Re: Instability in current -git tree

[Michal Hocko <mhocko@kernel.org>]

On Mon 16-07-18 08:09:19, Pavel Tatashin wrote:
> 
> 
> On 07/16/2018 08:06 AM, Michal Hocko wrote:
> > On Sat 14-07-18 09:39:29, Pavel Tatashin wrote:
> > [...]
> >> From 95259841ef79cc17c734a994affa3714479753e3 Mon Sep 17 00:00:00 2001
> >> From: Pavel Tatashin <pasha.tatashin@oracle.com>
> >> Date: Sat, 14 Jul 2018 09:15:07 -0400
> >> Subject: [PATCH] mm: zero unavailable pages before memmap init
> >>
> >> We must zero struct pages for memory that is not backed by physical memory,
> >> or kernel does not have access to.
> >>
> >> Recently, there was a change which zeroed all memmap for all holes in e820.
> >> Unfortunately, it introduced a bug that is discussed here:
> >>
> >> https://www.spinics.net/lists/linux-mm/msg156764.html
> >>
> >> Linus, also saw this bug on his machine, and confirmed that pulling
> >> commit 124049decbb1 ("x86/e820: put !E820_TYPE_RAM regions into memblock.reserved")
> >> fixes the issue.
> >>
> >> The problem is that we incorrectly zero some struct pages after they were
> >> setup.
> > 
> > I am sorry but I simply do not see it. zero_resv_unavail should be
> > touching only reserved memory ranges and those are not initialized
> > anywhere. So who has reused them and put them to normal available
> > memory to be initialized by free_area_init_node[s]?
> > 
> > The patch itself should be safe because reserved and available memory
> > ranges should be disjoint so the ordering shouldn't matter. The fact
> > that it matters is the crux thing to understand and document. So the
> > change looks good to me but I do not understand _why_ it makes any
> > difference. There must be somebody to put (memblock) reserved memory
> > available to the page allocator behind our backs.
> 
> Thats exactly right, and I am also not sure why this is happening,
> there must be some overlapping happening that just should not. I will
> study it later.

Maybe a stupid question, but I do not see it from the code (this init
code is just to complex to keep it cached in head so I always have to
study the code again and again, sigh). So what exactly prevents
memmap_init_zone to stumble over reserved regions? We do play some ugly
games to find a first !reserved pfn in the node but I do not really see
anything in the init path to properly skip over reserved holes inside
the node.

-- 
Michal Hocko
SUSE Labs