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The reserve_mem kernel command line option may pass back a physical address, but the memory is still part of the normal memory just like using memblock_alloc() would be. This means that the physical memory returned by the reserve_mem command line option can be converted directly to virtual memory by simply using phys_to_virt(). When freeing the buffer there's no need to call vunmap() anymore as the memory allocated by reserve_mem is freed by the call to reserve_mem_release_by_name(). Because the persistent ring buffer can also be allocated via the memmap option, which *is* different than normal memory as it cannot be added back to the buddy system, it must be treated differently. It still needs to be virtually mapped to have access to it. It also can not be freed nor can it ever be memory mapped to user space. Create a new trace_array flag called TRACE_ARRAY_FL_MEMMAP which gets set if the buffer is created by the memmap option, and this will prevent the buffer from being memory mapped by user space. Also increment the ref count for memmap'ed buffers so that they can never be freed. Link: https://lore.kernel.org/all/Z-wFszhJ_9o4dc8O@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: Jann Horn <jannh@google.com> Link: https://lore.kernel.org/20250402144953.583750106@goodmis.org Suggested-by: Mike Rapoport <rppt@kernel.org> Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.