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

Mike Rapoport
2021-07-21T10:11:27Z

On Thu, Jul 15, 2021 at 04:35:40AM +0100, Matthew Wilcox (Oracle) wrote:
> This allows us to map a portion of a folio. Callers can only expect
> to access up to the next page boundary.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> include/linux/highmem-internal.h | 11 +++++++++
> include/linux/highmem.h | 38 ++++++++++++++++++++++++++++++++
> 2 files changed, 49 insertions(+)
>
> diff --git a/include/linux/highmem-internal.h b/include/linux/highmem-internal.h
> index 7902c7d8b55f..d5d6f930ae1d 100644
> --- a/include/linux/highmem-internal.h
> +++ b/include/linux/highmem-internal.h
> @@ -73,6 +73,12 @@ static inline void *kmap_local_page(struct page *page)
> return __kmap_local_page_prot(page, kmap_prot);
> }
>
> +static inline void *kmap_local_folio(struct folio *folio, size_t offset)
> +{
> + struct page *page = folio_page(folio, offset / PAGE_SIZE);
> + return __kmap_local_page_prot(page, kmap_prot) + offset % PAGE_SIZE;
> +}
> +
> static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
> {
> return __kmap_local_page_prot(page, prot);
> @@ -160,6 +166,11 @@ static inline void *kmap_local_page(struct page *page)
> return page_address(page);
> }
>
> +static inline void *kmap_local_folio(struct folio *folio, size_t offset)
> +{
> + return page_address(&folio->page) + offset;
> +}
> +
> static inline void *kmap_local_page_prot(struct page *page, pgprot_t prot)
> {
> return kmap_local_page(page);
> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 8c6e8e996c87..85de3bd0b47d 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -96,6 +96,44 @@ static inline void kmap_flush_unused(void);
> */
> static inline void *kmap_local_page(struct page *page);
>
> +/**
> + * 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.
> + */
> +static inline void *kmap_local_folio(struct folio *folio, size_t offset);
> +
> /**
> * kmap_atomic - Atomically map a page for temporary usage - Deprecated!
> * @page: Pointer to the page to be mapped
> --
> 2.30.2
>
>

--
Sincerely yours,
Mike.