linux-kernel - Re: [PATCH v3 1/5] mm: page_alloc: remain memblock_next_valid_pfn() when CONFIG_HAVE_ARCH_PFN

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Message-ID: <20180403001401.GA45531@WeideMacBook-Pro.local>
Date:   Tue, 3 Apr 2018 08:14:01 +0800
From:   Wei Yang <richard.weiyang@...il.com>
To:     Jia He <hejianet@...il.com>
Cc:     Wei Yang <richard.weiyang@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mel Gorman <mgorman@...e.de>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Pavel Tatashin <pasha.tatashin@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        AKASHI Takahiro <takahiro.akashi@...aro.org>,
        Gioh Kim <gi-oh.kim@...fitbricks.com>,
        Steven Sistare <steven.sistare@...cle.com>,
        Daniel Vacek <neelx@...hat.com>,
        Eugeniu Rosca <erosca@...adit-jv.com>,
        Vlastimil Babka <vbabka@...e.cz>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, James Morse <james.morse@....com>,
        Steve Capper <steve.capper@....com>, x86@...nel.org,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        Philippe Ombredanne <pombredanne@...b.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Kemi Wang <kemi.wang@...el.com>,
        Petr Tesarik <ptesarik@...e.com>,
        YASUAKI ISHIMATSU <yasu.isimatu@...il.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Nikolay Borisov <nborisov@...e.com>,
        Jia He <jia.he@...-semitech.com>
Subject: Re: [PATCH v3 1/5] mm: page_alloc: remain memblock_next_valid_pfn()
 when CONFIG_HAVE_ARCH_PFN_VALID is enable

On Mon, Apr 02, 2018 at 05:17:35PM +0800, Jia He wrote:
>
>
>On 4/2/2018 4:12 PM, Wei Yang Wrote:
>> On Wed, Mar 28, 2018 at 05:49:23PM +0800, Jia He wrote:
>> > 
>> > On 3/28/2018 5:18 PM, Wei Yang Wrote:
>> > > Oops, I should reply this thread. Forget about the reply on another thread.
>> > > 
>> > > On Sun, Mar 25, 2018 at 08:02:15PM -0700, Jia He wrote:
>> > > > Commit b92df1de5d28 ("mm: page_alloc: skip over regions of invalid pfns
>> > > > where possible") optimized the loop in memmap_init_zone(). But it causes
>> > > > possible panic bug. So Daniel Vacek reverted it later.
>> > > > 
>> > > Why this has a bug? Do you have some link about it?
>> > > 
>> > > If the audience could know the potential risk, it would be helpful to review
>> > > the code and decide whether to take it back.
>> > Hi Wei
>> > Paul firstly submit a commit b92df1de5 to improve the loop in
>> > memmap_init_zone.
>> > And Daniel tried to fix a bug_on panic issue on X86 in commit 864b75f9d6b
>> > because
>> > there is evidence that this bug_on was caused by b92df1de5 [1].
>> > 
>> > But things didn't get better, 864b75f9d6b caused booting hang issue on
>> > arm{64} [2]
>> > So maintainer decided to reverted both b92df1de5 and 864b75f9d6b
>> > 
>> > [1] https://patchwork.kernel.org/patch/10251145/
>> > [2] https://lkml.org/lkml/2018/3/14/469
>> I took some time to look into the discussion, while the root cause seems not
>> clear now?
>> 
>Frankly speaking, to me the root cause of that bug_on is not completedly
>clear :-) Daniel ever gave me some hints as followed, but currently I have
>no x86 platform to understand the details.
>
>"On arm and arm64, memblock is used by default. But generic version of
>pfn_valid() is based on mem sections and memblock_next_valid_pfn()
>does not always return the next valid one but skips more resulting in
>some valid frames to be skipped (as if they were invalid). And that's
>why kernel was eventually crashing on some !arm machines."
>

This means a system with memblock is safe to use this function?

As I know, mem_section is based on memblock, so in which case
memblock_next_valid_pfn() skips a valid pfn? A little confused.

>-- 
>Cheers,
>Jia

-- 
Wei Yang
Help you, Help me