Messages in this thread | | | From | Ed L Cashin <> | Subject | Re: [PATCH] ATA over Ethernet driver for 2.6.9 (with changes) | Date | Tue, 14 Dec 2004 08:39:16 -0500 |
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Jens Axboe <axboe@suse.de> writes:
> On Mon, Dec 13 2004, Ed L Cashin wrote: >> * use mempool allocation in make_request_fn > > It's not good enough, if cannot use a higher allocation priority > that GFP_NOIO here - basically guarantee that your allocation will > not block on further io. Currently you have the very same deadlock > as before, the mempool does not help you since you call into the > allocator and deadlock before ever blocking on the mempool.
Do you mean that with GFP_KERNEL we may still deadlock on line 199 of the snippet below (from mm/mempool.c)? That alloc pointer points to mempool_alloc_slab, which gets called with __GFP_WAIT turned off. The kmem_cache allocator doesn't get called with the allocation priority we specify in our make_request_fn, so we won't block there.
190 void * mempool_alloc(mempool_t *pool, int gfp_mask) 191 { 192 void *element; 193 unsigned long flags; 194 DEFINE_WAIT(wait); 195 int gfp_nowait = gfp_mask & ~(__GFP_WAIT | __GFP_IO); 196 197 might_sleep_if(gfp_mask & __GFP_WAIT); 198 repeat_alloc: 199 element = pool->alloc(gfp_nowait|__GFP_NOWARN, pool->pool_data); 200 if (likely(element != NULL)) 201 return element; 202
If we block later on the pool, that's because there are 16 objects in use, which means that mempool_free is going to get called 16 times as I/O completes, so I/O is throttled and forward progress is guaranteed. Otherwise, how does the mempool mechanism help in preventing deadlock?
It looks like we can simply change GFP_KERNEL to GFP_IO in our make_request_fn, but I'd also like to understand why that's necessary when there's a dedicated pre-allocated pool per aoe device.
-- Ed L Cashin <ecashin@coraid.com>
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