[PATCH 13/13] mm: Document pXd_leaf() API

[peterx@redhat.com]

From: Peter Xu <peterx@redhat.com>

There's one small section already, but since we're going to remove
pXd_huge(), that comment may start to obsolete.

Rewrite that section with more information, hopefully with that the API is
crystal clear on what it implies.

Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
---
 include/linux/pgtable.h | 24 +++++++++++++++++++-----
 1 file changed, 19 insertions(+), 5 deletions(-)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 85fc7554cd52..6b0d222a7fad 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1770,11 +1770,25 @@ typedef unsigned int pgtbl_mod_mask;
 #endif
 
 /*
- * p?d_leaf() - true if this entry is a final mapping to a physical address.
- * This differs from p?d_huge() by the fact that they are always available (if
- * the architecture supports large pages at the appropriate level) even
- * if CONFIG_HUGETLB_PAGE is not defined.
- * Only meaningful when called on a valid entry.
+ * pXd_leaf() is the API to check whether a pgtable entry is a huge page
+ * mapping.  It should work globally across all archs, without any
+ * dependency on CONFIG_* options.  For architectures that do not support
+ * huge mappings on specific levels, below fallbacks will be used.
+ *
+ * A leaf pgtable entry should always imply the following:
+ *
+ * - It is a "present" entry.  IOW, before using this API, please check it
+ *   with pXd_present() first. NOTE: it may not always mean the "present
+ *   bit" is set.  For example, PROT_NONE entries are always "present".
+ *
+ * - It should _never_ be a swap entry of any type.  Above "present" check
+ *   should have guarded this, but let's be crystal clear on this.
+ *
+ * - It should contain a huge PFN, which points to a huge page larger than
+ *   PAGE_SIZE of the platform.  The PFN format isn't important here.
+ *
+ * - It should cover all kinds of huge mappings (e.g., pXd_trans_huge(),
+ *   pXd_devmap(), or hugetlb mappings).
  */
 #ifndef pgd_leaf
 #define pgd_leaf(x)	false
-- 
2.44.0