On Fri, Jun 09, 2023 at 05:09:08AM +0200, Fabio M. De Francesco wrote: > 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: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Ira Weiny <ira.weiny@xxxxxxxxx> > Cc: Jonathan Corbet <corbet@xxxxxxx> > Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx> > Cc: Mike Rapoport <rppt@xxxxxxxxxxxxx> > Cc: Peter Collingbourne <pcc@xxxxxxxxxx> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Vlastimil Babka <vbabka@xxxxxxx> > Cc: Will Deacon <will@xxxxxxxxxx> > Signed-off-by: Fabio M. De Francesco <fmdefrancesco@xxxxxxxxx> Acked-by: Mike Rapoport (IBM) <rppt@xxxxxxxxxx> > --- > Documentation/mm/highmem.rst | 27 +++++++++++++++------------ > 1 file changed, 15 insertions(+), 12 deletions(-) > > diff --git a/Documentation/mm/highmem.rst b/Documentation/mm/highmem.rst > index c964e0848702..bb9584f167a6 100644 > --- a/Documentation/mm/highmem.rst > +++ b/Documentation/mm/highmem.rst > @@ -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 require > + 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 > -- > 2.40.1 > -- Sincerely yours, Mike.