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).