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    SubjectRe: [PATCH] ATA over Ethernet driver for 2.6.9 (with changes)
    Date
    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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