Re: [PATCH] ext4: fix uniniatilized extend splitting error.

Mingming Cao
2008-01-11T01:08:50Z

On Thu, 2008-01-10 at 14:31 -0700, Andreas Dilger wrote:
> On Jan 10, 2008 17:31 +0300, Dmitry Monakhov wrote:
> > While playing with new fancy fallocate interface on ext4 i've triggered
> > bug which corrupted my grub :).
>
> I notice I'm CC'd on this, but in fact Amit wrote the code. I've CC'd
> him even though I expect he will notice it anyways.
>
> > My testcase:
> > ~~~~~~~~~~~~
> > blksize = 0x1000;
> > fd = open(argv[1], O_RDWR|O_CREAT, 0700);
> > unsigned long long sz = 0x10000000UL;
> > /* allocating big blocks chunk */
> > syscall(__NR_fallocate, fd, 0, 0UL, sz)
> >
> > /* grab all other available filesystem space */
> > tfd = open("tmp", O_RDWR|O_CREAT|O_DIRECT, 0700);
> > while( write(tfd, buf, 4096) > 0); /* loop untill ENOSPC */
> > fsync(fd); /* just in case */
> > while (pos < sz) {
> > /* each seek+ write operation result in splits uninitialized extent
> > in three extents. Splitting may result in new extent allocation
> > which probably will fail because of ENOSPC*/
> >
> > lseek(fd, blksize*2 -1, SEEK_CUR);
> > if ((ret = write(fd, 'a', 1)) != 1)
> > exit(1);
> > pos += blksize * 2;
> > }
>
> Interesting test, and well thought out...
>
> > Buggy place:
> > ~~~~~~~~~~~~
> > ext4_ext_get_blocks(..., bh_result,..)
> > {
> > err = 0;
> > allocated = 0;
> > ....
> > ret = ext4_ext_convert_to_initialized(...)
> > if (ret < 0)
> > << By occasion real error code was lost here.
> > goto out2
> > ....
> > out2:
> > ....
> > return err? err: allocated;
> > << Wow.. exit with "0", and caller assumes what bh_result was properly filled
> > << and then will submit it for write. But in fact bh contains random data in
> > << ->b_bdev, ->b_blocknr fileds :).
> > }
>
> The other item that Amit and I discussed in the past is in the case of
> ENOSPC it would be possible instead of splitting the extent to zero-fill
> the smaller extent (1 block in your test case) and write the whole thing
> as an initialized extent. This could then either be merged with the
> previous or following allocated extent, or the whole extent zeroed if that
> was not possible.
>
> It would add some latency in the worst case to do this in the kernel,
> but this would only happen if there is no free space at all. It might
> even be desirable to always zero-fill small extents instead of splitting
> uninitialized extents, because a random write of 64kB is not more expensive
> than 4kB and avoids overhead of splitting the nicely contiguous extent tree.
>

Zero-fill uninitialized extents all the time can cause extra IO, as that
zero-out (via get_block()) is happened at prepare write time rather than
page write out time.

I'd prefer the latency zero-out happens only when it needed.

> > Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
> > ---
> > fs/ext4/extents.c | 5 +++--
> > 1 files changed, 3 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
> > index 8528774..fc8e508 100644
> > --- a/fs/ext4/extents.c
> > +++ b/fs/ext4/extents.c
> > @@ -2320,9 +2320,10 @@ int ext4_ext_get_blocks(handle_t *handle, struct inode *inode,
> > ret = ext4_ext_convert_to_initialized(handle, inode,
> > path, iblock,
> > max_blocks);
> > - if (ret <= 0)
> > + if (ret <= 0) {
> > + err = ret;
> > goto out2;
> > - else
> > + } else
> > allocated = ret;
> > goto outnew;
> > }
> > --
> > 1.5.3.1.40.g6972-dirty
> >
> >
> > -
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>
> Cheers, Andreas
> --
> Andreas Dilger
> Sr. Staff Engineer, Lustre Group
> Sun Microsystems of Canada, Inc.
>
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