Re: [patch 18/39] remap_file_pages protection support: add VM_FAULT_SIGSEGV

[Nick Piggin <nickpiggin@yahoo.com.au>]

Russell King wrote:
> On Fri, Aug 12, 2005 at 08:21:45PM +0200, blaisorblade@yahoo.it wrote:
>
> > @@ -632,10 +632,11 @@ static inline int page_mapped(struct pag
> >  * Used to decide whether a process gets delivered SIGBUS or
> >  * just gets major/minor fault counters bumped up.
> >  */
> > -#define VM_FAULT_OOM	(-1)
> > -#define VM_FAULT_SIGBUS	0
> > -#define VM_FAULT_MINOR	1
> > -#define VM_FAULT_MAJOR	2
> > +#define VM_FAULT_OOM		(-1)
> > +#define VM_FAULT_SIGBUS		0
> > +#define VM_FAULT_MINOR		1
> > +#define VM_FAULT_MAJOR		2
> > +#define VM_FAULT_SIGSEGV	3
> >
> > #define offset_in_page(p)	((unsigned long)(p) & ~PAGE_MASK)
>
>
> Please arrange for "success" values to be numerically larger than "failure"
> values.  This will avoid breaking ARM.
>
> Is there a reason why we don't use -ve numbers for failure and +ve for
> success here?

Well there is now, and that is we are now using a bit in the 2nd
byte as flags. So I had to do away with -ve numbers there entirely.

You could achieve a similar thing by using another bit in that byte
#define VM_FAULT_FAILED 0x20
and make that bit present in VM_FAULT_OOM and VM_FAULT_SIGBUS, then
do an unlikely test for that bit in your handler and branch away to
the slow path.

--
SUSE Labs, Novell Inc.

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