Re: [PATCH] ext4: Fail migrate if we allocated new blocks via mmap write.

["Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com>]

On Thu, Mar 13, 2008 at 04:02:52PM -0700, Mingming Cao wrote:
> On Thu, 2008-03-13 at 14:08 +0530, Aneesh Kumar K.V wrote:
> > If we write to holes in the file via mmap, we endup allocating
> > new blocks. This block allocation happens without taking inode->i_mutex.
> > Since migrate is protected by i_mutex and migrate expect no
> > new blocks get allocated during migrate, fail migrate if new blocks
> > get allocated.
> > 
> > We can't take inode->i_mutex in the mmap write path because that
> > would result in a locking order violation between i_mutex and mmap_sem.
> > Also adding a seprate rw_sempahore for protecion is really high overhead
> > for a rare operation such as migrate.
> > 
> 
> Hi Aneesh,
> 
> Thanks for the update. I like this approach...some comments below.
> 
> > Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> > ---
> >  fs/ext4/inode.c         |   17 ++++++++++++-----
> >  fs/ext4/migrate.c       |   28 +++++++++++++++++++++++++---
> >  include/linux/ext4_fs.h |    1 +
> >  3 files changed, 38 insertions(+), 8 deletions(-)
> > 
> > diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> > index 059f2fc..f947251 100644
> > --- a/fs/ext4/inode.c
> > +++ b/fs/ext4/inode.c
> > @@ -986,6 +986,16 @@ int ext4_get_blocks_wrap(handle_t *handle, struct inode *inode, sector_t block,
> >  		retval = ext4_get_blocks_handle(handle, inode, block,
> >  				max_blocks, bh, create, extend_disksize);
> >  	}
> > +
> > +	if (retval > 0) {
> > +		/*
> > +		 * We allocated new blocks which will result in i_data
> > +		 * format to change.  Force the migrate to fail by
> > +		 * clearing migrate flags
> > +		 */
> > +		EXT4_I(inode)->i_flags = EXT4_I(inode)->i_flags &
> > +							~EXT4_EXT_MIGRATE;
> > +	}
> 
> We probably need to check buffer_new() for the resulting bh, as retval >
> 0 doesn't necessarily means ext4_ext_get_blocks() allocated new blocks.


Only if we request with create = 0 the API returns >0 and buffer head
unmapped.

> 
> And I think this check should only for ext3 type files, maybe checking
> the flag or move the "if" right after ext4_get_blocks_handle()?
> 
> >  	up_write((&EXT4_I(inode)->i_data_sem));
> >  	return retval;
> >  }
> > @@ -2962,7 +2972,8 @@ static int ext4_do_update_inode(handle_t *handle,
> >  	if (ext4_inode_blocks_set(handle, raw_inode, ei))
> >  		goto out_brelse;
> >  	raw_inode->i_dtime = cpu_to_le32(ei->i_dtime);
> > -	raw_inode->i_flags = cpu_to_le32(ei->i_flags);
> > +	/* clear the migrate flag in the raw_inode */
> > +	raw_inode->i_flags = cpu_to_le32(ei->i_flags & ~EXT4_EXT_MIGRATE);
> 
> Do we need to save this flag on-disk?


We don't need to. That's why i am clearing it in the raw_inode. We still
need to have it in ext4_inode_info so that an ongoing migrate doesn't
fail.

> 
> >  	if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
> >  	    cpu_to_le32(EXT4_OS_HURD))
> >  		raw_inode->i_file_acl_high =
> > @@ -3502,9 +3513,5 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> >  	 * access and zero out the page. The journal handle get initialized
> >  	 * in ext4_get_block.
> >  	 */
> > -	/* FIXME!! should we take inode->i_mutex ? Currently we can't because
> > -	 * it has a circular locking dependency with DIO. But migrate expect
> > -	 * i_mutex to ensure no i_data changes
> > -	 */
> >  	return block_page_mkwrite(vma, page, ext4_get_block);
> 
> If you update this patch, how about split this part to a separate fix
> and merge that with it's parent ext4-page-mkwrite() patch? 
> 
> >  }
> > diff --git a/fs/ext4/migrate.c b/fs/ext4/migrate.c
> > index 5c1e27d..f4c9e78 100644
> > --- a/fs/ext4/migrate.c
> > +++ b/fs/ext4/migrate.c
> > @@ -327,7 +327,7 @@ static int free_ind_block(handle_t *handle, struct inode *inode, __le32 *i_data)
> >  }
> > 
> >  static int ext4_ext_swap_inode_data(handle_t *handle, struct inode *inode,
> > -				struct inode *tmp_inode)
> > +						struct inode *tmp_inode)
> >  {
> >  	int retval;
> >  	__le32	i_data[3];
> > @@ -351,6 +351,18 @@ static int ext4_ext_swap_inode_data(handle_t *handle, struct inode *inode,
> > 
> >  	down_write(&EXT4_I(inode)->i_data_sem);
> >  	/*
> > +	 * if EXT4_EXT_MIGRATE is cleared a block allocation
> > +	 * happened after we started the migrate. We need to
> > +	 * fail the migrate
> > +	 */
> > +	if (!(EXT4_I(inode)->i_flags & EXT4_EXT_MIGRATE)) {
> > +		retval = -EAGAIN;
> > +		up_write(&EXT4_I(inode)->i_data_sem);
> > +		goto err_out;
> > +	} else
> > +		EXT4_I(inode)->i_flags = EXT4_I(inode)->i_flags &
> > +							~EXT4_EXT_MIGRATE;
> > +	/*
> 
> I could not see the caller of ext4_ext_swap_inode_data():
> ext4_ext_mirgrate() checks the return value from
> ext4_ext_swap_inode_data(). We probably should free allocated blocks,
> rebuild the extents tree for the tmp inode and do the swap again in the
> EAGAIN case. And for other error case probably need proper error
> handling too.

The ioctl will return EAGAIN and the application can issue the ioctl
again.

> 
> 
> >  	 * We have the extent map build with the tmp inode.
> >  	 * Now copy the i_data across
> >  	 */
> > @@ -508,6 +520,17 @@ int ext4_ext_migrate(struct inode *inode, struct file *filp,
> >  	 * switch the inode format to prevent read.
> >  	 */
> >  	mutex_lock(&(inode->i_mutex));
> > +	/*


-aneesh