Re: [Nbd] Deadlock Issues with Local NBD Server

[Alex Bligh <alex@...872...>]

--On 27 September 2011 20:26:48 -0700 Adam Cozzette <acozzette@...946...> 
wrote:

> So it seems to me that the deadlock situation is essentially the same
> whether you're swapping to a block device or just writing out dirty
> pages. Or am I mistaken about that? Is this something that is so unlikely
> to occur that there is no point in worrying about it?

I think it is *not* so unlikely there is no point worrying about it.

My excuse to date for not worrying about it is that in our application
nbd is accessed directly by hypervisors, so I /think/ we don't suffer
from that issue.

IIRC memory allocation on stock nbd-server in the write path is
minimal (but minimal != zero).

Now you mention it, I am trying to work out how it works even with a
remote server. If all the pages are dirty waiting to be written to
nbd, and a write is started which causes a GFP_KERNEL allocation to
allocate the skbuff for the tcp packet, and there is no free memory,
I am not sure how any progress is made. I get the feeling this at least
must be soluble, or nfs and iscsi would be dead in the water.

--
Alex Bligh