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Date: Thu, 13 Mar 2008 11:47:39 +0100
From: Jan Kara <jack@...e.cz>
To: "Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>
Cc: cmm@...ibm.com, tytso@....edu, adilger@....com,
linux-ext4@...r.kernel.org
Subject: Re: [PATCH] ext4: Fail migrate if we allocated new blocks via mmap
write.
On Thu 13-03-08 14:08:07, 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.
Yes, the patch looks fine. Thanks for all the changes :).
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.vnet.ibm.com>
Acked-by: Jan Kara <jack@...e.cz>
> ---
> 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;
> + }
> 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);
> 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);
> }
> 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;
> + /*
> * 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));
> + /*
> + * Even though we take i_mutex we can still cause block allocation
> + * via mmap write to holes. If we have allocated new blocks we fail
> + * migrate. New block allocation will clear EXT4_EXT_MIGRATE flag
> + * The flag is updated with i_data_sem held to prevent racing with
> + * block allocation.
> + */
> + down_read((&EXT4_I(inode)->i_data_sem));
> + EXT4_I(inode)->i_flags = EXT4_I(inode)->i_flags | EXT4_EXT_MIGRATE;
> + up_read((&EXT4_I(inode)->i_data_sem));
> +
> handle = ext4_journal_start(inode, 1);
>
> ei = EXT4_I(inode);
> @@ -560,8 +583,7 @@ err_out:
> */
> free_ext_block(handle, tmp_inode);
> else
> - retval = ext4_ext_swap_inode_data(handle, inode,
> - tmp_inode);
> + retval = ext4_ext_swap_inode_data(handle, inode, tmp_inode);
>
> /* We mark the tmp_inode dirty via ext4_ext_tree_init. */
> if (ext4_journal_extend(handle, 1) != 0)
> diff --git a/include/linux/ext4_fs.h b/include/linux/ext4_fs.h
> index 8f5a563..2d15f16 100644
> --- a/include/linux/ext4_fs.h
> +++ b/include/linux/ext4_fs.h
> @@ -240,6 +240,7 @@ struct flex_groups {
> #define EXT4_TOPDIR_FL 0x00020000 /* Top of directory hierarchies*/
> #define EXT4_HUGE_FILE_FL 0x00040000 /* Set to each huge file */
> #define EXT4_EXTENTS_FL 0x00080000 /* Inode uses extents */
> +#define EXT4_EXT_MIGRATE 0x00100000 /* Inode is migrating */
> #define EXT4_RESERVED_FL 0x80000000 /* reserved for ext4 lib */
>
> #define EXT4_FL_USER_VISIBLE 0x000BDFFF /* User visible flags */
> --
> 1.5.4.4.532.ga6828.dirty
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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