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

[Tang Chen]

Hi Andrew,

On 01/10/2013 07:11 AM, Andrew Morton wrote:
> On Wed, 9 Jan 2013 17:32:26 +0800
> Tang Chen<tangchen@xxxxxxxxxxxxxx>  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@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/