Re: [PATCH] reiser4: fix readv

["Vladimir V. Saveliev" <vs@namesys.com>]

Hello

On Monday 18 September 2006 16:30, Christoph Hellwig wrote:
> On Sun, Sep 17, 2006 at 10:39:07PM +0400, Vladimir V. Saveliev wrote:
> > It seems the problem can be fixed a bit simpler. Currently, there is a difference between read and readv: read calls f_op->read if it is defined,
> > but readv calls f_op->read if f_op->aio_read is not defined. The latest is a bit unlogical imho: 
> > wouldn't it be more consistent if readv worked via f_op->read if it is defined?
> > If we fixed readv (do_readv_writev) that way - reiser4 would not need anything from its aio_read but generic_file_aio_read.
> 
> This behaviour is historic baggae.  readv used to call ->readv and fall
> back to ->read when it wasn't available.  We now merged ->readv into
> ->aio_read and kept that behaviour.  I think it makes sense, though - if
> the filesystem supports vator operations via ->aio_read we should use
> them and not fall back to the manual looping around ->read.
> 
> I'd rather change read to do the same thing as readv so we have
> consistent behaviour.
> 

Can we put it that way: if a filesystem can use page_cache directly - it has to set f_op->aio_read to generic_file_aio_read
and set f_op->read to NULL. If the filesystem wants to try to screw things up a bit - 
it implements f_op->read and its f_op->read is called by sys_read and sys_readv
regardless to whether it has aio_read or not?

> why does this matter for reiser4?
> 

reiser4 reads some files via generic page cache routines. In that case reiser4' read calls do_sync_read. 
Therefore, it has to define f_op->aio_read. 
OTOH, there are files for which reiser4' read does not call do_sync_read.

In case of readv, f_op->aio_read is called directly (if it is defined), which may result in that reiser4' aio_read is called for files for which 
reiser4' read would never call do_sync_read. 
To avoid the problem we have to implement reiser4_aio_read which either calls generic_file_aio_read or does something very similar to do_loop_readv_writev.