On Mon, 12 Jul 2010, Jan Kara wrote:
> On Mon 12-07-10 17:58:46, Lukas Czerner wrote:
> > On Mon, 12 Jul 2010, Jan Kara wrote:
> >
> > > > Walk through each allocation group and trim all free extents. It can be
> > > > invoked through TRIM ioctl on the file system. The main idea is to
> > > > provide a way to trim the whole file system if needed, since some SSD's
> > > > may suffer from performance loss after the whole device was filled (it
> > > > does not mean that fs is full!).
> > > >
> > > > It search for free extents in each allocation group. When the free
> > > > extent is found, blocks are marked as used and then trimmed. Afterwards
> > > > these blocks are marked as free in per-group bitmap.
> > > >
> > > > Signed-off-by: Lukas Czerner <lczerner@xxxxxxxxxx>
> > > > ---
> > > > fs/ext3/balloc.c | 145 +++++++++++++++++++++++++++++++++++++++++++++++
> > > > fs/ext3/super.c | 1 +
> > > > include/linux/ext3_fs.h | 1 +
> > > > 3 files changed, 147 insertions(+), 0 deletions(-)
> > > >
> > > > diff --git a/fs/ext3/balloc.c b/fs/ext3/balloc.c
> > > > index a177122..bcee525 100644
> > > > --- a/fs/ext3/balloc.c
> > > > +++ b/fs/ext3/balloc.c
> > > ...
> > > > + /**
> > > > + * Allocate contiguous free extents by setting bits in the
> > > > + * block bitmap
> > > > + */
> > > > + while (next < max
> > > > + && !ext3_set_bit_atomic(sb_bgl_lock(sbi, group),
> > > > + next, bh->b_data)) {
> > > > + next++;
> > > > + }
> > > This is actually wrong. You completely ignore journalling here. You can't
> > > just go and modify metadata buffer - other process can be modifying it as well
> > > and writing it to disk and thus your changes will also get written. And if
> > > a crash happens afterwards before the bitmap is written again, you'll get an
> > > inconsistent filesystem.
> > > Also you have to check whether the block isn't actually still used by a
> > > running/committing transaction - look at fs/ext3/balloc.c:claim_block() to see
> > > how you have to allocate free blocks.
> >
> > I may be wrong, but I thought that since the trim command ensures that
> > every operation in queue completes before the trim proceed, I do not
> > need to care much about the journaling and running transaction. But I
> > will took at it once more..
> Consider just a simple race:
>
> thread A: thread B:
>
> allocate blocks in group G
> set bits for free blocks in group G
> transaction with allocation
> commits - bitmap has bits
> from thread B set
> ----------------------------------------------- crash
> After a journal replay we have just leaked blocks set in the bitmap
> by thread B...
> And there are probably races with worse consequences. This is just the
> simplest one.
>
> Honza
>
Ok, I was terribly wrong! I am going to fix it, as well as ext4 patch.
Thanks for clarifying that!
-Lukas
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