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SubjectRe: [PATCH V5 00/30] loop: Issue O_DIRECT aio using bio_vec
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On Wed, 2013-01-09 at 13:58 -0600, Dave Kleikamp wrote:
> This patchset was begun by Zach Brown and was originally submitted for
> review in October, 2009. Feedback was positive, and I have picked up
> where he left off, porting his patches to the latest mainline kernel
> and adding support more file systems.
>
> This patch series adds a kernel interface to fs/aio.c so that kernel code can
> issue concurrent asynchronous IO to file systems. It adds an aio command and
> file system methods which specify io memory with pages instead of userspace
> addresses.
>
> This series was written to reduce the current overhead loop imposes by
> performing synchronus buffered file system IO from a kernel thread. These
> patches turn loop into a light weight layer that translates bios into iocbs.
>
> It introduces new file ops, read_iter() and write_iter(), that replace the
> aio_read() and aio_write() operations. The iov_iter structure can now contain
> either a user-space iovec or a kernel-space bio_vec. Since it would be
> overly complicated to replace every instance of aio_read() and aio_write(),
> the old operations are not removed, but file systems implementing the new
> ones need not keep the old ones.
>
> Changes from V4 include:
> * moved iov-iter.c from mm/ to fs/
> * removed dio_aligned helper
> * insured that FUA write to loop device is committed to media
> * removed no-longer-used REQ_KERNEL define
>
> These patches apply to 3.8-rc2 and are also available at:
> git://github.com/kleikamp/linux-shaggy.git loop_2013_01_04
>
> My hopes are that this patchset is finally ready for linux-next.

Just a note that we at parallels are anxiously awaiting this too. Our
rewrite of the linux loop device to be more efficient (and not double
cache) depends on this work. I should also note that we've been testing
these patches (albeit backported to a RHEL kernel as a current work
base) with no problems reported so far.

James



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