Re: [PATCH] mm/highmem: Align-down to page the address for kunmap_flush_on_unmap()

[Helge Deller]

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@xxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Suggested-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> > Confirmed-by: Helge Deller <deller@xxxxxx>
> > Confirmed-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > Fixes: f3ba3c710ac5 ("mm/highmem: Provide kmap_local*")
> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx>
> > ---
> >
> > 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