On Fri, Jun 19, 2020 at 09:08:54AM +0200, Lukas Czerner wrote:
> On Fri, Jun 19, 2020 at 08:41:22AM +0200, Lukas Czerner wrote:
> > On Wed, Jun 17, 2020 at 02:19:04PM -0500, Eric Sandeen wrote:
> > > If for any reason a directory passed to do_split() does not have enough
> > > active entries to exceed half the size of the block, we can end up
> > > iterating over all "count" entries without finding a split point.
> > >
> > > In this case, count == move, and split will be zero, and we will
> > > attempt a negative index into map[].
> > >
> > > Guard against this by detecting this case, and falling back to
> > > split-to-half-of-count instead; in this case we will still have
> > > plenty of space (> half blocksize) in each split block.
> > >
> > > Fixes: ef2b02d3e617 ("ext34: ensure do_split leaves enough free space in both blocks")
> > > Signed-off-by: Eric Sandeen <sandeen@xxxxxxxxxx>
> > > ---
> > >
> > > diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
> > > index a8aca4772aaa..8b60881f07ee 100644
> > > --- a/fs/ext4/namei.c
> > > +++ b/fs/ext4/namei.c
> > > @@ -1858,7 +1858,7 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
> > > blocksize, hinfo, map);
> > > map -= count;
> > > dx_sort_map(map, count);
> > > - /* Split the existing block in the middle, size-wise */
> > > + /* Ensure that neither split block is over half full */
> > > size = 0;
> > > move = 0;
> > > for (i = count-1; i >= 0; i--) {
> > > @@ -1868,8 +1868,18 @@ static struct ext4_dir_entry_2 *do_split(handle_t *handle, struct inode *dir,
> > > size += map[i].size;
> > > move++;
> > > }
> > > - /* map index at which we will split */
> > > - split = count - move;
> > > + /*
> > > + * map index at which we will split
> > > + *
> > > + * If the sum of active entries didn't exceed half the block size, just
> > > + * split it in half by count; each resulting block will have at least
> > > + * half the space free.
> > > + */
> > > + if (i > 0)
> > > + split = count - move;
> > > + else
> > > + split = count/2;
> >
> > Won't we have exactly the same problem as we did before your commit
> > ef2b02d3e617cb0400eedf2668f86215e1b0e6af ? Since we do not know how much
> > space we actually moved we might have not made enough space for the new
> > entry ?
> >
> > Also since we have the move == count when the problem appears then it's
> > clear that we never hit the condition
> >
> > 1865 → → /* is more than half of this entry in 2nd half of the block? */
> > 1866 → → if (size + map[i].size/2 > blocksize/2)
> > 1867 → → → break;
> >
> > in the loop. This is surprising but it means the the entries must have
> > gaps between them that are small enough that we can't fit the entry
> > right in ? Should not we try to compact it before splitting, or is it
> > the case that this should have been done somewhere else ?
>
> The other possibility is that map[i].size is not right and indeed there
> seems to be a bug in dx_make_map()
>
> map_tail->size = le16_to_cpu(de->rec_len);
>
> should be
>
> map_tail->size = ext4_rec_len_from_disk(de->rec_len, blocksize));
>
> right ? Otherwise with large enough records the size will be smaller
> than it really is.
>
> A quick look at fs/ext4/namei.c reveals couple of places there rec_len
> is used without the conversion and we should check whether it needs
> fixing.
>
> -Lukas
And indeed the following patch seems to have fixed the issue we were
seeing. Eric I think that this might be a proper fix. But we still need
to check the other uses of rec_len to make sure it's ok as well.
diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c
index 94ec882..5509fdc 100644
--- a/fs/ext4/namei.c
+++ b/fs/ext4/namei.c
@@ -1068,7 +1068,7 @@ static int dx_make_map(struct ext4_dir_entry_2 *de, unsigned blocksize,
map_tail--;
map_tail->hash = h.hash;
map_tail->offs = ((char *) de - base)>>2;
- map_tail->size = le16_to_cpu(de->rec_len);
+ map_tail->size = ext4_rec_len_from_disk(le16_to_cpu(de->rec_len), blocksize);
count++;
cond_resched();
}
>
>
>
> >
> > If we really want ot be fair and we want to split it right in the middle
> > of the entries size-wise then we need to keep track of of sum of the
> > entries and decide based on that, not blocksize/2. But maybe the problem
> > could be solved by compacting the entries together because the condition
> > seems to rely on that.
> >
> > -Lukas
> >
> > > +
> > > hash2 = map[split].hash;
> > > continued = hash2 == map[split - 1].hash;
> > > dxtrace(printk(KERN_INFO "Split block %lu at %x, %i/%i\n",
> > >
> > >
> >
>
