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[dm-devel] Re: Why does __do_page_cache_readahead submit READ, not READA?



On Thu, Jul 30, 2009 at 08:06:49AM +0200, Jens Axboe wrote:
> On Wed, Jul 29 2009, Chris Mason wrote:
> > On Wed, Jul 29, 2009 at 11:18:45PM +0200, Jens Axboe wrote:
> > > On Wed, Jul 29 2009, Lars Ellenberg wrote:
> > > > I naively assumed, from the "readahead" in the name, that readahead
> > > > would be submitting READA bios. It does not.
> > > > 
> > > > I recently did some statistics on how many READ and READA requests
> > > > we actually see on the block device level.
> > > > I was suprised that READA is basically only used for file system
> > > > internal meta data (and not even for all file systems),
> > > > but _never_ for file data.
> > > > 
> > > > A simple
> > > > 	dd if=bigfile of=/dev/null bs=4k count=1
> > > > will absolutely cause readahead of the configured amount, no problem.
> > > > But on the block device level, these are READ requests, where I'd
> > > > expected them to be READA requests, based on the name.
> > > > 
> > > > This is because __do_page_cache_readahead() calls read_pages(),
> > > > which in turn is mapping->a_ops->readpages(), or, as fallback,
> > > > mapping->a_ops->readpage().
> > > > 
> > > > On that level, all variants end up submitting as READ.
> > > > 
> > > > This may even be intentional.
> > > > But if so, I'd like to understand that.
> > > 
> > > I don't think it's intentional, and if memory serves, we used to use
> > > READA when submitting read-ahead. Not sure how best to improve the
> > > situation, since (as you describe), we lose the read-ahead vs normal
> > > read at that level. I did some experimentation some time ago for
> > > flagging this, see:
> > > 
> > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commitdiff;h=16cfe64e3568cda412b3cf6b7b891331946b595e
> > > 
> > > which should pass down READA properly.
> > 
> > One of the problems in the past was that reada would fail if there
> > wasn't a free request when we actually wanted it to go ahead and wait.
> > Or something.  We've switched it around a few times I think.
> 
> Yes, we did used to do that, whether it was 2.2 or 2.4 I
> don't recall :-)
> 
> It should be safe to enable know, whether there's a prettier way
> than the above, I don't know. It works by detecting the read-ahead
> marker, but it's a bit of a fragile design.

I dug through my old email and found this fun bug w/buffer heads and
reada.

1) submit reada ll_rw_block on ext3 directory block
2) decide that we really really need to wait on this block
3) wait_on_buffer(bh) ; check up to date bit when done

The problem in the bugzilla was that reada was returning EAGAIN or
EWOULDBLOCK, and the whole filesystem world expects that if we
wait_on_buffer and don't find the buffer up to date, its time
set things read only and run around screaming.

The expectations in the code at the time were that the caller needs to
be aware the request may fail with EAGAIN/EWOULDBLOCK, but the reality
was that everyone who found that locked buffer also needed to be able to
check for it.  This one bugzilla had a teeny window where the reada
buffer head was leaked to the world.

So, I think we can start using it again if it is just a hint to the
elevator about what to do with the IO, and we never actually turn the
READA into a transient failure (which I think is mostly true today, there
weren't many READA tests in the code I could see).

-chris


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