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Date: Tue, 20 Jul 2021 16:56:10 +0100
From: Matthew Wilcox <willy@infradead.org>
To: Christoph Hellwig <hch@infradead.org>
Subject: Re: [PATCH] iomap: Add missing flush_dcache_page
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Cc: Andreas Gruenbacher <agruenba@redhat.com>,
 "Darrick J . Wong" <djwong@kernel.org>, stable@vger.kernel.org,
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On Mon, Jul 19, 2021 at 09:39:17AM +0100, Christoph Hellwig wrote:
> On Fri, Jul 16, 2021 at 06:28:10PM +0100, Matthew Wilcox wrote:
> > > >  	memcpy(addr, iomap->inline_data, size);
> > > >  	memset(addr + size, 0, PAGE_SIZE - size);
> > > >  	kunmap_atomic(addr);
> > > > +	flush_dcache_page(page);
> > > 
> > > .. and all writes into a kmap also need such a flush, so this needs to
> > > move a line up.  My plan was to add a memcpy_to_page_and_pad helper
> > > ala memcpy_to_page to get various file systems and drivers out of the
> > > business of cache flushing as much as we can.
> > 
> > hm?  It's absolutely allowed to flush the page after calling kunmap.
> > Look at zero_user_segments(), for example.
> 
> Documentation/core-api/cachetlb.rst states that any user page obtained
> using kmap needs a flush_kernel_dcache_page after modification.
> flush_dcache_page is a strict superset of flush_kernel_dcache_page.

Looks like (the other) Christoph broke this in 2008 with commit
eebd2aa35569 ("Pagecache zeroing: zero_user_segment, zero_user_segments
and zero_user"):

It has one line about it in the changelog:

    Also extract the flushing of the caches to be outside of the kmap.

... which doesn't even attempt to justify why it's safe to do so.

-               memset((char *)kaddr + (offset), 0, (size));    \
-               flush_dcache_page(page);                        \
-               kunmap_atomic(kaddr, (km_type));                \
+       kunmap_atomic(kaddr, KM_USER0);
+       flush_dcache_page(page);

Looks like it came from
https://lore.kernel.org/lkml/20070911060425.472862098@sgi.com/
but there was no discussion of this ... plenty of discussion about
other conceptual problems with the entire patchset.

> That beeing said flushing after kmap updates is a complete mess.
> arm as probably the poster child for dcache challenged plus highmem
> architectures always flushed caches from kunmap and, and arc has
> a flush_dcache_page that doesn't work at all on a highmem page that
> is not kmapped (where kmap_atomic and kmap_local_page don't count as
> kmapped as they don't set page->virtual).