Hi Kazu,
On Thu, 7 Apr 2022 06:43:00 +0000
HAGIO KAZUHITO(萩尾 一仁) <k-hagio-ab@xxxxxxx> wrote:
> -----Original Message-----
> > Once the last dumpable page was processed there is no need to finish the
> > loop to the last page. Thus exit early to improve performance.
> >
> > Signed-off-by: Philipp Rudo <prudo@xxxxxxxxxx>
> > ---
> > makedumpfile.c | 6 ++++++
> > 1 file changed, 6 insertions(+)
> >
> > diff --git a/makedumpfile.c b/makedumpfile.c
> > index 2ef3458..c944d0e 100644
> > --- a/makedumpfile.c
> > +++ b/makedumpfile.c
> > @@ -8884,6 +8884,12 @@ write_kdump_pages_cyclic(struct cache_data *cd_header, struct cache_data *cd_pag
> >
> > for (pfn = start_pfn; pfn < end_pfn; pfn++) {
> >
> > + /*
> > + * All dumpable pages have been processed. No need to continue.
> > + */
> > + if (num_dumped == info->num_dumpable)
> > + break;
>
> This patch is likely to increase the possibility of failure to capture
> /proc/kcore, although this is an unofficial functionality...
>
> # makedumpfile -ld31 /proc/kcore kcore.snap
> # crash vmlinux kcore.snap
> ...
> crash: page incomplete: kernel virtual address: ffff916fbeffed00 type: "pglist node_id"
>
> In cyclic mode, makedumpfile first calculates only info->num_dumpable [1] and
> frees the used bitmap, and later creates 2nd bitmap again [2] at this time.
>
> create_dumpfile
> create_dump_bitmap
> info->num_dumpable = get_num_dumpable_cyclic() <<-- [1]
> writeout_dumpfile
> write_kdump_pages_and_bitmap_cyclic
> foreach cycle
> create_2nd_bitmap <<-- [2]
> write_kdump_pages_cyclic
>
> So with live system, num_dumped can exceed info->num_dumpable.
> If it stops at info->num_dumpable, some necessary data can be missed.
thanks for the explanation! I haven't considered that case and assumed
info->num_dumpable is constant.
> Capturing /proc/kcore is still fragile and not considered enough, but
> sometimes useful... when I want to capture a snapshot of memory.
>
> (the bitmap is allocated as block, so probably it's working as some buffer?)
>
> So I will merge the 1/2 patch, but personally would not like to merge
> this patch. How necessary is this?
I don't think this patch is absolutely necessary. It's only a small
performance improvement but shouldn't have a huge impact. If you like
you can drop the patch.
Thanks
Philipp
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