Re: [PATCH] splice support #2

[Andrew Morton <akpm@osdl.org>]

Nick Piggin <nickpiggin@yahoo.com.au> wrote:
>
> Also,
>  might you be able to do a single file-sized file<->file splice,
>  and have it do a remote copy on a suitable network fs

splice() may not be suitable for such filesystems.  Quoting Robert Peterson
from earlier this week:


> The problem is that
> loop.c circumvents proper locking by going directly to prepare_write
> rather than following the normal process.  If I added some kind of
> library callbacks to allow cluster locking, it would break the
> normal locking sequence of an ordinary write.
>
> The normal sequence of an ordinary write typically looks like:
>
> 1. write
> 2.   (Take care of cluster locking if necessary)
> 3.   generic_file_write_nolock
> 4.      generic_file_aio_write_nolock
> 5.         generic_file_buffered_write
> 6.            a_ops->prepare_write
> etc.
>
> Right now, loop.c is circumventing step 2 (optional cluster locking
> if needed by the underlying fs) by bypassing the "write" op.  If I 
> added callbacks to prepare_write at (7) as you suggest, it would either
> (a) introduce deadlocks conflicting with step 2 above or
> (b) require the underlying fs to use its own versions of 3,4,5, and 6,
> thus bypassing a great deal of vfs, which is not good for anyone.
>
> Some could argue that loop.c should always use write rather than
> prepare_write/commit_write, but most fs's don't have special
> locking needs and therefore using them is a performance boost.
>