Re: Bug with "fix partial page writes" [3.2-rc regression]

[Allison Henderson]

On 12/05/2011 08:44 PM, Yongqiang Yang wrote:
> On Tue, Dec 6, 2011 at 11:33 AM, Tao Ma<tm@xxxxxx>  wrote:
> > On 12/06/2011 11:08 AM, Yongqiang Yang wrote:
> > > Hi Allison,
> > >
> > > I noticed another problem which has nothing to do with punching hole.
> > >   __block_write_begin does not zero buffers beyond EOF.(I guess you
> > yes, that is expected.
> > > tried to zero them in your code, am I right? )  When users mapread
> > > beyond EOF,  users get non-zero data.  I am not sure zero or non-zero
> > > data should be, but fsx thinks they should be zero data and reports an
> > > error.
> > why users can read the data passing EOF? I am also puzzled. Punching
> > hole will do this? I don't think it's right.
> According to code, fiemap_fault handles the case right.   But I met
> the error - 'non-zero data beyond EOF' reported by fsx.  It is
> strange.  It seems that uptodate status is set wrong.  Just a guess:-)
>
> I am guessing Allison met the problem before and tried to fix it in
> write path by zeroing buffers beyond EOF.

Yes I did run into something similar.  I found 2 cases that involved EOF:
1. A truncate shortens EOF, but only zeroed to the end of the block, but 
not to the end of the page.  This was corrected by "[PATCH 5/6 v7] ext4: 
fix fsx truncate failure"

2. A write extends EOF, but does not zero all of the page beyond EOF, 
and that was what "[PATCH 6/6 v7] ext4: fix partial page writes" was 
supposed to address.

I am still digging through tracing output at the moment, so I dont have 
a very good explanation right now, but I will keep folks posted if I 
find something.

Allison Henderson


> Yongqiang.
> > Thanks
> > Tao
> > > It I understand the problem right, it happens more often with punch hole.
> > >
> > > Yongqiang.
> > > On Tue, Dec 6, 2011 at 9:40 AM, Allison Henderson
> > > <achender@xxxxxxxxxxxxxxxxxx>  wrote:
> > > > On 12/05/2011 04:38 PM, Hugh Dickins wrote:
> > > > > On Mon, 21 Nov 2011, Hugh Dickins wrote:
> > > > > > On Mon, 21 Nov 2011, Ted Ts'o wrote:
> > > > > > > On Sun, Nov 20, 2011 at 12:59:10PM -0800, Hugh Dickins wrote:
> > > > > > > > On Tue, 8 Nov 2011, Curt Wohlgemuth wrote:
> > > > > > > > It appears that there's a bug with this patch:
> > > > >
> > > > >
> > > > > This has been outstanding for a month now, and we've heard no progress:
> > > > > please revert commit 02fac1297eb3 "ext4: fix partial page writes" for rc5.
> > > > >
> > > > > The problems appear on a 1k-blocksize filesystem under memory pressure:
> > > > > the hunk in ext4_da_write_end() causes oops, because it's playing with
> > > > > a page after generic_write_end() dropped our last reference to it; and
> > > > > backing out the hunk in ext4_da_write_begin() is then found to stop
> > > > > rare data corruption seen when kbuilding.
> > > > >
> > > > > Although I earlier reported that backing out the patch caused an fsx
> > > > > test to fail earlier, I've since found great variation in how soon it
> > > > > fails, and seen it fail just as quickly with 02fac1297eb3 still in.
> > > > > I also reported that I had to go back to 2.6.38 for fsx not to fail
> > > > > under memory pressure: you won't be surprised that that turned out to
> > > > > be because 2.6.38 defaults nomblk_io_submit but 2.6.39 mblk_io_submit.
> > > > >
> > > > > Thanks,
> > > > > Hugh
> > > >
> > > >
> > > > Hi there,
> > > >
> > > > Have you tried Yongqiang's patch "[PATCH 1/2] ext4: let mpage_submit_io
> > > > works well when blocksize<  pagesize" ?  I have tried it and it does seem to
> > > > help, but I am still running into some failures that I am trying to debug,
> > > > but let please let us know if it helps the issues that you are seeing.  Thx!
> > > >
> > > > Allison Henderson

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