Re: [PATCH v2 1/2] ext4: only update i_reserved_data_blocks on successful block allocation

[Baokun Li]

On 2023/4/11 17:08, Jan Kara wrote:
> On Thu 06-04-23 21:28:33, Baokun Li wrote:
> > In our fault injection test, we create an ext4 file, migrate it to
> > non-extent based file, then punch a hole and finally trigger a WARN_ON
> > in the ext4_da_update_reserve_space():
> >
> > EXT4-fs warning (device sda): ext4_da_update_reserve_space:369:
> > ino 14, used 11 with only 10 reserved data blocks
> >
> > When writing back a non-extent based file, if we enable delalloc, the
> > number of reserved blocks will be subtracted from the number of blocks
> > mapped by ext4_ind_map_blocks(), and the extent status tree will be
> > updated. We update the extent status tree by first removing the old
> > extent_status and then inserting the new extent_status. If the block range
> > we remove happens to be in an extent, then we need to allocate another
> > extent_status with ext4_es_alloc_extent().
> >
> >         use old    to remove   to add new
> >      |----------|------------|------------|
> >                old extent_status
> >
> > The problem is that the allocation of a new extent_status failed due to a
> > fault injection, and __es_shrink() did not get free memory, resulting in
> > a return of -ENOMEM. Then do_writepages() retries after receiving -ENOMEM,
> > we map to the same extent again, and the number of reserved blocks is again
> > subtracted from the number of blocks in that extent. Since the blocks in
> > the same extent are subtracted twice, we end up triggering WARN_ON at
> > ext4_da_update_reserve_space() because used > ei->i_reserved_data_blocks.
> >
> > For non-extent based file, we update the number of reserved blocks after
> > ext4_ind_map_blocks() is executed, which causes a problem that when we call
> > ext4_ind_map_blocks() to create a block, it doesn't always create a block,
> > but we always reduce the number of reserved blocks. So we move the logic
> > for updating reserved blocks to ext4_ind_map_blocks() to ensure that the
> > number of reserved blocks is updated only after we do succeed in allocating
> > some new blocks.
> >
> > Fixes: 5f634d064c70 ("ext4: Fix quota accounting error with fallocate")
> > Signed-off-by: Baokun Li <libaokun1@xxxxxxxxxx>
> Looks good, just one nit below.
>
> > diff --git a/fs/ext4/indirect.c b/fs/ext4/indirect.c
> > index c68bebe7ff4b..9acab70ddf5e 100644
> > --- a/fs/ext4/indirect.c
> > +++ b/fs/ext4/indirect.c
> > @@ -651,6 +651,14 @@ int ext4_ind_map_blocks(handle_t *handle, struct inode *inode,
> >   
> >   	ext4_update_inode_fsync_trans(handle, inode, 1);
> >   	count = ar.len;
> > +
> > +	/*
> > +	 * Update reserved blocks/metadata blocks after successful block
> > +	 * allocation which had been deferred till now.
> > +	 */
> > +	if ((count > 0) && (flags & EXT4_GET_BLOCKS_DELALLOC_RESERVE))
> You don't need the count > 0 condition here. It should be always true.
>
> 								Honza
Indeed!

Thanks a million!

I will remove this condition in patch v3.

--
With Best Regards,
Baokun Li
.