Commit e4c9eabc authored by Fabio M. De Francesco's avatar Fabio M. De Francesco Committed by Jonathan Corbet
Browse files

Documentation/highmem: Add information about kmap_local_folio() 



The differences between kmap_local_page() and kmap_local_folio() consist
only in the first taking a pointer to a page and the second taking two
arguments, a pointer to a folio and the byte offset within the folio which
identifies the page.

The two API's can be explained at the same time in the "Temporary Virtual
Mappings" section of the Highmem's documentation.

Add information about kmap_local_folio() in the same subsection that
explains kmap_local_page().

Cc: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: default avatarIra Weiny <ira.weiny@intel.com>
Reviewed-by: default avatarMike Rapoport (IBM) <rppt@kernel.org>
Signed-off-by: default avatarFabio M. De Francesco <fmdefrancesco@gmail.com>
Signed-off-by: default avatarJonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230708121719.8270-1-fmdefrancesco@gmail.com
parent 9e6e2ed2

Documentation/mm/highmem.rst

+15 −12
Original line number Diff line number Diff line
@@ -51,11 +51,14 @@ Temporary Virtual Mappings 
The kernel contains several ways of creating temporary mappings. The following

list shows them in order of preference of use.



* kmap_local_page().  This function is used to require short term mappings.

  It can be invoked from any context (including interrupts) but the mappings

  can only be used in the context which acquired them.



  This function should always be used, whereas kmap_atomic() and kmap() have

* kmap_local_page(), kmap_local_folio() - These functions are used to create

  short term mappings. They can be invoked from any context (including

  interrupts) but the mappings can only be used in the context which acquired

  them. The only differences between them consist in the first taking a pointer

  to a struct page and the second taking a pointer to struct folio and the byte

  offset within the folio which identifies the page.



  These functions should always be used, whereas kmap_atomic() and kmap() have

  been deprecated.



  These mappings are thread-local and CPU-local, meaning that the mapping
@@ -72,17 +75,17 @@ list shows them in order of preference of use. 
  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.

  kmap_local_page(), as well as kmap_local_folio() always returns valid virtual

  kernel addresses and it is assumed that kunmap_local() will never fail.



  On CONFIG_HIGHMEM=n kernels and for low memory pages this returns the

  On CONFIG_HIGHMEM=n kernels and for low memory pages they return 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().

  always safe to use kmap_local_{page,folio}() / kunmap_local().



  While it is significantly faster than kmap(), for the highmem case it

  comes with restrictions about the pointers validity. Contrary to kmap()

  While they are significantly faster than kmap(), for the highmem case they

  come 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
@@ -91,7 +94,7 @@ list shows them in order of preference of use. 
  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().

  kmap_local_page() or kmap_local_folio().



  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