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Re: [PATCH 1/2] kexec: update VMCOREINFO for compound_order/dtor

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  Re: [PATCH 1/2] kexec: update VMCOREINFO for compound_order/dtor Andrew Morton <akpm@linux-foundation.org> - 2016-04-01 22:30 +0200
    RE: [PATCH 1/2] kexec: update VMCOREINFO for compound_order/dtor Atsushi Kumagai <ats-kumagai@wm.jp.nec.com> - 2016-04-04 03:10 +0200

#1369582 — Re: [PATCH 1/2] kexec: update VMCOREINFO for compound_order/dtor

On Tue, 1 Mar 2016 06:14:32 +0000 Atsushi Kumagai <ats-kumagai@wm.jp.nec.com> wrote:

> makedumpfile refers page.lru.next to get the order of compound pages
> for page filtering. However, now the order is stored in page.compound_order,
> hence VMCOREINFO should be updated to export the offset of
> page.compound_order.
> 
> The fact is, page.compound_order was introduced already in kernel 4.0,
> but the offset of it was the same as page.lru.next until kernel 4.3,
> so this was not actual problem.
> 
> The above can be said also for page.lru.prev and page.compound_dtor,
> it's necessary to detect hugetlbfs pages. Further, the content was
> changed from direct address to the ID which means dtor.

It's unclear which kernels need the patch and why.  I *think* that the
patch is needed in 4.3.x, 4.4.x, 4.5.x and 4.6 in order to make
makedumpfile work correctly.  Is that right?

And it appears that [patch 2/2] is needed in 4.0+?

However in both cases I am uncertain - what are the end-user visible
effects of these regressions?  Why can bugs remain in place for so long
without having been observed?


Please make all these things clear when perparing changelogs for
bugfixes: which kernel versions need fixing and why (ie: what are the
end-user visible effects of the bug).

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#1370320

>On Tue, 1 Mar 2016 06:14:32 +0000 Atsushi Kumagai <ats-kumagai@wm.jp.nec.com> wrote:
>
>> makedumpfile refers page.lru.next to get the order of compound pages
>> for page filtering. However, now the order is stored in page.compound_order,
>> hence VMCOREINFO should be updated to export the offset of
>> page.compound_order.
>>
>> The fact is, page.compound_order was introduced already in kernel 4.0,
>> but the offset of it was the same as page.lru.next until kernel 4.3,
>> so this was not actual problem.
>>
>> The above can be said also for page.lru.prev and page.compound_dtor,
>> it's necessary to detect hugetlbfs pages. Further, the content was
>> changed from direct address to the ID which means dtor.
>
>It's unclear which kernels need the patch and why.  I *think* that the
>patch is needed in 4.3.x, 4.4.x, 4.5.x and 4.6 in order to make
>makedumpfile work correctly.  Is that right?

The patch is necessary for 4.4.x, 4.5.x and 4.6.
4.3.x don't have the problem.

>And it appears that [patch 2/2] is needed in 4.0+?

[patch 2/2] is for 4.5.x and latter:

  $ git name-rev 1c290f642101e6
  1c290f642101e6 tags/v4.5-rc1~77^2~129
  $

>However in both cases I am uncertain - what are the end-user visible
>effects of these regressions?  Why can bugs remain in place for so long
>without having been observed?

The problem is that unnecessary hugepages wouldn't be removed from
a dump file in the older kernels. This means that extra disk space
would be consumed.
It's a problem, but not critical.

>Please make all these things clear when perparing changelogs for
>bugfixes: which kernel versions need fixing and why (ie: what are the
>end-user visible effects of the bug).

Sure, I will be careful about it.


Thanks,
Atsushi Kumagai

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