Re: bio_chain: proposed solution for bio_alloc failure and large IO simplification

["Adam J. Richter" <adam@yggdrasil.com>]

Jens Axboe wrote:
>The I/O path allocations all use GFP_NOIO (or GFP_NOFS), which all have
>__GFP_WAIT set. So the bio allocations will try normal allocation first,
>then fall back to the bio pool. If the bio pool is also empty, we will
>block waiting for entries to be freed there. So there never will be a
>failure.

	I did not realize that allocation with __GFP_WAIT was guaranteed
to _never_ fail.

	Even so, if __GFP_WAIT never fails, then it can deadlock (for
example, some other device driver has a memory leak).  Under a
scheme like bio_chain (provided that it is changed not to call a
memory allocator that can deadlock), the only way you deadlock is
if there really is deadlock bug in the lower layers that process
the underlying request.

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