Re: [Intel IOMMU 06/10] Avoid memory allocation failures in dma map api calls

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 20 Jun 2007 10:06:39 +0200 Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, 2007-06-19 at 14:37 -0700, Keshavamurthy, Anil S wrote:
> > plain text document attachment (intel_iommu_pf_memalloc.patch)
> > Intel IOMMU driver needs memory during DMA map calls to setup its internal
> > page tables and for other data structures. As we all know that these DMA 
> > map calls are mostly called in the interrupt context or with the spinlock 
> > held by the upper level drivers(network/storage drivers), so in order to 
> > avoid any memory allocation failure due to low memory issues,
> > this patch makes memory allocation by temporarily setting PF_MEMALLOC
> > flags for the current task before making memory allocation calls.
> > 
> > We evaluated mempools as a backup when kmem_cache_alloc() fails
> > and found that mempools are really not useful here because
> >  1) We don;t know for sure how much to reserve in advance
> 
> So you just unleashed an unbounded allocation context on PF_MEMALLOC?
> seems like a really really bad idea.
> 
> >  2) And mempools are not useful for GFP_ATOMIC case (as we call 
> >     memory alloc functions with GFP_ATOMIC)
> 
> Mempools work as intended with GFP_ATOMIC, it gets filled up to the
> specified number of elements using GFP_KERNEL (at creation time). This
> gives GFP_ATOMIC allocations nr_elements extra items once it would start
> failing.

Yup.  Changelog is buggy.

> > With PF_MEMALLOC flag set in the current->flags, the VM subsystem avoids
> > any watermark checks before allocating memory thus guarantee'ing the 
> > memory till the last free page.
> 
> PF_MEMALLOC as is, is meant to salvage the VM from the typical VM
> deadlock. Using it as you do now is not something a driver should ever
> do, and I'm afraid I will have to strongly oppose this patch.
> 
> You really really want to calculate an upper bound on your memory
> consumption and reserve this.
> 
> So, I'm afraid I'll have to..
> 
> NACK!

err, PF_MEMALLOC doesn't actually do anything if in_interrupt(), so your
reason-for-nacking isn't legitimate.  And neither is Anil's patch ;)

So I'm thinking that if this patch passed all his testing, a patch which
didn't play these PF_MEMALLOC games would pass the same tests.