Re: [PATCH] mm: skip reserved page for kmem leak scanning

[Zhaoyang Huang]

On Tue, Aug 30, 2022 at 4:03 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 30.08.22 04:41, Zhaoyang Huang wrote:
> > On Mon, Aug 29, 2022 at 8:19 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >>
> >> On 26.08.22 05:23, Zhaoyang Huang wrote:
> >>> On Fri, Aug 26, 2022 at 11:13 AM zhaoyang.huang
> >>> <zhaoyang.huang@xxxxxxxxxx> wrote:
> >>>>
> >>>> From: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >>>>
> >>>> It is no need to scan reserved page, skip it.
> >>>>
> >>>> Signed-off-by: Zhaoyang Huang <zhaoyang.huang@xxxxxxxxxx>
> >>>> ---
> >>>>  mm/kmemleak.c | 2 +-
> >>>>  1 file changed, 1 insertion(+), 1 deletion(-)
> >>>>
> >>>> diff --git a/mm/kmemleak.c b/mm/kmemleak.c
> >>>> index a182f5d..c546250 100644
> >>>> --- a/mm/kmemleak.c
> >>>> +++ b/mm/kmemleak.c
> >>>> @@ -1471,7 +1471,7 @@ static void kmemleak_scan(void)
> >>>>                         if (page_zone(page) != zone)
> >>>>                                 continue;
> >>>>                         /* only scan if page is in use */
> >>>> -                       if (page_count(page) == 0)
> >>>> +                       if (page_count(page) == 0 || PageReserved(page))
> >>> Sorry for previous stupid code by my faint, correct it here
> >>
> >> Did you even test the initial patch?
> >>
> >> I wonder why we should consider this change
> >>
> >> (a) I doubt it's a performance issue. If it is, please provide numbers
> >>     before/after.
> > For Android-like SOC systems where AP(cpu runs linux) are one of the
> > memory consumers which are composed of other processors such as modem,
> > isp,wcn etc. The reserved memory occupies a certain number of
> > memory(could be 30% of MemTotal) which makes scan reserved pages
> > pointless.
>
> But we only scan the memmap (struct page) here and not the actual
> memory. Do you have any performance numbers showing that there is even
> an observable change?
>
> >> (b) We'll stop scanning early allocations. As the memmap is usually
> >>     allocated early during boot ... we'll stop scanning essentially the
> >>     whole mmap and that whole loop would be dead code? What am i
> >>     missing?
> > memmap refers to pages here? If we can surpass these as it exist
> > permanently during life period. Besides, I wonder if PageLRU should
> > also be skipped?
> > -                       if (page_count(page) == 0)
> > +                       if (page_count(page) == 0 ||
> > PageReserved(page) || PageLRU(page))
>
> I think we need a really good justification to start poking holes into
> the memmap scanner. I'm no expert on this code (and under which
> circumstances we actually might find referenced objects in a memmap),
> though.
>
> But we should be careful with that.
Agree. It may be helpless as kmemleak is for debugging purposes. Nack
this patch by myself. Sorry for interrupt.
>
> --
> Thanks,
>
> David / dhildenb
>