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Date: Mon, 19 Jul 2021 09:39:17 +0100
From: Christoph Hellwig <hch@infradead.org>
To: Matthew Wilcox <willy@infradead.org>
Subject: Re: [PATCH] iomap: Add missing flush_dcache_page
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Cc: linux-xfs@vger.kernel.org, Andreas Gruenbacher <agruenba@redhat.com>,
 "Darrick J . Wong" <djwong@kernel.org>, stable@vger.kernel.org,
 Christoph Hellwig <hch@infradead.org>, linux-fsdevel@vger.kernel.org,
 Gao Xiang <xiang@kernel.org>, linux-erofs@lists.ozlabs.org
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 <linux-erofs-bounces+linux-erofs=archiver.kernel.org@lists.ozlabs.org>

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.
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).