linux-kernel - Re: [PATCH] mm/spase: never partially remove memmap for early section

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Date:   Wed, 24 Jun 2020 09:48:43 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>
Cc:     akpm@...ux-foundation.org, osalvador@...e.de,
        dan.j.williams@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/spase: never partially remove memmap for early section

On 23.06.20 17:18, Michal Hocko wrote:
> On Tue 23-06-20 17:42:58, Wei Yang wrote:
>> For early sections, we assumes its memmap will never be partially
>> removed. But current behavior breaks this.
>>
>> Let's correct it.
>>
>> Fixes: ba72b4c8cf60 ("mm/sparsemem: support sub-section hotplug")
>> Signed-off-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
> 
> Can a user trigger this or is this a theoretical bug?

I tried to reproduce it, but somehow I get unexpected behavior.
With a hacked QEMU I can get

$ cat /proc/iomem
[...]
100000000-143ffffff : System RAM
144000000-343dfffff : Persistent Memory
  144000000-1441fffff : namespace0.0
  144200000-144ffffff : dax0.0

After
$ ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M

I get

$ cat /proc/iomem
[...]
100000000-143ffffff : System RAM
144000000-343dfffff : Persistent Memory
  144000000-1441fffff : namespace0.0
  144200000-144ffffff : dax0.0

I can trigger remove+re-add via 
$ ndctl create-namespace --force --reconfig=namespace0.0 --mode=devdax --size=16M

So we clearly have an overlap between System RAM and dax0.0 within a section.
However, I never get early_section() to trigger in
section_activate()/section_deactivate() ? That's unexpected


Definitely something seems to go wrong after the first "ndctl create-namespace".
Using a random page walker:

[root@...alhost ~]# ./page-types 
[  387.019229] general protection fault, probably for non-canonical address 0xfdfdfdfdfdfdfdfc: 0000 [#1] SMP NOPTI
[  387.020172] CPU: 17 PID: 1314 Comm: page-types Kdump: loaded Tainted: G        W         5.8.0-rc2 #20
[  387.021015] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.4
[  387.022056] RIP: 0010:stable_page_flags+0x27/0x3f0
[  387.022519] Code: 00 00 00 66 66 66 66 90 48 85 ff 0f 84 d2 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48f
[  387.024291] RSP: 0018:ffff9f8781057e58 EFLAGS: 00010202
[  387.024775] RAX: fdfdfdfdfdfdfdfc RBX: fdfdfdfdfdfdfdfd RCX: 00007ffc4f4f1f78
[  387.025423] RDX: 0000000000000001 RSI: 0000002000000000 RDI: ffffc590c5100000
[  387.026052] RBP: ffffc590c5100000 R08: 0000000000000001 R09: 0000000000000000
[  387.026696] R10: 0000000000000000 R11: 0000000000000000 R12: 00007ffc4f4f1f80
[  387.027324] R13: 0000000000040000 R14: 00007ffc4f4d1f80 R15: ffffffffa1577ee0
[  387.027974] FS:  00007f91a0f9c580(0000) GS:ffff888b3f7c0000(0000) knlGS:0000000000000000
[  387.028699] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  387.029223] CR2: 0000000000449b00 CR3: 000000007a7fc000 CR4: 00000000000006e0
[  387.029864] Call Trace:
[  387.030108]  kpageflags_read+0xcc/0x160
[  387.030473]  proc_reg_read+0x53/0x80
[  387.030809]  vfs_read+0x9d/0x150
[  387.031114]  ksys_pread64+0x65/0xa0
[  387.031449]  do_syscall_64+0x4d/0x90
[  387.031783]  entry_SYSCALL_64_after_hwframe+0x44/0xa9


-- 
Thanks,

David / dhildenb