Re: [dm-devel] [PATCH 12/13] Make generic_make_request handlearbitrarily large bios

[Vivek Goyal]

On Mon, May 21, 2012 at 10:55:42AM -0700, Kent Overstreet wrote:
> On Mon, May 21, 2012 at 01:17:06PM -0400, Vivek Goyal wrote:
> > May be I am missing something, hence I will ask.  Is punting to workqueue
> > will really solve the issue raised by Neil. 
> > 
> > Due to spliting required, we will be holding some bios in the stack and
> > these bios can't be submitted till further allocation from pool happens. So
> > will it matter whether we are waiting for allocation in submitting process
> > context or in worker thread context.
> 
> Punting to workqueue allows all the bio splits that have already been
> allocated to be submitted.
>
> > IOW, say you have a pool of 2 bios. We allocate 1 bio (say bio A),  and submit
> > it for IO (now 1 bio left in pool). Now, bio A needs to be split up, so we
> > allocate bio B and submit it (pool is empty now). Now we try to submit bio
> > B and this also needs to be split. There are no more free bios so we will
> > wait for some to get free but none of the bios (A and B) have actually
> > been submitted for IO so nothing will get freed and we have a deadlock
> > (This is assuming that memory is tight enough that we are not able to do
> > any allocations from the slab backing the mempool).
> 
> You're talking about a different issue. You can't safely split a split
> from the same bio pool - but this code doesn't do that since it adds a
> separate bio pool for each request_queue that's only for splits.

Ok, I missed the fact that there are per queue pools for bio allocation.
Thanks for clarifying.

Vivek
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