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Subject: Re: What am I doing wrong?  submit_bio() suddenly stops working...
From: Peter Zijlstra <peterz@infradead.org>
To: Jens Axboe <axboe@kernel.dk>
Cc: Boaz Harrosh <bharrosh@panasas.com>, "Ted Ts'o" <tytso@mit.edu>,
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Date: Fri, 22 Oct 2010 15:04:20 +0200
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On Thu, 2010-10-21 at 20:07 +0200, Jens Axboe wrote:
> 
> >> +    do {
> >> +            bio = bio_alloc(GFP_NOIO, nvecs);
> >> +            nvecs >>= 1;
> >> +    } while (bio == NULL);
> > 
> > This is surly bad. bio_alloc must be allowed to fail
> > (Specially with GFP_NOIO). You should only loop down to
> > 1 and then prepare to return -ENOMEM from this function
> > and handle it properly in callers. (Or schedule and wait
> > like below)
> 
> Since __GFP_WAIT is set, it'll never return NULL. And as long as you
> don't allocate more than 1 before doing you submit_bio(), it should be
> OK in this case. 

__GFP_WAIT can return NULL, on OOM and when the size is over a magic
threshold.

I find it bad form to rely on any allocation not failing.