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

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Message-ID: <258d0ddb-6c82-0c95-a15e-b085b59d2142@redhat.com>
Date:   Mon, 4 Oct 2021 13:55:30 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Naoya Horiguchi <naoya.horiguchi@...ux.dev>, linux-mm@...ck.org
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Alistair Popple <apopple@...dia.com>,
        Peter Xu <peterx@...hat.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Bin Wang <wangbin224@...wei.com>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1] mm, pagemap: expose hwpoison entry

On 04.10.21 13:50, Naoya Horiguchi wrote:
> From: Naoya Horiguchi <naoya.horiguchi@....com>
> 
> A hwpoison entry is a non-present page table entry to report
> memory error events to userspace. If we have an easy way to know
> which processes have hwpoison entries, that might be useful for
> user processes to take proper actions. But we don't have it now.
> So make pagemap interface expose hwpoison entries to userspace.

Noting that this is only a way to inspect hwpoison set for private 
anonymous memory. You cannot really identify anything related to shared 
memory.

Do you also handle private hugetlb pages?

> 
> 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@....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


-- 
Thanks,

David / dhildenb