Re: semi-stable page writes

On Tue, Oct 30, 2012 at 09:01:22AM +1100, Dave Chinner wrote:

On Fri, Oct 26, 2012 at 03:19:09AM -0700, Darrick J. Wong wrote:

quoted

Hi everyone,

Are people still annoyed about writes taking unexpectedly long amounts of tme
due to the stable page write patchset?  I'm guessing yes...

I haven't heard anyone except th elunatic fringe complain
recently...

quoted

I'm close to posting a patchset that (a) gates the wait_on_page_writeback calls
on a flag that you can set in the bdi to indicate that you need stable writes
(which blk_integrity_register will set);

I'd prefer stable pages by default (e.g. btrfs needs it for sane
data crc calculations), with an option to turn it off.

quoted

(b) (ab)uses a page flag bit (PG_slab)
to indicate that a page is actually being sent out to disk hardware; and (c)

I don't think you can do that. You can send slab allocated memory to
disk (e.g. kmalloc()d memory) and XFS definitely does that for
sub-page sized metadata. I'm pretty sure that means the PG_slab
flag is not available for (ab)use in the IO path....

I gave up on PG_slab and declared my own PG_ bit.  Unfortunately, atm I can't
remember which bit of code marks the page ptes so that they have to go back
through page_mkwrite, where we can trap the write.  Hopefully for a shorter
duration.

Also, I was wondering -- is it possible to pursue a dual strategy?  If we can
obtain a memory page without sleeping or causing any writeback, then use the
page as a bounce buffer.  Otherwise, just wait like we do now.  It looks as
though one could use __GFP_NORETRY | __GFP_NO_MEMALLOC to see if the allocator
can give out a page without having to run reclaim...?

--D

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com
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