Date:   Mon, 9 Sep 2019 05:48:34 +0000
From:   Toshiki Fukasawa <t-fukasawa@...jp.nec.com>
To:     David Hildenbrand <david@...hat.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "dan.j.williams@...el.com" <dan.j.williams@...el.com>
CC:     Toshiki Fukasawa <t-fukasawa@...jp.nec.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        "adobriyan@...il.com" <adobriyan@...il.com>,
        "hch@....de" <hch@....de>,
        "longman@...hat.com" <longman@...hat.com>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>,
        "mst@...hat.com" <mst@...hat.com>,
        Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
        Junichi Nomura <j-nomura@...jp.nec.com>
Subject: Re: [RFC PATCH v2] mm: initialize struct pages reserved by
 ZONE_DEVICE driver.

On 2019/09/06 19:35, David Hildenbrand wrote:
> On 06.09.19 12:02, Toshiki Fukasawa wrote:
>> Thank you for your feedback.
>>
>> On 2019/09/06 17:45, David Hildenbrand wrote:
>>> On 06.09.19 10:09, Toshiki Fukasawa wrote:
>>>> A kernel panic is observed during reading
>>>> /proc/kpage{cgroup,count,flags} for first few pfns allocated by
>>>> pmem namespace:
>>>>
>>>> BUG: unable to handle page fault for address: fffffffffffffffe
>>>> [  114.495280] #PF: supervisor read access in kernel mode
>>>> [  114.495738] #PF: error_code(0x0000) - not-present page
>>>> [  114.496203] PGD 17120e067 P4D 17120e067 PUD 171210067 PMD 0
>>>> [  114.496713] Oops: 0000 [#1] SMP PTI
>>>> [  114.497037] CPU: 9 PID: 1202 Comm: page-types Not tainted 5.3.0-rc1
>>>> [  114.497621] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.11.0-0-g63451fca13-prebuilt.qemu-project.org 04/01/2014
>>>> [  114.498706] RIP: 0010:stable_page_flags+0x27/0x3f0
>>>> [  114.499142] Code: 82 66 90 66 66 66 66 90 48 85 ff 0f 84 d1 03 00 00 41 54 55 48 89 fd 53 48 8b 57 08 48 8b 1f 48 8d 42 ff 83 e2 01 48 0f 44 c7 <48> 8b 00 f6 c4 02 0f 84 57 03 00 00 45 31 e4 48 8b 55 08 48 89 ef
>>>> [  114.500788] RSP: 0018:ffffa5e601a0fe60 EFLAGS: 00010202
>>>> [  114.501373] RAX: fffffffffffffffe RBX: ffffffffffffffff RCX: 0000000000000000
>>>> [  114.502009] RDX: 0000000000000001 RSI: 00007ffca13a7310 RDI: ffffd07489000000
>>>> [  114.502637] RBP: ffffd07489000000 R08: 0000000000000001 R09: 0000000000000000
>>>> [  114.503270] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000240000
>>>> [  114.503896] R13: 0000000000080000 R14: 00007ffca13a7310 R15: ffffa5e601a0ff08
>>>> [  114.504530] FS:  00007f0266c7f540(0000) GS:ffff962dbbac0000(0000) knlGS:0000000000000000
>>>> [  114.505245] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>>>> [  114.505754] CR2: fffffffffffffffe CR3: 000000023a204000 CR4: 00000000000006e0
>>>> [  114.506401] Call Trace:
>>>> [  114.506660]  kpageflags_read+0xb1/0x130
>>>> [  114.507051]  proc_reg_read+0x39/0x60
>>>> [  114.507387]  vfs_read+0x8a/0x140
>>>> [  114.507686]  ksys_pread64+0x61/0xa0
>>>> [  114.508021]  do_syscall_64+0x5f/0x1a0
>>>> [  114.508372]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>> [  114.508844] RIP: 0033:0x7f0266ba426b
>>>>
>>>> The first few pages of ZONE_DEVICE expressed as the range
>>>> (altmap->base_pfn) to (altmap->base_pfn + altmap->reserve) are
>>>> skipped by struct page initialization. Some pfn walkers like
>>>> /proc/kpage{cgroup, count, flags} can't handle these uninitialized
>>>> struct pages, which causes the error.
>>>>
>>>> In previous discussion, Dan seemed to have concern that the struct
>>>> page area of some pages indicated by vmem_altmap->reserve may not
>>>> be allocated. (See https://lore.kernel.org/lkml/CAPcyv4i5FjTOnPbXNcTzvt+e6RQYow0JRQwSFuxaa62LSuvzHQ@mail.gmail.com/)
>>>> However, arch_add_memory() called by devm_memremap_pages() allocates
>>>> struct page area for pages containing addresses in the range
>>>> (res.start) to (res.start + resource_size(res)), which include the
>>>> pages indicated by vmem_altmap->reserve. If I read correctly, it is
>>>> allocated as requested at least on x86_64. Also, memmap_init_zone()
>>>> initializes struct pages in the same range.
>>>> So I think the struct pages should be initialized.>
>>>
>>> For !ZONE_DEVICE memory, the memmap is valid with SECTION_IS_ONLINE -
>>> for the whole section. For ZONE_DEVICE memory we have no such
>>> indication. In any section that is !SECTION_IS_ONLINE and
>>> SECTION_MARKED_PRESENT, we could have any subsections initialized. >
>>> The only indication I am aware of is pfn_zone_device_reserved() - which
>>> seems to check exactly what you are trying to skip here.
>>>
>>> Can't you somehow use pfn_zone_device_reserved() ? Or if you considered
>>> that already, why did you decide against it?
>>
>> No, in current approach this function is no longer needed.
>> The reason why we change the approach is that all pfn walkers
>> have to be aware of the uninitialized struct pages.
> 
> We should use the same strategy for all pfn walkers then (effectively
> get rid of pfn_zone_device_reserved() if that's what we want).

True, but this patch replaces "/proc/kpageflags: do not use uninitialized
struct pages". If we initialize the uninitialized struct pages, no pfn walker
will need to be aware of them.

> 
>>
>> As for SECTION_IS_ONLINE, I'm not sure now.
>> I will look into it next week.
> 
> SECTION_IS_ONLINE does currently not apply to ZONE_DEVICE and due to
> sub-section support for ZONE_DEVICE, it cannot easily be reused.
> 
It seems that SECTION_IS_ONLINE and SECTION_MARKED_PRESENT can be used to
distinguish uninitialized struct pages if we can apply them to ZONE_DEVICE,
but that is no longer necessary with this approach.

Thanks,
Toshiki Fukasawa

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