ring-buffer: Use flush_kernel_vmap_range() over flush_dcache_folio()

JIRA: https://issues.redhat.com/browse/RHEL-115358

commit e4d4b86
Author: Steven Rostedt <rostedt@goodmis.org>
Date:   Wed Apr 2 10:49:07 2025 -0400

    ring-buffer: Use flush_kernel_vmap_range() over flush_dcache_folio()

    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: 117c392 ("ring-buffer: Introducing ring-buffer mapping functions");
    Suggested-by: Jann Horn <jannh@google.com>
    Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>

Author: jeromemarchand

kernel/trace/ring_buffer.c

@@ -5994,7 +5994,7 @@ static void rb_update_meta_page(struct ring_buffer_per_cpu *cpu_buffer)
 	meta->read = cpu_buffer->read;
 
 	/* Some archs do not have data cache coherency between kernel and user-space */
-	flush_dcache_folio(virt_to_folio(cpu_buffer->meta_page));
+	flush_kernel_vmap_range(cpu_buffer->meta_page, PAGE_SIZE);
 }
 
 static void
@@ -7309,7 +7309,8 @@ int ring_buffer_map_get_reader(struct trace_buffer *buffer, int cpu)
 
 out:
 	/* Some archs do not have data cache coherency between kernel and user-space */
-	flush_dcache_folio(virt_to_folio(cpu_buffer->reader_page->page));
+	flush_kernel_vmap_range(cpu_buffer->reader_page->page,
+				buffer->subbuf_size + BUF_PAGE_HDR_SIZE);
 
 	rb_update_meta_page(cpu_buffer);