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authorFabio M. De Francesco <fmdefrancesco@gmail.com>2022-07-28 18:48:44 +0300
committerAndrew Morton <akpm@linux-foundation.org>2022-08-09 04:06:46 +0300
commita9e9c93966afdaae74a6a7533552391646b93f2c (patch)
tree393c1b289b1c3dd236dd973553846a88fe650d48
parent72f1c55adf70fd08ceac6b67455238db2014894a (diff)
downloadlinux-a9e9c93966afdaae74a6a7533552391646b93f2c.tar.xz
Documentation/mm: add details about kmap_local_page() and preemption
What happens if a thread is preempted after mapping pages with kmap_local_page() was questioned recently.[1] Commit f3ba3c710ac5 ("mm/highmem: Provide kmap_local*") from Thomas Gleixner explains clearly that on context switch, the maps of an outgoing task are removed and the map of the incoming task are restored and that kmap_local_page() can be invoked from both preemptible and atomic contexts.[2] Therefore, for the purpose to make it clearer that users can call kmap_local_page() from contexts that allow preemption, rework a couple of sentences and add further information in highmem.rst. [1] https://lore.kernel.org/lkml/5303077.Sb9uPGUboI@opensuse/ [2] https://lore.kernel.org/all/20201118204007.468533059@linutronix.de/ Link: https://lkml.kernel.org/r/20220728154844.10874-8-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>
-rw-r--r--Documentation/mm/highmem.rst13
1 files changed, 9 insertions, 4 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst
index e045a4b7b3da..0f731d9196b0 100644
--- a/Documentation/mm/highmem.rst
+++ b/Documentation/mm/highmem.rst
@@ -60,14 +60,19 @@ list shows them in order of preference of use.
This function should be preferred, where feasible, over all the others.
These mappings are thread-local and CPU-local, meaning that the mapping
- can only be accessed from within this thread and the thread is bound the
- CPU while the mapping is active. Even if the thread is preempted (since
- preemption is never disabled by the function) the CPU can not be
- unplugged from the system via CPU-hotplug until the mapping is disposed.
+ can only be accessed from within this thread and the thread is bound to the
+ CPU while the mapping is active. Although preemption is never disabled by
+ this function, the CPU can not be unplugged from the system via
+ CPU-hotplug until the mapping is disposed.
It's valid to take pagefaults in a local kmap region, unless the context
in which the local mapping is acquired does not allow it for other reasons.
+ As said, pagefaults and preemption are never disabled. There is no need to
+ disable preemption because, when context switches to a different task, the
+ maps of the outgoing task are saved and those of the incoming one are
+ restored.
+
kmap_local_page() always returns a valid virtual address and it is assumed
that kunmap_local() will never fail.