linux-kernel - Re: [PATCH v2 2/3] doc/vm: add information about page_table_check warn

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Date:   Sat, 22 Jul 2023 23:37:10 -0400
From:   Pasha Tatashin <pasha.tatashin@...een.com>
To:     Randy Dunlap <rdunlap@...radead.org>
Cc:     akpm@...ux-foundation.org, corbet@....net, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        rick.p.edgecombe@...el.com
Subject: Re: [PATCH v2 2/3] doc/vm: add information about page_table_check
 warn_on behavior

On Sat, Jul 22, 2023 at 7:59 PM Randy Dunlap <rdunlap@...radead.org> wrote:
>
> Hi--
>
> On 7/22/23 16:15, Pasha Tatashin wrote:
> > The default behavior of page table check was changed from panicking
> > kernel to printing a warning.
> >
> > Add a note how to still panic the kernel when error is detected.
> >
> > Signed-off-by: Pasha Tatashin<pasha.tatashin@...een.com>
> > ---
> >   Documentation/mm/page_table_check.rst | 5 +++--
> >   1 file changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/Documentation/mm/page_table_check.rst b/Documentation/mm/page_table_check.rst
> > index c12838ce6b8d..f534c80ee9c9 100644
> > --- a/Documentation/mm/page_table_check.rst
> > +++ b/Documentation/mm/page_table_check.rst
> > @@ -14,13 +14,14 @@ Page table check performs extra verifications at the time when new pages become
> >   accessible from the userspace by getting their page table entries (PTEs PMDs
> >   etc.) added into the table.
> >
> > -In case of detected corruption, the kernel is crashed. There is a small
> > +In case of detected corruption, a warning is printed. There is a small
> >   performance and memory overhead associated with the page table check. Therefore,
> >   it is disabled by default, but can be optionally enabled on systems where the
> >   extra hardening outweighs the performance costs. Also, because page table check
> >   is synchronous, it can help with debugging double map memory corruption issues,
> >   by crashing kernel at the time wrong mapping occurs instead of later which is
> > -often the case with memory corruptions bugs.
> > +often the case with memory corruptions bugs. In order to crash kernel sysctl
> > +panic_on_warn should be set to 1.
>
> Better as:
>    In order to crash the kernel, the sysctl panic_on_warn should be set
> to 1.

Will update in the next version.

Thanks,
Pasha

>