On Wed, Jul 21, 2021 at 05:22:16PM +0300, Mike Rapoport wrote: > On Wed, Jul 21, 2021 at 03:12:03PM +0100, Matthew Wilcox wrote: > > On Wed, Jul 21, 2021 at 12:58:24PM +0300, Mike Rapoport wrote: > > > > +/** > > > > + * kmap_local_folio - Map a page in this folio for temporary usage > > > > + * @folio: The folio to be mapped. > > > > + * @offset: The byte offset within the folio. > > > > + * > > > > + * Returns: The virtual address of the mapping > > > > + * > > > > + * Can be invoked from any context. > > > > > > Context: Can be invoked from any context. > > > > > > > + * > > > > + * Requires careful handling when nesting multiple mappings because the map > > > > + * management is stack based. The unmap has to be in the reverse order of > > > > + * the map operation: > > > > + * > > > > + * addr1 = kmap_local_folio(page1, offset1); > > > > + * addr2 = kmap_local_folio(page2, offset2); > > > > > > Please s/page/folio/g here and in the description below > > > > > > > + * ... > > > > + * kunmap_local(addr2); > > > > + * kunmap_local(addr1); > > > > + * > > > > + * Unmapping addr1 before addr2 is invalid and causes malfunction. > > > > + * > > > > + * Contrary to kmap() mappings the mapping is only valid in the context of > > > > + * the caller and cannot be handed to other contexts. > > > > + * > > > > + * 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. > > > > + * > > > > + * While it is significantly faster than kmap() for the higmem case it > > > > + * comes with restrictions about the pointer validity. Only use when really > > > > + * necessary. > > > > + * > > > > + * On HIGHMEM enabled systems mapping a highmem page has the side effect of > > > > + * disabling migration in order to keep the virtual address stable across > > > > + * preemption. No caller of kmap_local_folio() can rely on this side effect. > > > > + */ > > > > kmap_local_folio() only maps one page from the folio. So it's not > > appropriate to s/page/folio/g. I fiddled with the description a bit to > > make this clearer: > > > > /** > > * kmap_local_folio - Map a page in this folio for temporary usage > > - * @folio: The folio to be mapped. > > - * @offset: The byte offset within the folio. > > - * > > - * Returns: The virtual address of the mapping > > - * > > - * Can be invoked from any context. > > + * @folio: The folio containing the page. > > + * @offset: The byte offset within the folio which identifies the page. > > * > > * Requires careful handling when nesting multiple mappings because the map > > * management is stack based. The unmap has to be in the reverse order of > > * the map operation: > > * > > - * addr1 = kmap_local_folio(page1, offset1); > > - * addr2 = kmap_local_folio(page2, offset2); > > + * addr1 = kmap_local_folio(folio1, offset1); > > + * addr2 = kmap_local_folio(folio2, offset2); > > * ... > > * kunmap_local(addr2); > > * kunmap_local(addr1); > > @@ -131,6 +127,9 @@ static inline void *kmap_local_page(struct page *page); > > * On HIGHMEM enabled systems mapping a highmem page has the side effect of > > * disabling migration in order to keep the virtual address stable across > > * preemption. No caller of kmap_local_folio() can rely on this side effect. > > + * > > + * Context: Can be invoked from any context. > > + * Return: The virtual address of @offset. > > */ > > static inline void *kmap_local_folio(struct folio *folio, size_t offset) > > This is clearer, thanks! > > Maybe just add page to Return: description: > > * Return: The virtual address of page @offset. No, it really does return the virtual address of @offset. If you ask for offset 0x1234 within a (sufficiently large) folio, it will map the second page of that folio and return the address of the 0x234'th byte within it.
