Re: Delayed allocation and page_lock vs transaction start ordering

Jan Kara
2008-05-26T17:21:26Z

On Wed 21-05-08 13:51:09, Aneesh Kumar K.V wrote:
> On Tue, Apr 15, 2008 at 06:14:30PM +0200, Jan Kara wrote:
> > Hi,
> >
> > I've ported my patch inversing locking ordering of page_lock and
> > transaction start to ext4 (on top of ext4 patch queue). Everything except
> > delayed allocation is converted (the patch is below for interested
> > readers). The question is how to proceed with delayed allocation. Its
> > current implementation in VFS is designed to work well with the old
> > ordering (page lock first, then start a transaction). We could bend it to
> > work with the new locking ordering but I really see no point since ext4 is
> > the only user. Also XFS has AFAIK ordering first start transaction, then
> > lock pages so if we should ever merge delayed alloc implementations the new
> > ordering would make it easier.
> > So what do people think here? Do you agree with reimplementing current
> > mpage_da_... functions? Eric, I guess you have the best clue how XFS does
> > this, do you have some advices? Also maybe pointers into XFS code would be
> > useful if it is reasonably readable :). Thanks.
> >
> > Honza
>
>
> [....snip....]
>
> > */
> > -static int ext4_ordered_writepage(struct page *page,
> > +static int __ext4_ordered_writepage(struct page *page,
> > struct writeback_control *wbc)
> > {
> > struct inode *inode = page->mapping->host;
> > @@ -1723,22 +1694,6 @@ static int ext4_ordered_writepage(struct page *page,
> > int ret = 0;
> > int err;
> >
> > - J_ASSERT(PageLocked(page));
> > -
> > - /*
> > - * We give up here if we're reentered, because it might be for a
> > - * different filesystem.
> > - */
> > - if (ext4_journal_current_handle())
> > - goto out_fail;
> > -
> > - handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));
> > -
> > - if (IS_ERR(handle)) {
> > - ret = PTR_ERR(handle);
> > - goto out_fail;
> > - }
> > -
> > if (!page_has_buffers(page)) {
> > create_empty_buffers(page, inode->i_sb->s_blocksize,
> > (1 << BH_Dirty)|(1 << BH_Uptodate));
> > @@ -1762,114 +1717,139 @@ static int ext4_ordered_writepage(struct page *page,
> > * and generally junk.
> > */
> > if (ret == 0) {
> > - err = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
> > + handle = ext4_journal_start(inode,
> > + ext4_writepage_trans_blocks(inode));
> > + if (IS_ERR(handle)) {
> > + ret = PTR_ERR(handle);
> > + goto out_put;
> > + }
> > +
> > + ret = walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE,
> > NULL, jbd2_journal_dirty_data_fn);
> > + err = ext4_journal_stop(handle);
> > if (!ret)
> > ret = err;
> > }
> > - walk_page_buffers(handle, page_bufs, 0,
> > - PAGE_CACHE_SIZE, NULL, bput_one);
> > - err = ext4_journal_stop(handle);
> > - if (!ret)
> > - ret = err;
> > +out_put:
> > + walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE, NULL,
> > + bput_one);
> > return ret;
> > +}
> > +
> > +static int ext4_ordered_writepage(struct page *page,
> > + struct writeback_control *wbc)
> > +{
> > + J_ASSERT(PageLocked(page));
> > +
> > + /*
> > + * We give up here if we're reentered, because it might be for a
> > + * different filesystem.
> > + */
> > + if (!ext4_journal_current_handle())
> > + return __ext4_ordered_writepage(page, wbc);
> >
> > -out_fail:
> > redirty_page_for_writepage(wbc, page);
> > unlock_page(page);
> > - return ret;
> > + return 0;
> > }
>
>
> How about change below to make sure we don't have a deadlock.
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index 9d1d07b..85de163 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -1718,6 +1718,10 @@ static int jbd2_journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
> return 0;
> }
>
> +static int ext4_bh_unmapped(handle_t *handle, struct buffer_head *bh)
> +{
> + return !buffer_mapped(bh);
> +}
> /*
> * Note that we don't need to start a transaction unless we're journaling
> * data because we should have holes filled from ext4_page_mkwrite(). If
> @@ -1767,20 +1771,33 @@ static int jbd2_journal_dirty_data_fn(handle_t *handle, struct buffer_head *bh)
> * us.
> *
> */
> -static int __ext4_ordered_writepage(struct page *page,
> - struct writeback_control *wbc)
> +static int __ext4_ordered_alloc_and_writepage(struct page *page,
> + struct writeback_control *wbc, int alloc)
> {
> - struct inode *inode = page->mapping->host;
> - struct buffer_head *page_bufs;
> + int ret = 0, err;
> + unsigned long len;
> handle_t *handle = NULL;
> - int ret = 0;
> - int err;
> + struct buffer_head *page_bufs;
> + struct inode *inode = page->mapping->host;
> + loff_t size = i_size_read(inode);
>
> if (!page_has_buffers(page)) {
> create_empty_buffers(page, inode->i_sb->s_blocksize,
> (1 << BH_Dirty)|(1 << BH_Uptodate));
> }
> page_bufs = page_buffers(page);
> +
> + if (page->index == size >> PAGE_CACHE_SHIFT)
> + len = size & ~PAGE_CACHE_MASK;
> + else
> + len = PAGE_CACHE_SIZE;
> +
> + if (!alloc && walk_page_buffers(NULL, page_bufs, 0,
> + len, NULL, ext4_bh_unmapped)) {
> + printk(KERN_CRIT "%s called with unmapped buffer\n",
> + __func__);
> + BUG();
> + }
> walk_page_buffers(handle, page_bufs, 0,
> PAGE_CACHE_SIZE, NULL, bget_one);
>
> @@ -1828,7 +1845,7 @@ static int ext4_ordered_writepage(struct page *page,
> * different filesystem.
> */
> if (!ext4_journal_current_handle())
> - return __ext4_ordered_writepage(page, wbc);
> + return __ext4_ordered_alloc_and_writepage(page, wbc, 0);
>
> redirty_page_for_writepage(wbc, page);
> unlock_page(page);
> @@ -3777,10 +3794,6 @@ int ext4_change_inode_journal_flag(struct inode *inode, int val)
> return err;
> }
>
> -static int ext4_bh_unmapped(handle_t *handle, struct buffer_head *bh)
> -{
> - return !buffer_mapped(bh);
> -}
>
> int ext4_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> {
> @@ -3837,7 +3850,7 @@ int ext4_page_mkwrite(struct vm_area_struct *vma, struct page *page)
> if (ext4_should_writeback_data(inode))
> ret = __ext4_writeback_writepage(page, &wbc);
> else if (ext4_should_order_data(inode))
> - ret = __ext4_ordered_writepage(page, &wbc);
> + ret = __ext4_ordered_alloc_and_writepage(page, &wbc, 1);
> else
> ret = __ext4_journalled_writepage(page, &wbc);
> /* Page got unlocked in writepage */
>
>
>
> ie we call __ext4_ordered_alloc_and_writepage with alloc = 1 only in
> case of page_mkwrite. All the other case we should have all the buffer
> heads mapped. Otherwise we will try to allocate new blocks which starts
> a new transaction holding page lock.
When do we try to allocate new blocks in writepage now? ext4_page_mkwrite()
should have done the allocation before writepage() was called so there
should be no need to allocate anything... But maybe I miss something.

> > -static int ext4_writeback_writepage(struct page *page,
> > +static int __ext4_writeback_writepage(struct page *page,
> > struct writeback_control *wbc)
> > {
> > struct inode *inode = page->mapping->host;
> > +
> > + if (test_opt(inode->i_sb, NOBH))
> > + return nobh_writepage(page, ext4_get_block, wbc);
> > + else
> > + return block_write_full_page(page, ext4_get_block, wbc);
> > +}
> > +
> > +
> > +static int ext4_writeback_writepage(struct page *page,
> > + struct writeback_control *wbc)
> > +{
> > + if (!ext4_journal_current_handle())
> > + return __ext4_writeback_writepage(page, wbc);
> > +
> > + redirty_page_for_writepage(wbc, page);
> > + unlock_page(page);
> > + return 0;
> > +}
> > +
> > +static int __ext4_journalled_writepage(struct page *page,
> > + struct writeback_control *wbc)
> > +{
> > + struct address_space *mapping = page->mapping;
> > + struct inode *inode = mapping->host;
> > + struct buffer_head *page_bufs;
> > handle_t *handle = NULL;
> > int ret = 0;
> > int err;
> >
> > - if (ext4_journal_current_handle())
> > - goto out_fail;
> > + ret = block_prepare_write(page, 0, PAGE_CACHE_SIZE, ext4_get_block);
> > + if (ret != 0)
> > + goto out_unlock;
> > +
> > + page_bufs = page_buffers(page);
> > + walk_page_buffers(handle, page_bufs, 0, PAGE_CACHE_SIZE, NULL,
> > + bget_one);
> > + /* As soon as we unlock the page, it can go away, but we have
> > + * references to buffers so we are safe */
> > + unlock_page(page);
> >
> > handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));
> > if (IS_ERR(handle)) {
> > ret = PTR_ERR(handle);
> > - goto out_fail;
> > + goto out;
> > }
> >
> > - if (test_opt(inode->i_sb, NOBH) && ext4_should_writeback_data(inode))
> > - ret = nobh_writepage(page, ext4_get_block, wbc);
> > - else
> > - ret = block_write_full_page(page, ext4_get_block, wbc);
> > + ret = walk_page_buffers(handle, page_bufs, 0,
> > + PAGE_CACHE_SIZE, NULL, do_journal_get_write_access);
> >
> > + err = walk_page_buffers(handle, page_bufs, 0,
> > + PAGE_CACHE_SIZE, NULL, write_end_fn);
> > + if (ret == 0)
> > + ret = err;
> > err = ext4_journal_stop(handle);
> > if (!ret)
> > ret = err;
> > - return ret;
> >
> > -out_fail:
> > - redirty_page_for_writepage(wbc, page);
> > + walk_page_buffers(handle, page_bufs, 0,
> > + PAGE_CACHE_SIZE, NULL, bput_one);
> > + EXT4_I(inode)->i_state |= EXT4_STATE_JDATA;
> > + goto out;
> > +
> > +out_unlock:
> > unlock_page(page);
> > +out:
> > return ret;
> > }
> >
> > static int ext4_journalled_writepage(struct page *page,
> > struct writeback_control *wbc)
> > {
> > - struct inode *inode = page->mapping->host;
> > - handle_t *handle = NULL;
> > - int ret = 0;
> > - int err;
> > -
> > if (ext4_journal_current_handle())
> > goto no_write;
> >
> > - handle = ext4_journal_start(inode, ext4_writepage_trans_blocks(inode));
> > - if (IS_ERR(handle)) {
> > - ret = PTR_ERR(handle);
> > - goto no_write;
> > - }
> > -
> > if (!page_has_buffers(page) || PageChecked(page)) {
>
>
> This will never happen with writepage right ? And we don't call
> ext4_journalled_writepage from page_mkwrite. So is this needed ?
> If not __ext4_journalled_writepage can handle everything in a single
> transaction right and assume that it is called within a transaction.
I'm not sure I understand you. PageChecked() can happen from writepage
call path. We set PageChecked() when we do set_page_dirty() as far as I
remember... Basically, we use this flag to decide whether writepage should
do checkpointing or write into the journal.

> > /*
> > * It's mmapped pagecache. Add buffers and journal it. There
> > * doesn't seem much point in redirtying the page here.
> > */
> > ClearPageChecked(page);
> > - ret = block_prepare_write(page, 0, PAGE_CACHE_SIZE,
> > - ext4_get_block);
> > - if (ret != 0) {
> > - ext4_journal_stop(handle);
> > - goto out_unlock;
> > - }
> > - ret = walk_page_buffers(handle, page_buffers(page), 0,
> > - PAGE_CACHE_SIZE, NULL, do_journal_get_write_access);
> > -
> > - err = walk_page_buffers(handle, page_buffers(page), 0,
> > - PAGE_CACHE_SIZE, NULL, write_end_fn);
> > - if (ret == 0)
> > - ret = err;
> > - EXT4_I(inode)->i_state |= EXT4_STATE_JDATA;
> > - unlock_page(page);
> > + return __ext4_journalled_writepage(page, wbc);
> > } else {
> > /*
> > * It may be a page full of checkpoint-mode buffers. We don't
> > * really know unless we go poke around in the buffer_heads.
> > * But block_write_full_page will do the right thing.
> > */
> > - ret = block_write_full_page(page, ext4_get_block, wbc);
> > + return block_write_full_page(page, ext4_get_block, wbc);
> > }
> > - err = ext4_journal_stop(handle);
> > - if (!ret)
> > - ret = err;
> > -out:
> > - return ret;
> > -
> > no_write:
> > redirty_page_for_writepage(wbc, page);
> > -out_unlock:
> > unlock_page(page);
> > - goto out;
> > + return 0;
> > }

Honza
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR