Re: [PATCH v14 054/138] mm: Add kmap_local_folio()

Mike Rapoport
2021-07-21T14:26:33Z

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.

--
Sincerely yours,
Mike.