linux-kernel - Re: [v6 01/15] x86/mm: reserve only exiting low pages

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Date:   Fri, 11 Aug 2017 11:24:55 -0400
From:   Pasha Tatashin <pasha.tatashin@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, sparclinux@...r.kernel.org,
        linux-mm@...ck.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        x86@...nel.org, kasan-dev@...glegroups.com, borntraeger@...ibm.com,
        heiko.carstens@...ibm.com, davem@...emloft.net,
        willy@...radead.org, ard.biesheuvel@...aro.org,
        will.deacon@....com, catalin.marinas@....com, sam@...nborg.org
Subject: Re: [v6 01/15] x86/mm: reserve only exiting low pages

>> Struct pages are initialized by going through __init_single_page(). Since
>> the existing physical memory in memblock is represented in memblock.memory
>> list, struct page for every page from this list goes through
>> __init_single_page().
> 
> By a page _from_ this list you mean struct pages backing the physical
> memory of the memblock lists?

Correct: "for every page from this list...", for every page represented 
by this list the struct page is initialized through __init_single_page()

>> In this patchset we will stop zeroing struct page memory during allocation.
>> Therefore, this bug must be fixed in order to avoid random assert failures
>> caused by CONFIG_DEBUG_VM_PGFLAGS triggers.
>>
>> The fix is to reserve memory from the first existing PFN.
> 
> Hmm, I assume this is a result of some assert triggering, right? Which
> one? Why don't we need the same treatment for other than x86 arch?

Correct, the pgflags asserts were triggered when we were setting 
reserved flags to struct page for PFN 0 in which was never initialized 
through __init_single_page(). The reason they were triggered is because 
we set all uninitialized memory to ones in one of the debug patches.

>> Signed-off-by: Pavel Tatashin <pasha.tatashin@...cle.com>
>> Reviewed-by: Steven Sistare <steven.sistare@...cle.com>
>> Reviewed-by: Daniel Jordan <daniel.m.jordan@...cle.com>
>> Reviewed-by: Bob Picco <bob.picco@...cle.com>
> 
> I guess that the review happened inhouse. I do not want to question its
> value but it is rather strange to not hear the specific review comments
> which might be useful in general and moreover even not include those
> people on the CC list so they are aware of the follow up discussion.

I will bring this up with my colleagues to how to handle this better in 
the future. I will also CC the reviewers when I sent out the updated 
patch series.