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Some architectures do not have data cache coherency between user and kernel space. For these architectures, the cache needs to be flushed on both the kernel and user addresses so that user space can see the updates the kernel has made. Instead of using flush_dcache_folio() and playing with virt_to_folio() within the call to that function, use flush_kernel_vmap_range() which takes the virtual address and does the work for those architectures that need it. This also fixes a bug where the flush of the reader page only flushed one page. If the sub-buffer order is 1 or more, where the sub-buffer size would be greater than a page, it would miss the rest of the sub-buffer content, as the "reader page" is not just a page, but the size of a sub-buffer. Link: https://lore.kernel.org/all/CAG48ez3w0my4Rwttbc5tEbNsme6tc0mrSN95thjXUFaJ3aQ6SA@mail.gmail.com/ Cc: stable@vger.kernel.org Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Masami Hiramatsu <mhiramat@kernel.org> Cc: Mark Rutland <mark.rutland@arm.com> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Vincent Donnefort <vdonnefort@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Mike Rapoport <rppt@kernel.org> Link: https://lore.kernel.org/20250402144953.920792197@goodmis.org Fixes: 117c39200d9d7 ("ring-buffer: Introducing ring-buffer mapping functions"); Suggested-by: Jann Horn <jannh@google.com> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>