[PATCH man-pages v2 1/4] madvise.2: update THP file/shmem documentation for +5.4

[Zach OKeefe <zokeefe@google.com>]

From: Zach O'Keefe <zokeefe@google.com>

Since Linux 5.4, Transparent Huge Pages now support both file-backed
memory and shmem memory. Update MADV_HUGEPAGE advice description to
reflect this.

Additionally, expand the description of requirements for memory to be
considered eligible for THP: alignment / mapping requirements, VMA
flags, prctl(2) settings, inode status, etc.

Signed-off-by: Zach O'Keefe <zokeefe@google.com>
---
 man2/madvise.2 | 38 +++++++++++++++++++++++++++++++++++---
 1 file changed, 35 insertions(+), 3 deletions(-)

diff --git a/man2/madvise.2 b/man2/madvise.2
index 81cce56af..64f788ace 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2
@@ -320,8 +320,6 @@ Enable Transparent Huge Pages (THP) for pages in the range specified by
 .I addr
 and
 .IR length .
-Currently, Transparent Huge Pages work only with private anonymous pages (see
-.BR mmap (2)).
 The kernel will regularly scan the areas marked as huge page candidates
 to replace them with huge pages.
 The kernel will also allocate huge pages directly when the region is
@@ -354,12 +352,46 @@ an access pattern that the developer knows in advance won't risk
 to increase the memory footprint of the application when transparent
 hugepages are enabled.
 .IP
+.\" commit 99cb0dbd47a15d395bf3faa78dc122bc5efe3fc0
+Since Linux 5.4,
+automatic scan of eligible areas and replacement by huge pages works with
+private anonymous pages (see
+.BR mmap (2)),
+shmem pages,
+and file-backed pages.
+For all memory types,
+memory may only be replaced by huge pages on hugepage-aligned boundaries.
+For file-mapped memory \(em including tmpfs (see
+.BR tmpfs (2))
+\(em the mapping must also be naturally hugepage-aligned within the file.
+Additionally,
+for file-backed,
+non-tmpfs memory,
+the file must not be open for write and the mapping must be executable.
+.IP
+The VMA must not be marked
+.BR VM_NOHUGEPAGE ,
+.BR VM_HUGETLB ,
+.BR VM_IO ,
+.BR VM_DONTEXPAND ,
+.BR VM_MIXEDMAP ,
+or
+.BR VM_PFNMAP ,
+nor can it be stack memory or backed by a DAX-enabled device
+(unless the DAX device is hot-plugged as System RAM).
+The process must also not have
+.B PR_SET_THP_DISABLE
+set (see
+.BR prctl (2) ).
+.IP
 The
 .B MADV_HUGEPAGE
 and
 .B MADV_NOHUGEPAGE
 operations are available only if the kernel was configured with
-.BR CONFIG_TRANSPARENT_HUGEPAGE .
+.B CONFIG_TRANSPARENT_HUGEPAGE
+and file/shmem memory is only supported if the kernel was configured with
+.BR CONFIG_READ_ONLY_THP_FOR_FS .
 .TP
 .BR MADV_NOHUGEPAGE " (since Linux 2.6.38)"
 Ensures that memory in the address range specified by
-- 
2.38.0.413.g74048e4d9e-goog