mm/mempolicy: rewrite alloc_pages documentation

mainline inclusion
from mainline-5.13-rc1
commit 6421ec76
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZVL2
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6421ec764a62c51f810c5dc40cd45eeb15801ad9

-------------------------------------------------

Document alloc_pages() for both NUMA and non-NUMA cases as kernel-doc
doesn't care.

Link: https://lkml.kernel.org/r/20210225150642.2582252-6-willy@infradead.orgSigned-off-by: NMatthew Wilcox (Oracle) <willy@infradead.org>
Acked-by: NMike Rapoport <rppt@linux.ibm.com>
Acked-by: NMichal Hocko <mhocko@suse.com>
Acked-by: NVlastimil Babka <vbabka@suse.cz>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 6421ec76)
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>
Reviewed-by: NTong Tiangen <tongtiangen@huawei.com>
Signed-off-by: NZheng Zengkai <zhengzengkai@huawei.com>

mm/mempolicy.c

@@ -2239,19 +2239,18 @@ alloc_pages_vma(gfp_t gfp, int order, struct vm_area_struct *vma,
 EXPORT_SYMBOL(alloc_pages_vma);
 
 /**
- * 	alloc_pages - Allocate pages.
+ * alloc_pages - Allocate pages.
+ * @gfp: GFP flags.
+ * @order: Power of two of number of pages to allocate.
  *
- *	@gfp:
- *		%GFP_USER   user allocation,
- *      	%GFP_KERNEL kernel allocation,
- *      	%GFP_HIGHMEM highmem allocation,
- *      	%GFP_FS     don't call back into a file system.
- *      	%GFP_ATOMIC don't sleep.
- *	@order: Power of two of allocation size in pages. 0 is a single page.
+ * Allocate 1 << @order contiguous pages.  The physical address of the
+ * first page is naturally aligned (eg an order-3 allocation will be aligned
+ * to a multiple of 8 * PAGE_SIZE bytes).  The NUMA policy of the current
+ * process is honoured when in process context.
  *
- *	Allocate a page from the kernel page pool.  When not in
- *	interrupt context and apply the current process NUMA policy.
- *	Returns NULL when no page can be allocated.
+ * Context: Can be called from any context, providing the appropriate GFP
+ * flags are used.
+ * Return: The page on success or NULL if allocation fails.
  */
 struct page *alloc_pages(gfp_t gfp, unsigned order)
 {