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kmap() and kmap_atomic() are being deprecated in favor of
kmap_local_page().
There are two main problems with kmap(): (1) It comes with an overhead as
mapping space is restricted and protected by a global lock for
synchronization and (2) it also requires global TLB invalidation when the
kmap's pool wraps and it might block when the mapping space is fully
utilized until a slot becomes available.
kmap_local_page() is safe from any context and is therefore redundant with
kmap_atomic() with the exception of any pagefault or preemption disable
requirements. However, using kmap_atomic() for these side effects makes
the code less clear. So any requirement for pagefault or preemption
disable should be made explicitly.
With kmap_local_page() the mappings are per thread, CPU local, can take
page faults, and can be called from any context (including interrupts).
It is faster than kmap() in kernels with HIGHMEM enabled. Furthermore,
the tasks can be preempted and, when they are scheduled to run again, the
kernel virtual addresses are restored.
Link: https://lkml.kernel.org/r/20220813220034.806698-1-ira.weiny@intel.com
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Suggested-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
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