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, count the total blocks
before and after migrate. If they differ fail migrate with EAGAIN.
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.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxxxxxxx>
---
fs/ext4/inode.c | 4 ----
fs/ext4/migrate.c | 25 +++++++++++++++++++++++--
2 files changed, 23 insertions(+), 6 deletions(-)
diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 059f2fc..a52904b 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -3502,9 +3502,5 @@ 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..363f4f9 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, blkcnt_t total_blocks)
{
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);
/*
+ * total number of blocks will be different if a mmap write
+ * happened to holes in the file. We don't take
+ * inode->i_lock spin lock here because block allocation
+ * can't happen if we are holding i_data_sem
+ */
+ if (total_blocks != inode->i_blocks) {
+ retval = -EAGAIN;
+ up_write(&EXT4_I(inode)->i_data_sem);
+ goto err_out;
+
+ }
+ /*
* We have the extent map build with the tmp inode.
* Now copy the i_data across
*/
@@ -445,6 +457,7 @@ int ext4_ext_migrate(struct inode *inode, struct file *filp,
struct inode *tmp_inode = NULL;
struct list_blocks_struct lb;
unsigned long max_entries;
+ blkcnt_t total_blocks = 0;
if (!test_opt(inode->i_sb, EXTENTS))
/*
@@ -508,6 +521,14 @@ 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.
+ */
+ spin_lock(&inode->i_lock);
+ total_blocks = inode->i_blocks;
+ spin_unlock(&inode->i_lock);
handle = ext4_journal_start(inode, 1);
ei = EXT4_I(inode);
@@ -561,7 +582,7 @@ err_out:
free_ext_block(handle, tmp_inode);
else
retval = ext4_ext_swap_inode_data(handle, inode,
- tmp_inode);
+ tmp_inode, total_blocks);
/* We mark the tmp_inode dirty via ext4_ext_tree_init. */
if (ext4_journal_extend(handle, 1) != 0)
--
1.5.4.4.481.g5075.dirty
--
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