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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Message-Id: <20070816235250.d1e39b79.akpm@linux-foundation.org>
Date:	Thu, 16 Aug 2007 23:52:50 -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 Fri, 17 Aug 2007 06:24:47 +0400 Alex Tomas <alex@...sterfs.com> wrote:

> Andrew Morton wrote:
> > 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.
> 
> I tend to think it's still a bit different: set of pages doesn't change with
> t_sync_datalist. with sb->inode->page approach even silly dd will be able to
> *add* a bunch of new pages while we're syncing first ones. why shouldn't we
> fix this?
> 

Sort-of.  But the per-superpblock, per-inode writeback code is pretty
careful to avoid livelocks.  The per-inode writeback is a strict single
linear sweep across the file.  It'll basically write out anything which was
dirty when it was called.  The per-superblock inode walk isn't as accurate
as that, becuase of the difficulties of juggling list_heads.  But we're
slowly working on that, and I suspect it'll be ggod enough for ext3
purposes already.


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