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    SubjectRe: [PATCH] block: blk_queue_bounce_limits can actually sleep
    On Tue, 20 May 2008 12:45:56 -0700
    Andrew Morton <akpm@linux-foundation.org> wrote:

    > On Tue, 20 May 2008 21:29:59 +0200
    > Jens Axboe <jens.axboe@oracle.com> wrote:
    >
    > > On Mon, May 19 2008, Arjan van de Ven wrote:
    > > > From: Arjan van de Ven <arjan@linux.intel.com>
    > > > Subject: [PATCH] block: blk_queue_bounce_limits can actually sleep
    > > >
    > > > blk_queue_bounce_limit can call init_emergency_isa_pool, which
    > > > does sleeping allocations... document it as such by adding
    > > > might_sleep() to the driver
    > >
    > > Isn't that superflous, as mempool_create() -> kmalloc(...,
    > > __GFP_WAIT) ends up spewing that warning anyway?
    >
    > It's largely superfluous given the way in which Arjan implemented it.
    >
    > One situation which we regularly hit is:
    >
    > foo()
    > {
    > ...
    > if (some_unlikely_condition())
    > do_something_which_sleeps();
    > ...
    > }
    >
    > and then we go and call that code under spinlock and ship it out, when
    > of course a handful of testers hit the unlikely condition.
    >
    > The solution to that is to add a might_sleep() _outside_ the test of
    > some_unlikely_condition(). ie:
    >
    > --- a/block/blk-settings.c~a
    > +++ a/block/blk-settings.c
    > @@ -140,6 +140,8 @@ void blk_queue_bounce_limit(struct reque
    > unsigned long b_pfn = dma_addr >> PAGE_SHIFT;
    > int dma = 0;
    >
    > + might_sleep();
    > +
    > q->bounce_gfp = GFP_NOIO;
    > #if BITS_PER_LONG == 64
    > /* Assume anything <= 4GB can be handled by IOMMU.
    > _
    >
    > but it's all vague and waffly because Arjan forgot to tell us why he's
    > bothering to patch this code at all???

    the sata_nv driver calls this from an invalid context ... and spews a
    ton of warnings as a result... made me think this is a common mistake
    to make.

    I'd love to make it do your version instead, but I was afraid it would
    trigger too often....



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