linux-kernel - Re: [PATCH 1/2] kexec: update VMCOREINFO for compound

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Message-Id: <20160401132421.7cadaeb967dc342f643d8c38@linux-foundation.org>
Date:	Fri, 1 Apr 2016 13:24:21 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Atsushi Kumagai <ats-kumagai@...jp.nec.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [PATCH 1/2] kexec: update VMCOREINFO for compound_order/dtor

On Tue, 1 Mar 2016 06:14:32 +0000 Atsushi Kumagai <ats-kumagai@...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).