linux-kernel - Re: [PATCH v14 054/138] mm: Add kmap_local

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Date:   Wed, 21 Jul 2021 15:12:03 +0100
From:   Matthew Wilcox <willy@...radead.org>
To:     Mike Rapoport <rppt@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v14 054/138] mm: Add kmap_local_folio()

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);