linux-ext4 - Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

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Date:	Thu, 16 Aug 2007 11:46:05 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alex Tomas <alex@...sterfs.com>
Cc:	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS
 is under heavy write load (massive starvation)

On Thu, 16 Aug 2007 22:20:06 +0400
Alex Tomas <alex@...sterfs.com> wrote:

> Andrew Morton wrote:
> >>> But under this proposal, t_sync_datalist just gets removed: the new
> >>> ordered-data mode _only_ need to do the sb->inode->page walk.  So if I'm
> >>> understanding you, the way in which we'd handle any such race is to make
> >>> kjournald's writeback of the dirty pages block in lock_page().  Once it
> >>> gets the page lock it can look to see if some other thread has mapped the
> >>> page to disk.
> >> if I'm right holding number of pages locked, then they won't be locked, but
> >> writeback. of course kjournald can block on writeback as well, but how does
> >> it find pages with *newly allocated* blocks only?
> > 
> > I don't think we'd want kjournald to do that.  Even if a page was dirtied
> > by an overwrite, we'd want to write it back during commit, just from a
> > quality-of-implementation point of view.  If we were to leave these pages
> > unwritten during commit then a post-recovery file could have a mix of
> > up-to-five-second-old data and up-to-30-seconds-old data.
> 
> trying to implement this I've got to think that there is one significant
> difference between t_sync_datalist and sb->inode->page walk: t_sync_datalist
> is per-transaction. IOW, it doesn't change once transaction is closed. in
> contrast, nothing (currently) would prevent others to modify pages while
> commit is in progress.

That can happen at present - there's nothing to stop a process from modifying
a page which is undergoing ordered-data commit-time writeout.
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