helge
>> and Matthew. However, this is not the first time that I see non-standard
>> tags in patches applied upstream (I too had a non-standard
>> "Analysed-by" tag in patch which fixes a SAC bug). Any objections?
>
> I think you can add Matthew and Helge as Suggested-by:  All 3 had input on
> the solution.
>
>>
>> 2) I'm not sure whether or not the "Fixes" tag is appropriate in this
>> patch. Can someone either confirm or deny it?
>
> This 'fixes' looks correct to me.  I don't know how many folks are running
> highmem with parisc but if they are I am sure they would appreciate the
> extra knowledge.

It seems nobody is running highmem on parisc, because it can't be enabled.
AFAICS, it's not in any parisc related Kconfig file.

> I do wonder if this should be cc'ed to stable to ensure it gets
> backported?  Helge do you think there is a need for that?

For correctness I think it's nevertheless good to backport it.

>>   include/linux/highmem-internal.h | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
>> index 034b1106d022..e247c9ac4583 100644
>> --- a/include/linux/highmem-internal.h
>> +++ b/include/linux/highmem-internal.h
>> @@ -200,7 +200,7 @@ static inline void *kmap_local_pfn(unsigned long pfn)
>>   static inline void __kunmap_local(const void *addr)
>>   {
>>   #ifdef ARCH_HAS_FLUSH_ON_KUNMAP
>> -	kunmap_flush_on_unmap(addr);
>> +	kunmap_flush_on_unmap(PTR_ALIGN_DOWN(addr, PAGE_SIZE));
>>   #endif
>>   }

That would be another possibility:

diff --git a/arch/parisc/include/asm/cacheflush.h b/arch/parisc/include/asm/cacheflush.h
index 0bdee6724132..ce5d1f8a23bd 100644
--- a/arch/parisc/include/asm/cacheflush.h
+++ b/arch/parisc/include/asm/cacheflush.h
@@ -77,6 +77,7 @@ void flush_anon_page(struct vm_area_struct *vma, struct page *page, unsigned lon
  #define ARCH_HAS_FLUSH_ON_KUNMAP
  static inline void kunmap_flush_on_unmap(const void *addr)
  {
+       addr = PTR_ALIGN_DOWN(addr, PAGE_SIZE);
         flush_kernel_dcache_page_addr(addr);
  }


Helge