linux-kernel - Re: [PATCH 0/2] mm, memory_hotplug: fix uninitialized pages fallouts.

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Date:   Tue, 29 Jan 2019 14:14:47 +0100
From:   Gerald Schaefer <gerald.schaefer@...ibm.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Mikhail Zaslonko <zaslonko@...ux.ibm.com>,
        Mikhail Gavrilov <mikhail.v.gavrilov@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pavel Tatashin <pasha.tatashin@...een.com>,
        schwidefsky@...ibm.com, heiko.carstens@...ibm.com,
        <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] mm, memory_hotplug: fix uninitialized pages
 fallouts.

On Mon, 28 Jan 2019 15:45:04 +0100
Michal Hocko <mhocko@...nel.org> wrote:

> Hi,
> Mikhail has posted fixes for the two bugs quite some time ago [1]. I
> have pushed back on those fixes because I believed that it is much
> better to plug the problem at the initialization time rather than play
> whack-a-mole all over the hotplug code and find all the places which
> expect the full memory section to be initialized. We have ended up with
> 2830bf6f05fb ("mm, memory_hotplug: initialize struct pages for the full
> memory section") merged and cause a regression [2][3]. The reason is
> that there might be memory layouts when two NUMA nodes share the same
> memory section so the merged fix is simply incorrect.
> 
> In order to plug this hole we really have to be zone range aware in
> those handlers. I have split up the original patch into two. One is
> unchanged (patch 2) and I took a different approach for `removable'
> crash. It would be great if Mikhail could test it still works for his
> memory layout.
> 
> [1] http://lkml.kernel.org/r/20181105150401.97287-2-zaslonko@linux.ibm.com
> [2] https://bugzilla.redhat.com/show_bug.cgi?id=1666948
> [3] http://lkml.kernel.org/r/20190125163938.GA20411@dhcp22.suse.cz

I verified that both patches fix the issues we had with valid_zones
(with mem=2050M) and removable (with mem=3075M).

However, the call trace in the description of your patch 1 is wrong.
You basically have the same call trace for test_pages_in_a_zone in
both patches. The "removable" patch should have the call trace for
is_mem_section_removable from Mikhails original patches:

 CONFIG_DEBUG_VM_PGFLAGS=y
 kernel parameter mem=3075M
 --------------------------
 page:000003d08300c000 is uninitialized and poisoned
 page dumped because: VM_BUG_ON_PAGE(PagePoisoned(p))
 Call Trace:
 ([<000000000038596c>] is_mem_section_removable+0xb4/0x190)
  [<00000000008f12fa>] show_mem_removable+0x9a/0xd8
  [<00000000008cf9c4>] dev_attr_show+0x34/0x70
  [<0000000000463ad0>] sysfs_kf_seq_show+0xc8/0x148
  [<00000000003e4194>] seq_read+0x204/0x480
  [<00000000003b53ea>] __vfs_read+0x32/0x178
  [<00000000003b55b2>] vfs_read+0x82/0x138
  [<00000000003b5be2>] ksys_read+0x5a/0xb0
  [<0000000000b86ba0>] system_call+0xdc/0x2d8
 Last Breaking-Event-Address:
  [<000000000038596c>] is_mem_section_removable+0xb4/0x190
 Kernel panic - not syncing: Fatal exception: panic_on_oops