mm: document pXd_leaf() API

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.

Link: https://lkml.kernel.org/r/20240318200404.448346-15-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Cc: Alistair Popple <apopple@nvidia.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: "Aneesh Kumar K.V" <aneesh.kumar@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Bjorn Andersson <andersson@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Fabio Estevam <festevam@denx.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Konrad Dybcio <konrad.dybcio@linaro.org>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Cc: Lucas Stach <l.stach@pengutronix.de>
Cc: Mark Salter <msalter@redhat.com>
Cc: "Matthew Wilcox (Oracle)" <willy@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Naoya Horiguchi <nao.horiguchi@gmail.com>
Cc: "Naveen N. Rao" <naveen.n.rao@linux.ibm.com>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 19 insertions, 5 deletions

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index bd9c7180718c..2a1c044ae467 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -1768,11 +1768,25 @@ 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