mm/highmem: fix kernel-doc warnings in highmem*.h

Patch series "Extend and reorganize Highmem's documentation", v4.

This series has the purpose to extend and reorganize Highmem's
documentation.

This is a work in progress because some information should still be moved
from highmem.rst to highmem.h and highmem-internal.h.  Specifically I'm
talking about moving the "how to" information to the relevant headers, as
it as been suggested by Ira Weiny (Intel).

Also, this is a work in progress because some kdocs in highmem.h and
highmem-internal.h should be improved.


This patch (of 4):

`scripts/kernel-doc -v -none include/linux/highmem*` reports the following
warnings:

include/linux/highmem.h:160: warning: expecting prototype for kunmap_atomic(). Prototype was for nr_free_highpages() instead
include/linux/highmem.h:204: warning: No description found for return value of 'alloc_zeroed_user_highpage_movable'
include/linux/highmem-internal.h:256: warning: Function parameter or member '__addr' not described in 'kunmap_atomic'
include/linux/highmem-internal.h:256: warning: Excess function parameter 'addr' description in 'kunmap_atomic'

Fix these warnings by (1) moving the kernel-doc comments from highmem.h to
highmem-internal.h (which is the file were the kunmap_atomic() macro is
actually defined), (2) extending and merging it with the comment which was
already in highmem-internal.h, and (3) using correct parameter names (4)
correcting a few technical inaccuracies in comments, and (5) adding a
deprecation notice in kunmap_atomic() for consistency with kmap_atomic().

Link: https://lkml.kernel.org/r/20220428212455.892-1-fmdefrancesco@gmail.com
Link: https://lkml.kernel.org/r/20220428212455.892-2-fmdefrancesco@gmail.comSigned-off-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Peter Collingbourne <pcc@google.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/highmem-internal.h

@@ -236,9 +236,21 @@ static inline unsigned long totalhigh_pages(void) { return 0UL; }
 
 #endif /* CONFIG_HIGHMEM */
 
-/*
- * Prevent people trying to call kunmap_atomic() as if it were kunmap()
- * kunmap_atomic() should get the return value of kmap_atomic, not the page.
+/**
+ * kunmap_atomic - Unmap the virtual address mapped by kmap_atomic() - deprecated!
+ * @__addr:       Virtual address to be unmapped
+ *
+ * Unmaps an address previously mapped by kmap_atomic() and re-enables
+ * pagefaults. Depending on PREEMP_RT configuration, re-enables also
+ * migration and preemption. Users should not count on these side effects.
+ *
+ * Mappings should be unmapped in the reverse order that they were mapped.
+ * See kmap_local_page() for details on nesting.
+ *
+ * @__addr can be any address within the mapped page, so there is no need
+ * to subtract any offset that has been added. In contrast to kunmap(),
+ * this function takes the address returned from kmap_atomic(), not the
+ * page passed to it. The compiler will warn you if you pass the page.
  */
 #define kunmap_atomic(__addr)					\
 do {								\

include/linux/highmem.h

@@ -37,7 +37,7 @@ static inline void *kmap(struct page *page);
 
 /**
  * kunmap - Unmap the virtual address mapped by kmap()
- * @addr:	Virtual address to be unmapped
+ * @page:	Pointer to the page which was mapped by kmap()
  *
  * Counterpart to kmap(). A NOOP for CONFIG_HIGHMEM=n and for mappings of
  * pages in the low memory area.
@@ -138,24 +138,16 @@ static inline void *kmap_local_folio(struct folio *folio, size_t offset);
  *
  * Returns: The virtual address of the mapping
  *
- * Effectively a wrapper around kmap_local_page() which disables pagefaults
- * and preemption.
+ * In fact a wrapper around kmap_local_page() which also disables pagefaults
+ * and, depending on PREEMPT_RT configuration, also CPU migration and
+ * preemption. Therefore users should not count on the latter two side effects.
+ *
+ * Mappings should always be released by kunmap_atomic().
  *
  * Do not use in new code. Use kmap_local_page() instead.
  */
 static inline void *kmap_atomic(struct page *page);
 
-/**
- * kunmap_atomic - Unmap the virtual address mapped by kmap_atomic()
- * @addr:	Virtual address to be unmapped
- *
- * Counterpart to kmap_atomic().
- *
- * Effectively a wrapper around kunmap_local() which additionally undoes
- * the side effects of kmap_atomic(), i.e. reenabling pagefaults and
- * preemption.
- */
-
 /* Highmem related interfaces for management code */
 static inline unsigned int nr_free_highpages(void);
 static inline unsigned long totalhigh_pages(void);
@@ -191,6 +183,8 @@ static inline void clear_user_highpage(struct page *page, unsigned long vaddr)
  * @vma: The VMA the page is to be allocated for
  * @vaddr: The virtual address the page will be inserted into
  *
+ * Returns: The allocated and zeroed HIGHMEM page
+ *
  * This function will allocate a page for a VMA that the caller knows will
  * be able to migrate in the future using move_pages() or reclaimed
  *