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Date: Tue, 30 Jan 2024 17:09:44 +0000
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Subject: Re: [PATCH v7 1/1] mm: report per-page metadata information
From: Yosry Ahmed <yosryahmed@google.com>
To: Greg KH <gregkh@linuxfoundation.org>
Cc: Sourav Panda <souravpanda@google.com>, corbet@lwn.net, rafael@kernel.org, 
	akpm@linux-foundation.org, mike.kravetz@oracle.com, muchun.song@linux.dev, 
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On Mon, Jan 29, 2024 at 02:52:23PM -0800, Greg KH wrote:
> On Mon, Jan 29, 2024 at 02:42:04PM -0800, Sourav Panda wrote:
> > Adds two new per-node fields, namely nr_page_metadata and
> > nr_page_metadata_boot, to /sys/devices/system/node/nodeN/vmstat
> > and a global PageMetadata field to /proc/meminfo. This information can
> > be used by users to see how much memory is being used by per-page
> > metadata, which can vary depending on build configuration, machine
> > architecture, and system use.
> > 
> > Per-page metadata is the amount of memory that Linux needs in order to
> > manage memory at the page granularity. The majority of such memory is
> > used by "struct page" and "page_ext" data structures. In contrast to
> > most other memory consumption statistics, per-page metadata might not
> > be included in MemTotal. For example, MemTotal does not include memblock
> > allocations but includes buddy allocations. In this patch, exported
> > field nr_page_metadata in /sys/devices/system/node/nodeN/vmstat would
> > exclusively track buddy allocations while nr_page_metadata_boot would
> > exclusively track memblock allocations. Furthermore, PageMetadata in
> > /proc/meminfo would exclusively track buddy allocations allowing it to
> > be compared against MemTotal.
> > 
> > This memory depends on build configurations, machine architectures, and
> > the way system is used:
> > 
> > Build configuration may include extra fields into "struct page",
> > and enable / disable "page_ext"
> > Machine architecture defines base page sizes. For example 4K x86,
> > 8K SPARC, 64K ARM64 (optionally), etc. The per-page metadata
> > overhead is smaller on machines with larger page sizes.
> > System use can change per-page overhead by using vmemmap
> > optimizations with hugetlb pages, and emulated pmem devdax pages.
> > Also, boot parameters can determine whether page_ext is needed
> > to be allocated. This memory can be part of MemTotal or be outside
> > MemTotal depending on whether the memory was hot-plugged, booted with,
> > or hugetlb memory was returned back to the system.
> > 
> > Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> > Signed-off-by: Sourav Panda <souravpanda@google.com>
> > ---
> >  Documentation/filesystems/proc.rst |  3 +++
> >  fs/proc/meminfo.c                  |  4 ++++
> >  include/linux/mmzone.h             |  4 ++++
> >  include/linux/vmstat.h             |  4 ++++
> >  mm/hugetlb_vmemmap.c               | 19 ++++++++++++++----
> >  mm/mm_init.c                       |  3 +++
> >  mm/page_alloc.c                    |  1 +
> >  mm/page_ext.c                      | 32 +++++++++++++++++++++---------
> >  mm/sparse-vmemmap.c                |  8 ++++++++
> >  mm/sparse.c                        |  7 ++++++-
> >  mm/vmstat.c                        | 26 +++++++++++++++++++++++-
> >  11 files changed, 96 insertions(+), 15 deletions(-)
> > 
> > diff --git a/Documentation/filesystems/proc.rst b/Documentation/filesystems/proc.rst
> > index 49ef12df631b..d5901d04e082 100644
> > --- a/Documentation/filesystems/proc.rst
> > +++ b/Documentation/filesystems/proc.rst
> > @@ -993,6 +993,7 @@ Example output. You may not have all of these fields.
> >      AnonPages:       4654780 kB
> >      Mapped:           266244 kB
> >      Shmem:              9976 kB
> > +    PageMetadata:     513419 kB
> >      KReclaimable:     517708 kB
> >      Slab:             660044 kB
> >      SReclaimable:     517708 kB
> 
> Why are you adding it to the middle of the file?  Are you sure the
> userspace tools that parse this file today can handle an unknown field
> here, and not just at the end of the file?

FWIW, looking at git blame for fs/proc/meminfo.c, it seems like people
have generally been adding items where it makes sense semantically, not
at the end of the file. So maybe that's okay for userspace tools.