[PATCH v2] madvise.2: Clarify addr/length and update hugetlb support

* [PATCH v2] madvise.2: Clarify addr/length and update hugetlb support
@ 2022-05-26 18:09 Mike Kravetz
  2022-05-26 18:21 ` Peter Xu
                   ` (2 more replies)
  0 siblings, 3 replies; 10+ messages in thread
From: Mike Kravetz @ 2022-05-26 18:09 UTC (permalink / raw)
  To: linux-man, linux-mm
  Cc: Alejandro Colomar, Michael Kerrisk, David Hildenbrand,
	Axel Rasmussen, Peter Xu, Mike Kravetz

Clarify that madvise only works on full pages, and remove references
to 'bytes'.

Update MADV_DONTNEED and MADV_REMOVE sections to remove notes that
HugeTLB mappings are not supported.  Indicate the releases when they
were first supported as well as alignment restrictions.

Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
---
v1 -> v2 Added releases when Huge TLB support was added and moved
	alignment requirements to corresponding section.  (Peter)

 man2/madvise.2 | 31 +++++++++++++++++++++----------
 1 file changed, 21 insertions(+), 10 deletions(-)

diff --git a/man2/madvise.2 b/man2/madvise.2
index f1f384c0c..50686e7e3 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2
@@ -61,9 +61,13 @@ system call is used to give advice or directions to the kernel
 about the address range beginning at address
 .I addr
 and with size
+.IR length .
+.BR madvise ()
+only operates on whole pages, therefore
+.I addr
+must be page-aligned.  The value of
 .I length
-bytes
-In most cases,
+is rounded up to a multiple of page size.  In most cases,
 the goal of such advice is to improve system or application performance.
 .PP
 Initially, the system call supported a set of "conventional"
@@ -143,7 +147,7 @@ The resident set size (RSS) of the calling process will be immediately
 reduced however.
 .IP
 .B MADV_DONTNEED
-cannot be applied to locked pages, Huge TLB pages, or
+cannot be applied to locked pages, or
 .BR VM_PFNMAP
 pages.
 (Pages marked with the kernel-internal
@@ -153,6 +157,11 @@ flag are special memory areas that are not managed
 by the virtual memory subsystem.
 Such pages are typically created by device drivers that
 map the pages into user space.)
+.IP
+Support for Huge TLB pages was added in Linux v5.18.  Addresses within a
+mapping backed by Huge TLB pages must be aligned to the underlying Huge TLB
+page size, and the range length is rounded up to a multiple of the underlying
+Huge TLB page size.
 .\"
 .\" ======================================================================
 .\"
@@ -170,24 +179,24 @@ Note that some of these operations change the semantics of memory accesses.
 .\" commit f6b3ec238d12c8cc6cc71490c6e3127988460349
 Free up a given range of pages
 and its associated backing store.
-This is equivalent to punching a hole in the corresponding byte
+This is equivalent to punching a hole in the corresponding
 range of the backing store (see
 .BR fallocate (2)).
 Subsequent accesses in the specified address range will see
-bytes containing zero.
+pages containing zero.
 .\" Databases want to use this feature to drop a section of their
 .\" bufferpool (shared memory segments) - without writing back to
 .\" disk/swap space.  This feature is also useful for supporting
 .\" hot-plug memory on UML.
 .IP
 The specified address range must be mapped shared and writable.
-This flag cannot be applied to locked pages, Huge TLB pages, or
+This flag cannot be applied to locked pages, or
 .BR VM_PFNMAP
 pages.
 .IP
 In the initial implementation, only
 .BR tmpfs (5)
-was supported
+supported
 .BR MADV_REMOVE ;
 but since Linux 3.5,
 .\" commit 3f31d07571eeea18a7d34db9af21d2285b807a17
@@ -196,10 +205,12 @@ any filesystem which supports the
 .BR FALLOC_FL_PUNCH_HOLE
 mode also supports
 .BR MADV_REMOVE .
-Hugetlbfs fails with the error
-.BR EINVAL
-and other filesystems fail with the error
+Filesystems which do not support
+.BR MADV_REMOVE
+fail with the error
 .BR EOPNOTSUPP .
+.IP
+Support for Huge TLB filesystem was added in Linux v4.3.
 .TP
 .BR MADV_DONTFORK " (since Linux 2.6.16)"
 .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4
-- 
2.35.3


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