Re: delay in block_read_full_page()

[Andrew Morton <akpm@osdl.org>]

Michael Mesnier <mmesnier@ece.cmu.edu> wrote:
>
> Hello,
> 
> Please cc: me directly in your response.
> 
> I'm running into some trouble with an installable file system I'm 
> writing. In myfs_readpage() I simply return block_read_full_page() which 
> subsequently calls myfs_get_block().  However, there's a delay before 
> the I/O actually gets issued to the device.  Running sync from the 
> command line causes the I/O to get issued immediately, so the sync call 
> (even it there aren't dirty buffers) also manages to schedule any 
> outstanding read I/Os. How should my fs indicate to the vfs that these 
> read I/Os need to be issued immediately after my_readpage() is called?

Normally we leave the I/O pending in the expectation that more readpage()
requests will occur.  This allow us to merge things in the disk request
queues.  We'll actually submit the I/O to the device if:

a) There's a lot of it pending or

b) There haven't been any more readpage() calls for a while or

c) Someone actually wants to wait on the I/O (say, via lock_page())

It is unusual that you want this I/O to kick off immediately.  You will
probably find that blk_run_backing_dev() will do what you want.


That's all for a 2.6 kernel - you didn't specify.  It it's a 2.4 kernel
then you'll need to use run_task_queue(&tq_disk) to flush the queued I/O
requests.