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SubjectRe: [PATCH 0/6] Handle bio_alloc failure
On Tue, Apr 14 2009, Theodore Tso wrote:
> On Tue, Apr 14, 2009 at 08:20:49PM +0200, Jens Axboe wrote:
> >
> > It's a bio_alloc() guarantee, it uses a mempool backing. And if you use
> > a mempool backing, any allocation that can wait will always be
> > satisfied.
> >
> Am I missing something? I don't see anything in
> include/linux/mempool.h or mm/mempool.c, or in block/blk-core.c or
> include/linux/bio.h which documents that GFP_WAIT implies that
> bio_alloc() will always succeed.

Read mempool.c:mempool_alloc(). If __GFP_WAIT is set, it'll never turn
without having done the allocation. It's a bit weird that it isn't
documented in bio_alloc() itself, but there are several other places in
bio.c that references the fact that it cannot fail.

> My concern is that at some point in the future, someone either in the
> block device layer or in mm/mempool.c will consider this an
> implementation detail, and all of sudden calls to bio_alloc() with
> GFP_WAIT will start failing and the resulting hilarty which ensues
> won't be easily predicted by the developer making this change.

It's the entire premise of a mempool, so trust me, it'll never go away.
It is the reason they were added in the first place, for eg swap you
need the mempool guarantee or you risk deadlocking.

Jens Axboe

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