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authorFabio M. De Francesco <fmdefrancesco@gmail.com>2022-07-28 18:48:41 +0300
committerAndrew Morton <akpm@linux-foundation.org>2022-08-09 04:06:45 +0300
commit6b3afe2eeec27a148b83235484149f42b22ef793 (patch)
treed1e02a13185f2d1ab7cac003c33c7a811ccb69d7 /Documentation/mm
parent516ea046ec5558882b4a8e5f6b0c9b4c957db279 (diff)
downloadlinux-6b3afe2eeec27a148b83235484149f42b22ef793.tar.xz
Documentation/mm: avoid invalid use of addresses from kmap_local_page()
Users of kmap_local_page() must be absolutely sure to not hand kernel virtual address obtained calling kmap_local_page() on highmem pages to other contexts because those pointers are thread local, therefore, they are no longer valid across different contexts. Extend the documentation of kmap_local_page() to warn users about the above-mentioned potential invalid use of pointers returned by kmap_local_page(). Link: https://lkml.kernel.org/r/20220728154844.10874-5-fmdefrancesco@gmail.com Signed-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com> Suggested-by: Ira Weiny <ira.weiny@intel.com> Cc: Matthew Wilcox (Oracle) <willy@infradead.org> Cc: Mike Rapoport <rppt@linux.ibm.com> Cc: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Jonathan Corbet <corbet@lwn.net> Cc: Peter Collingbourne <pcc@google.com> Cc: Vlastimil Babka <vbabka@suse.cz> Cc: Will Deacon <will@kernel.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Diffstat (limited to 'Documentation/mm')
-rw-r--r--Documentation/mm/highmem.rst7
1 files changed, 7 insertions, 0 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst
index 34d7097d3ce8..71dc09563ff8 100644
--- a/Documentation/mm/highmem.rst
+++ b/Documentation/mm/highmem.rst
@@ -77,6 +77,13 @@ list shows them in order of preference of use.
for pages which are known to not come from ZONE_HIGHMEM. However, it is
always safe to use kmap_local_page() / kunmap_local().
+ While it is significantly faster than kmap(), for the higmem case it
+ comes with restrictions about the pointers validity. Contrary to kmap()
+ mappings, the local mappings are only valid in the context of the caller
+ and cannot be handed to other contexts. This implies that users must
+ be absolutely sure to keep the use of the return address local to the
+ thread which mapped it.
+
Nesting kmap_local_page() and kmap_atomic() mappings is allowed to a certain
extent (up to KMAP_TYPE_NR) but their invocations have to be strictly ordered
because the map implementation is stack based. See kmap_local_page() kdocs