Re: Why does __do_page_cache_readahead submit READ, not READA?

[Chris Mason <chris.mason@oracle.com>]

On Thu, Jul 30, 2009 at 12:47:21PM -0400, Jeff Moyer wrote:
> Chris Mason <chris.mason@oracle.com> writes:
> 
> > 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).
> 
> Well, is it a hint to the elevator or to the driver (or both)?

I would say both as long as they don't fail it.  IOW a priority decision
instead of a discard this request at will decision.

> The one
> bug I remember regarding READA failing was due to the FAILFAST bit
> getting set for READA I/O, and the powerpath driver returning a failure.
> Is that the bug to which you are referring?

This was a rhel bug with ext3 and (both dm and powerpath ) multipath, but in
theory it could be triggered on regular drives.  I don't think we ever
managed to, but removing READA definitely fixed it.

It was bug 213921 in the RH bugzilla, and I think it had been fixed in
other ways in mainline by the time we found it.

-chris

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