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From: David Hildenbrand <david@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
Cc: Vlastimil Babka <vbabka@suse.cz>,
	linux-mm@kvack.org, Steven Rostedt <rostedt@goodmis.org>,
	Ingo Molnar <mingo@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Michal Hocko <mhocko@suse.com>, Huang Ying <ying.huang@intel.com>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Pavel Tatashin <pasha.tatashin@oracle.com>,
	Miles Chen <miles.chen@mediatek.com>,
	Mel Gorman <mgorman@techsingularity.net>,
	Rik van Riel <riel@redhat.com>, James Hogan <jhogan@kernel.org>,
	"Levin, Alexander (Sasha Levin)" <alexander.levin@verizon.com>,
	open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC 2/8] mm: introduce PG_offline
Date: Sun, 22 Apr 2018 17:13:52 +0200	[thread overview]
Message-ID: <903ab7f7-88ce-9bc3-036b-261cce1bb26c@redhat.com> (raw)
In-Reply-To: <20180422140246.GA30714@bombadil.infradead.org>

On 22.04.2018 16:02, Matthew Wilcox wrote:
> On Sun, Apr 22, 2018 at 10:17:31AM +0200, David Hildenbrand wrote:
>> On 22.04.2018 05:01, Matthew Wilcox wrote:
>>> On Sat, Apr 21, 2018 at 06:52:18PM +0200, Vlastimil Babka wrote:
>>>> Sounds like your newly introduced "page types" could be useful here? I
>>>> don't suppose those offline pages would be using mapcount which is
>>>> aliased there?
>>>
>>> Oh, that's a good point!  Yes, this is a perfect use for page_type.
>>> We have something like twenty bits available there.
>>>
>>> Now you've got me thinking that we can move PG_hwpoison and PG_reserved
>>> to be page_type flags too.  That'll take us from 23 to 21 bits (on 32-bit,
>>> with PG_UNCACHED)
>>
>> Some things to clarify here. I modified the current RFC to also allow
>> PG_offline on allocated (ballooned) pages (e.g. virtio-balloon).
>>
>> kdump based dump tools can then easily identify which pages are not to
>> be dumped (either because the content is invalid or not accessible).
>>
>> I previously stated that ballooned pages would be marked as PG_reserved,
>> which is not true (at least not for virtio-balloon). However this allows
>> me to detect if all pages in a section are offline by looking at
>> (PG_reserved && PG_offline). So I can actually tell if a page is marked
>> as offline and allocated or really offline.
>>
>>
>> 1. The location (not the number!) of PG_hwpoison is basically ABI and
>> cannot be changed. Moving it around will most probably break dump tools.
>> (see kernel/crash_core.c)
> 
> It's not ABI.  It already changed after 4.9 when PG_waiters was introduced
> by commit 62906027091f.

It is, please have a look at the file I pointed you to.

We export the *value* of PG_hwpoison in the ELF file, therefore the
*value* can change, but the *location* (page_flags, mapcount, whatever)
must not change. Or am I missing something here? I don't think we can
move PG_hwpoison that easily.

Also, I can read "For pages that are never mapped to userspace,
page->mapcount may be used for storing extra information about page
type" - is that true for PG_hwpoison/PG_reserved? I am skeptical.

And we need something similar for PG_offline, because it will become
ABI. (I can see that PAGE_BUDDY_MAPCOUNT_VALUE is also exported in an
ELF file, so maybe a new page type might work for marking a page offline
- but I have to look at the details first tomorrow)

> 
>> 2. Exposing PG_offline via kdump will make it ABI as well. And we don't
>> want any complicated validity checks ("is the bit valid or not?"),
>> because that would imply having to make these bits ABI as well. So
>> having PG_offline just like PG_hwpoison part of page_flags is the right
>> thing to do. (see patch nr 4)
>>
>> 3. For determining if all pages of a section are offline (see patch nr
>> 5), I will have to be able to check 1. PG_offline and 2. PG_reserved on
>> any page. Will this be possible by moving e.g. PG_reserved to page
>> types? (especially if some field is suddenly aliased?)
> 
> It's possible to tell whether the field is in use as mapcount or
> page_types; mapcount should always be non-negative, and page_types
> reserves a few bits to detect under/overflow of mapcount.  The slab/slob
> users of the field will also be positive uses.
> 

Thanks for the info!

-- 

Thanks,

David / dhildenb

  reply	other threads:[~2018-04-22 15:13 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20180413131632.1413-1-david@redhat.com>
2018-04-13 13:16 ` [PATCH RFC 1/8] mm/memory_hotplug: Revert "mm/memory_hotplug: optimize memory hotplug" David Hildenbrand
2018-04-13 13:16 ` [PATCH RFC 2/8] mm: introduce PG_offline David Hildenbrand
2018-04-13 13:40   ` Michal Hocko
2018-04-13 13:46     ` David Hildenbrand
2018-04-17 11:50     ` David Hildenbrand
2018-04-13 17:11   ` Matthew Wilcox
2018-04-16  8:31     ` David Hildenbrand
2018-04-21 16:52     ` Vlastimil Babka
2018-04-22  3:01       ` Matthew Wilcox
2018-04-22  8:17         ` David Hildenbrand
2018-04-22 14:02           ` Matthew Wilcox
2018-04-22 15:13             ` David Hildenbrand [this message]
2018-04-29 21:08               ` Michal Hocko
2018-04-30  6:31                 ` David Hildenbrand
2018-04-20  7:30   ` David Hildenbrand
2018-04-13 13:16 ` [PATCH RFC 3/8] mm: use PG_offline in online/offlining code David Hildenbrand
2018-04-13 13:31 ` [PATCH RFC 4/8] kdump: expose PG_offline David Hildenbrand
2018-04-13 13:33 ` [PATCH RFC 5/8] mm: only mark section offline when all pages are offline David Hildenbrand
2018-04-13 13:33 ` [PATCH RFC 6/8] mm: offline_pages() is also limited by MAX_ORDER David Hildenbrand
2018-04-13 13:33 ` [PATCH RFC 7/8] mm: allow to control onlining/offlining of memory by a driver David Hildenbrand
2018-04-13 15:59   ` Michal Hocko
2018-04-13 16:32     ` David Hildenbrand
2018-04-13 13:33 ` [PATCH RFC 8/8] mm: export more functions used to online/offline memory David Hildenbrand

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