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:	Fri, 17 Aug 2007 06:24:47 +0400
From:	Alex Tomas <alex@...sterfs.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
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)

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?

thanks, Alex

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