Re: [PATCH v1] mm, pagemap: expose hwpoison entry

[Naoya Horiguchi <naoya.horiguchi@linux.dev>]

On Mon, Oct 04, 2021 at 11:32:28PM +0900, Naoya Horiguchi wrote:
> On Mon, Oct 04, 2021 at 01:55:30PM +0200, David Hildenbrand wrote:
> > On 04.10.21 13:50, Naoya Horiguchi wrote:
...
> > >
> > > Hwpoison entry for hugepage is also exposed by this patch. The below
> > > example shows how pagemap is visible in the case where a memory error
> > > hit a hugepage mapped to a process.
> > >
> > >      $ ./page-types --no-summary --pid $PID --raw --list --addr 0x700000000+0x400
> > >      voffset offset  len     flags
> > >      700000000       12fa00  1       ___U_______Ma__H_G_________________f_______1
> > >      700000001       12fa01  1ff     ___________Ma___TG_________________f_______1
> > >      700000200       12f800  1       __________B________X_______________f______w_
> > >      700000201       12f801  1       ___________________X_______________f______w_   // memory failure hit this page
> > >      700000202       12f802  1fe     __________B________X_______________f______w_
> > >
> > > The entries with both of "X" flag (hwpoison flag) and "w" flag (swap
> > > flag) are considered as hwpoison entries.  So all pages in 2MB range
> > > are inaccessible from the process.  We can get actual error location
> > > by page-types in physical address mode.
> > >
> > >      $ ./page-types --no-summary --addr 0x12f800+0x200 --raw --list
> > >      offset  len     flags
> > >      12f800  1       __________B_________________________________
> > >      12f801  1       ___________________X________________________
> > >      12f802  1fe     __________B_________________________________
> > >
> > > Signed-off-by: Naoya Horiguchi <naoya.horiguchi@nec.com>
> > > ---
> > >   fs/proc/task_mmu.c      | 41 ++++++++++++++++++++++++++++++++---------
> > >   include/linux/swapops.h | 13 +++++++++++++
> > >   tools/vm/page-types.c   |  7 ++++++-
> > >   3 files changed, 51 insertions(+), 10 deletions(-)
> >
> >
> > Please also update the documentation located at
> >
> > Documentation/admin-guide/mm/pagemap.rst
>
> I will do this in the next post.

Reading the document, I found that swap type is already exported so we
could identify hwpoison entry with it (without new PM_HWPOISON bit).
One problem is that the format of swap types (like SWP_HWPOISON) depends
on a few config macros like CONFIG_DEVICE_PRIVATE and CONFIG_MIGRATION,
so we also need to export how the swap type field is interpreted.

I thought of adding new interfaces for example under /sys/kernel/mm/swap/type_format/,
which shows info like below (assuming that all CONFIG_{DEVICE_PRIVATE,MIGRATION,MEMORY_FAILURE}
is enabled):

  $ ls /sys/kernel/mm/swap/type_format/
  hwpoison
  migration_read
  migration_write
  device_write
  device_read
  device_exclusive_write
  device_exclusive_read
  
  $ cat /sys/kernel/mm/swap/type_format/hwpoison
  25
  
  $ cat /sys/kernel/mm/swap/type_format/device_write
  28

Does it make sense or any better approach?

Thanks,
Naoya Horiguchi