On Tue 28-08-12 20:21:41, Dmitry Monakhov wrote:
> Currently ext4_da_write_begin may deadlock if called with opened journal
> transaction. Real life example:
> ->move_extent_per_page()
> ->ext4_journal_start()-> hold journal transaction
> ->write_begin()
> ->ext4_da_write_begin()
> ->ext4_nonda_switch()
> ->writeback_inodes_sb_if_idle() --> will wait for journal_stop()
>
> This bug may be easily fixed by code reordering,
> But in some cases it should be possible to call write_begin()
> while holding journal's transaction, in this case caller may avoid
> recoursion by passing AOP_FLAG_NOFS flag.
Well, I find calling ext4_write_begin() with a transaction started a bug.
Possibly ext4_write_begin() can be tweaked to handle that but things would
be simpler if we didn't have to.
Looking into move_extent_per_page(), calling ->write_begin() doesn't seem
to be quite right there anyway. For example it results in filling holes
under that page which is not desirable. I'm not even sure why do we call
->write_begin() there at all. The data in the page is unchanged. So it
should be enough to just remap buffers and mark the page dirty (but I might
be missing some subtlety here). Fujita-san, can you possibly explain?
Honza
> ---
> fs/ext4/inode.c | 28 +++++++++++++++++-----------
> 1 files changed, 17 insertions(+), 11 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 6324f74..d12d30e 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -889,6 +889,11 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
> struct page *page;
> pgoff_t index;
> unsigned from, to;
> + int nofs = flags & AOP_FLAG_NOFS;
> +
> + /* We cannot recurse into the filesystem if the transaction is already
> + * started */
> + BUG_ON(!nofs && journal_current_handle());
>
> trace_ext4_write_begin(inode, pos, len, flags);
> /*
> @@ -906,9 +911,6 @@ retry:
> ret = PTR_ERR(handle);
> goto out;
> }
> -
> - /* We cannot recurse into the filesystem as the transaction is already
> - * started */
> flags |= AOP_FLAG_NOFS;
>
> page = grab_cache_page_write_begin(mapping, index, flags);
> @@ -957,7 +959,8 @@ retry:
> }
> }
>
> - if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> + if (!nofs && ret == -ENOSPC &&
> + ext4_should_retry_alloc(inode->i_sb, &retries))
> goto retry;
> out:
> return ret;
> @@ -2447,7 +2450,7 @@ out_writepages:
> }
>
> #define FALL_BACK_TO_NONDELALLOC 1
> -static int ext4_nonda_switch(struct super_block *sb)
> +static int ext4_nonda_switch(struct super_block *sb, int writeback_allowed)
> {
> s64 free_blocks, dirty_blocks;
> struct ext4_sb_info *sbi = EXT4_SB(sb);
> @@ -2475,7 +2478,7 @@ static int ext4_nonda_switch(struct super_block *sb)
> * Even if we don't switch but are nearing capacity,
> * start pushing delalloc when 1/2 of free blocks are dirty.
> */
> - if (free_blocks < 2 * dirty_blocks)
> + if (writeback_allowed && free_blocks < 2 * dirty_blocks)
> writeback_inodes_sb_if_idle(sb, WB_REASON_FS_FREE_SPACE);
>
> return 0;
> @@ -2490,10 +2493,14 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
> pgoff_t index;
> struct inode *inode = mapping->host;
> handle_t *handle;
> + int nofs = flags & AOP_FLAG_NOFS;
>
> index = pos >> PAGE_CACHE_SHIFT;
> + /* We cannot recurse into the filesystem if the transaction is already
> + * started */
> + BUG_ON(!nofs && journal_current_handle());
>
> - if (ext4_nonda_switch(inode->i_sb)) {
> + if (ext4_nonda_switch(inode->i_sb, !nofs)) {
> *fsdata = (void *)FALL_BACK_TO_NONDELALLOC;
> return ext4_write_begin(file, mapping, pos,
> len, flags, pagep, fsdata);
> @@ -2512,8 +2519,6 @@ retry:
> ret = PTR_ERR(handle);
> goto out;
> }
> - /* We cannot recurse into the filesystem as the transaction is already
> - * started */
> flags |= AOP_FLAG_NOFS;
>
> page = grab_cache_page_write_begin(mapping, index, flags);
> @@ -2538,7 +2543,8 @@ retry:
> ext4_truncate_failed_write(inode);
> }
>
> - if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> + if (!nofs && ret == -ENOSPC &&
> + ext4_should_retry_alloc(inode->i_sb, &retries))
> goto retry;
> out:
> return ret;
> @@ -4791,7 +4797,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct vm_fault *vmf)
> /* Delalloc case is easy... */
> if (test_opt(inode->i_sb, DELALLOC) &&
> !ext4_should_journal_data(inode) &&
> - !ext4_nonda_switch(inode->i_sb)) {
> + !ext4_nonda_switch(inode->i_sb, 1)) {
> do {
> ret = __block_page_mkwrite(vma, vmf,
> ext4_da_get_block_prep);
> --
> 1.7.7.6
>
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--
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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