diff options
Diffstat (limited to 'Documentation/mm/highmem.rst')
| -rw-r--r-- | Documentation/mm/highmem.rst | 31 |
1 files changed, 27 insertions, 4 deletions
diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst index c9887f241c6c..0f731d9196b0 100644 --- a/Documentation/mm/highmem.rst +++ b/Documentation/mm/highmem.rst @@ -60,17 +60,40 @@ 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. + On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the + virtual address of the direct mapping. Only real highmem pages are + temporarily mapped. Therefore, users may call a plain page_address() + 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. + + Most code can be designed to use thread local mappings. User should + therefore try to design their code to avoid the use of kmap() by mapping + pages in the same thread the address will be used and prefer + kmap_local_page(). + 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 |