Re: [git pull] cpus4096 tree, part 3

Ingo Molnar
2009-01-26T20:10:39Z

* Andrew Morton <akpm@linux-foundation.org> wrote:

> but why did the current code pass testing at all??

i queued it up a week ago and beyond a same-day breakage i reported to
Nick (and which he fixed) this commit was problem-free and passed all
testing here.

Does it cause problems for you? If yes then please describe the kind of
problems.

Note: i see that -mm modifies a few other details of the x86 pagefault
handling path (there a pagefault-retry patch in there) - so there might be
contextual interactions there. But this particular cleanup/improvement
from Nick is working fine on a wide range of systems here.

Btw., regarding pagefault retry. The bits that are in -mm currently i
find a bit ugly:

> +++ a/arch/x86/mm/fault.c
> @@ -799,7 +799,7 @@ void __kprobes do_page_fault(struct pt_r
> struct vm_area_struct *vma;
> int write;
> int fault;
> - unsigned int retry_flag = FAULT_FLAG_RETRY;
> + int retry_flag = 1;
>
> tsk = current;
> mm = tsk->mm;
> @@ -951,6 +951,7 @@ good_area:
> }
>
> write |= retry_flag;
> +
> /*
> * If for any reason at all we couldn't handle the fault,
> * make sure we exit gracefully rather than endlessly redo
> @@ -969,8 +970,8 @@ good_area:
> * be removed or changed after the retry.
> */
> if (fault & VM_FAULT_RETRY) {
> - if (write & FAULT_FLAG_RETRY) {
> - retry_flag &= ~FAULT_FLAG_RETRY;
> + if (retry_flag) {
> + retry_flag = 0;
> goto retry;
> }
> BUG();

as this complicates every architecture with a 'can the fault be retried'
logic and open-coded retry loop.

But that logic is rather repetitive and once an architecture filters out
all its special in-kernel sources of faults and the hw quirks it has, the
handling of pte faults is rather generic and largely offloaded into
handle_pte_fault() already.

So when this patch was submitted a few weeks ago i suggested that retry
should be done purely in mm/memory.c instead, and the low level code
should at most be refactored to suit this method, but not complicated any
further.

Any deep reasons for why such a more generic approach is not desirable?

Ingo