linux-kernel - Re: [PATCH 0/2] Hibernate fixes for 'Fix memmap to be initialized for the entire section'

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Date:   Tue, 6 Dec 2016 17:38:11 +0000
From:   Will Deacon <will.deacon@....com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     James Morse <james.morse@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        Robert Richter <rrichter@...ium.com>,
        Catalin Marinas <catalin.marinas@....com>,
        David Daney <david.daney@...ium.com>,
        Mark Rutland <mark.rutland@....com>,
        Hanjun Guo <hanjun.guo@...aro.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] Hibernate fixes for 'Fix memmap to be initialized
 for the entire section'

On Mon, Dec 05, 2016 at 03:42:14PM +0000, Ard Biesheuvel wrote:
> On 2 December 2016 at 14:49, James Morse <james.morse@....com> wrote:
> > Patch "arm64: mm: Fix memmap to be initialized for the entire section"
> > changes pfn_valid() in a way that breaks hibernate. These patches fix
> > hibernate, and provided struct page's are allocated for nomap pages,
> > can be applied before [0].
> >
> > Hibernate core code belives 'valid' to mean "I can access this". It
> > uses pfn_valid() to test the page if the page is 'valid'.
> >
> > pfn_valid() needs to be changed so that all struct pages in a numa
> > node have the same node-id. Currently 'nomap' pages are skipped, and
> > retain their pre-numa node-ids, which leads to a later BUG_ON().
> >
> > These patches make hibernate's savable_page() take its escape route
> > via 'if (PageReserved(page) && pfn_is_nosave(pfn))'.
> >
> 
> This makes me feel slightly uneasy. Robert makes a convincing point,
> but I wonder if we can expect more fallout from the ambiguity of
> pfn_valid(). Now we are not only forced to assign non-existing (as far
> as the OS is concerned) pages to the correct NUMA node, we also need
> to set certain page flags.

Yes, I really don't know how to proceed here. Playing whack-a-mole with
pfn_valid() users doesn't sounds like an improvement on the current
situation to me.

Robert -- if we leave pfn_valid() as it is, would a point-hack to
memmap_init_zone help, or do you anticipate other problems?

Will