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| author | Fabio M. De Francesco <fmdefrancesco@gmail.com> | 2022-07-28 17:48:44 +0200 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2022-08-08 18:06:46 -0700 |
| commit | a9e9c93966afdaae74a6a7533552391646b93f2c (patch) | |
| tree | 393c1b289b1c3dd236dd973553846a88fe650d48 /Documentation/mm | |
| parent | 72f1c55adf70fd08ceac6b67455238db2014894a (diff) | |
| download | linux-a9e9c93966afdaae74a6a7533552391646b93f2c.tar.gz | |
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>
Diffstat (limited to 'Documentation/mm')
| -rw-r--r-- | Documentation/mm/highmem.rst | 13 |
1 files changed, 9 insertions, 4 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index e045a4b7b3da43..0f731d9196b010 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. |