linux-kernel - Re: [PATCH v6 02/15] memory-hotplug: check whether all memory blocks are offlined or not when removing memory

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Thu, 10 Jan 2013 13:56:07 +0800
From:	Tang Chen <tangchen@...fujitsu.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	rientjes@...gle.com, len.brown@...el.com, benh@...nel.crashing.org,
	paulus@...ba.org, cl@...ux.com, minchan.kim@...il.com,
	kosaki.motohiro@...fujitsu.com, isimatu.yasuaki@...fujitsu.com,
	wujianguo@...wei.com, wency@...fujitsu.com, hpa@...or.com,
	linfeng@...fujitsu.com, laijs@...fujitsu.com, mgorman@...e.de,
	yinghai@...nel.org, glommer@...allels.com, x86@...nel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-acpi@...r.kernel.org,
	linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
	linux-ia64@...r.kernel.org, cmetcalf@...era.com,
	sparclinux@...r.kernel.org
Subject: Re: [PATCH v6 02/15] memory-hotplug: check whether all memory blocks
 are offlined or not when removing memory

Hi Andrew,

On 01/10/2013 07:11 AM, Andrew Morton wrote:
> On Wed, 9 Jan 2013 17:32:26 +0800
> Tang Chen<tangchen@...fujitsu.com>  wrote:
>
>> We remove the memory like this:
>> 1. lock memory hotplug
>> 2. offline a memory block
>> 3. unlock memory hotplug
>> 4. repeat 1-3 to offline all memory blocks
>> 5. lock memory hotplug
>> 6. remove memory(TODO)
>> 7. unlock memory hotplug
>>
>> All memory blocks must be offlined before removing memory. But we don't hold
>> the lock in the whole operation. So we should check whether all memory blocks
>> are offlined before step6. Otherwise, kernel maybe panicked.
>
> Well, the obvious question is: why don't we hold lock_memory_hotplug()
> for all of steps 1-4?  Please send the reasons for this in a form which
> I can paste into the changelog.

In the changelog form:

Offlining a memory block and removing a memory device can be two
different operations. Users can just offline some memory blocks
without removing the memory device. For this purpose, the kernel has
held lock_memory_hotplug() in __offline_pages(). To reuse the code
for memory hot-remove, we repeat step 1-3 to offline all the memory
blocks, repeatedly lock and unlock memory hotplug, but not hold the
memory hotplug lock in the whole operation.

>
>
> Actually, I wonder if doing this would fix a race in the current
> remove_memory() repeat: loop.  That code does a
> find_memory_block_hinted() followed by offline_memory_block(), but
> afaict find_memory_block_hinted() only does a get_device().  Is the
> get_device() sufficiently strong to prevent problems if another thread
> concurrently offlines or otherwise alters this memory_block's state?

I think we already have memory_block->state_mutex to protect the
concurrently changing of memory_block's state.

The find_memory_block_hinted() here is to find the memory_block
corresponding to the memory section we are dealing with.

Thanks. :)

>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/