Re: [PATCH v4] madvise.2: Clarify addr/length and update hugetlb support

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Hi, Mike and Peter!

On 6/9/22 20:52, Mike Kravetz wrote:
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@xxxxxxxxxx>
Acked-by: Peter Xu <peterx@xxxxxxxxxx>

Thanks for the patch.  Applied.
And thanks, Peter, for reviewing it!

Cheers,

Alex

---
v3 -> v4 Formatting updates (Alex)
v2 -> v3 Rebased on man-pages-5.19-rc1.  Minor change to wording for
         sunsequent access of data after MADV_REMOVE.
v1 -> v2 Added releases when Huge TLB support was added and moved
         alignment requirements to corresponding section.  (Peter)
  man2/madvise.2 | 33 +++++++++++++++++++++++----------
  1 file changed, 23 insertions(+), 10 deletions(-)

diff --git a/man2/madvise.2 b/man2/madvise.2
index 2a8f1cd0a..7fc184e20 100644
--- a/man2/madvise.2
+++ b/man2/madvise.2
@@ -44,9 +44,14 @@ 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,

s/.  /.\n/

But I fixed & amended.

  the goal of such advice is to improve system or application performance.
  .PP
  Initially, the system call supported a set of "conventional"
@@ -126,7 +131,7 @@ 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
  .B VM_PFNMAP
  pages.
  (Pages marked with the kernel-internal
@@ -136,6 +141,12 @@ 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.
  .\"
  .\" ======================================================================
  .\"
@@ -153,24 +164,24 @@ 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.
+data with a value of 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
  .B 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
@@ -179,10 +190,12 @@ any filesystem which supports the
  .B FALLOC_FL_PUNCH_HOLE
  mode also supports
  .BR MADV_REMOVE .
-Hugetlbfs fails with the error
-.B EINVAL
-and other filesystems fail with the error
+Filesystems which do not support
+.B MADV_REMOVE
+fail with the error
  .BR EOPNOTSUPP .
+.IP
+Support for the Huge TLB filesystem was added in Linux v4.3.
  .TP
  .BR MADV_DONTFORK " (since Linux 2.6.16)"
  .\" commit f822566165dd46ff5de9bf895cfa6c51f53bb0c4


--
Alejandro Colomar
Linux man-pages comaintainer; http://www.kernel.org/doc/man-pages/
http://www.alejandro-colomar.es/

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