linux-kernel - Re: [PATCH v2] mm: allow deferred page init for vmemmap only

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Message-ID: <20180515125541.GH12670@dhcp22.suse.cz>
Date:   Tue, 15 May 2018 14:55:41 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Pavel Tatashin <pasha.tatashin@...cle.com>
Cc:     Steven Sistare <steven.sistare@...cle.com>,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>, tglx@...utronix.de,
        Linux Memory Management List <linux-mm@...ck.org>,
        mgorman@...hsingularity.net, mingo@...nel.org,
        peterz@...radead.org, Steven Rostedt <rostedt@...dmis.org>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Dennis Zhou <dennisszhou@...il.com>
Subject: Re: [PATCH v2] mm: allow deferred page init for vmemmap only

On Tue 15-05-18 08:17:27, Pavel Tatashin wrote:
> Hi Michal,
> 
> Thank you for your reply, my comments below:
> 
> > You are now disabling a potentially useful feature to SPARSEMEM users
> > without having any evidence that they do suffer from the issue which is
> > kinda sad. Especially when the only known offender is a UP pcp allocator
> > implementation.
> 
> True, but what is the use case for having SPARSEMEM without virtual mapping
> and deferred struct page init together. Is it a common case to have
> multiple gigabyte of memory and currently NUMA config to benefit from
> deferred page init and yet not having a memory for virtual mapping of
> struct pages? Or am I missing some common case here?

Well, I strongly suspect that this is more a momentum, then a real
reason to stick with SPARSEMEM_MANUAL. I would really love to reduce the
number of memory models we have. Getting rid of SPARSEMEM would be a
good start as VMEMMAP should be much better.
 
> > I will not insist of course but it seems like your fix doesn't really
> > prevent virt_to_page or other direct page access either.
> 
> I am not sure what do you mean, I do not prevent virt_to_page, but that is
> OK for SPARSEMEM_VMEMMAP case, because we do not need to access "struct
> page" for this operation, as translation is in page table. Yes, we do not
> prohibit other struct page accesses before mm_init(), but we now have a
> feature that checks for uninitialized struct page access, and if those will
> happen, we will learn about them.

This will always be a maze as the early boot tends to be. Sad but true.
That is why I am not really convinced we should use a large hammer and
disallow deferred page initialization just because UP implementation of
pcp does something too early. We should instead rule that one odd case.
Your patch simply doesn't rule a large class of potential issues. It
just rules out a potentially useful feature for an odd case. See my
point?
-- 
Michal Hocko
SUSE Labs