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.
> 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!
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