Re: [PATCH v3 x86 1/2] fix page faults by nmiaction in nmi if kmemcheck is enabled

Vegard Nossum
2012-03-06T10:27:26Z

On 6 March 2012 11:09, Li Zhong <zhong@linux.vnet.ibm.com> wrote:
> This patch tries to fix the problem of page fault exception caused by
> accessing nmiaction structure in nmi if kmemcheck is enabled.
>
> If kmemcheck is enabled, the memory allocated through slab are in pages
> that are marked non-present, so that some checks could be done in the
> page fault handling code ( e.g. whether the memory is read before
> written to ).
> As nmiaction is allocated in this way, so it resides in a non-present
> page. Then there is a page fault while the nmi code accessing the
> nmiaction structure, which would then cause a warning by
> WARN_ON_ONCE(in_nmi()) in kmemcheck_fault(), called by do_page_fault().
>
> v2: as Peter suggested, changed the nmiaction to use static storage.
>
> v3: as Peter suggested, use macro to shorten the codes. Also keep the
> original usage of register_nmi_handler, so users of this call doesn't
> need change.
>
> Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>

Looks like you've solved this now. Thanks.

For the record, another way to prevent the page fault from happening
in the first place is to set up a new slab cache with the flag
SLAB_NOTRACK. This is different from the GFP_NOTRACK flag which, as
you noted, doesn't prevent page faults, just inhibits
checking/warnings for those memory areas.

It's a bit of a hassle, I admit. Maybe we could create an additional,
separate set of slab caches (using SLAB_NOTRACK) and a new GFP flag
which selects this set of caches instead. This would allow anything
that takes a gfp_t to allocate memory that is guaranteed not to page
fault when using kmemcheck. Pekka, any thoughts?

Vegard