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

[Peter Xu <peterx@redhat.com>]

Hi, Mike,

Some minor nitpicks below.

On Tue, May 24, 2022 at 04:28:44PM -0700, 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.  They now are supported.
> 
> Under 'Linux notes' describe addr requirements and length handling
> for ranges in HugeTLB mappings.
> 
> Signed-off-by: Mike Kravetz <mike.kravetz@oracle.com>
> ---
>  man2/madvise.2 | 36 ++++++++++++++++++++++++++----------
>  1 file changed, 26 insertions(+), 10 deletions(-)
> 
> diff --git a/man2/madvise.2 b/man2/madvise.2
> index f1f384c0c..c3b0615cb 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.

This looks good, but since this will be a behavior change and we won't be
able to change the old kernels, I saw the man page normally does this with
things like:

  Since Linux 5.18, this madvise supports hugetlbfs pages.

Majorly it states starting from which version it'll work, and when it'll
not.

>  (Pages marked with the kernel-internal
> @@ -170,24 +174,24 @@ 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
> @@ -196,9 +200,9 @@ 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 .
>  .TP
>  .BR MADV_DONTFORK " (since Linux 2.6.16)"
> @@ -596,6 +600,18 @@ that are not mapped, the Linux version of
>  ignores them and applies the call to the rest (but returns
>  .B ENOMEM
>  from the system call, as it should).
> +.PP
> +If the specified address
> +.I addr
> +is within a mapping backed by Huge TLB pages, then
> +.I addr
> +must be aligned to the underlying Huge TLB page size.  If the range
> +specified by
> +.I addr
> +and
> +.I length
> +ends in a mapping backed by Huge TLB pages, then the end of the range
> +will be rounded up to a multiple of the underlying Huge TLB page size.

I'm slightly worried this could be hidden too deep, meanwhile it duplicates
part of the sentence of how start/end will be treated.

How about adding a short paragraph into each of MADV_DONTNEED and
MADV_REMOVE section (right after the new sentences upon hugetlbfs), with:

  For hugetlbfs, the start/end alignments on page sizes will be based on
  huge page size.

No strong opinions on any of these.  Anyway:

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

Thanks,

-- 
Peter Xu