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

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

[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>
Acked-by: Peter Xu <peterx@redhat.com>
---
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,
 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
-- 
2.35.3

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

[Alejandro Colomar]

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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@oracle.com>
> Acked-by: Peter Xu <peterx@redhat.com>

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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