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Date: Mon, 11 Feb 2013 17:35:49 +0100
From: Jan Kara <jack@...e.cz>
To: Theodore Ts'o <tytso@....edu>
Cc: Ext4 Developers List <linux-ext4@...r.kernel.org>
Subject: Re: [PATCH 06/12] ext4: grab page before starting transaction
handle in write_begin()
On Sat 09-02-13 16:53:46, Ted Tso wrote:
> The grab_cache_page_write_begin() function can potentially sleep for a
> long time, since it may need to do memory allocation which can block
> if the system is under significant memory pressure, and because it may
> be blocked on page writeback. If it does take a long time to grab the
> page, it's better that we not hold an active jbd2 handle.
>
> So grab a handle on the page first, and _then_ start the transaction
> handle.
>
> This commit fixes the following long transaction handle hold time:
>
> postmark-2917 [000] .... 196.435786: jbd2_handle_stats: dev 254,32
> tid 570 type 2 line_no 2541 interval 311 sync 0 requested_blocks 1
> dirtied_blocks 0
OK, the patch looks correct. ext4_write_begin() becomes a bit complex but
it's still well contained so I guess it's worth the improvement. You can
add:
Reviewed-by: Jan Kara <jack@...e.cz>
Honza
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> ---
> fs/ext4/inode.c | 111 ++++++++++++++++++++++++++++++++++----------------------
> 1 file changed, 68 insertions(+), 43 deletions(-)
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index c3c47e2..38164a8 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -906,32 +906,40 @@ static int ext4_write_begin(struct file *file, struct address_space *mapping,
> ret = ext4_try_to_write_inline_data(mapping, inode, pos, len,
> flags, pagep);
> if (ret < 0)
> - goto out;
> - if (ret == 1) {
> - ret = 0;
> - goto out;
> - }
> + return ret;
> + if (ret == 1)
> + return 0;
> }
>
> -retry:
> + /*
> + * grab_cache_page_write_begin() can take a long time if the
> + * system is thrashing due to memory pressure, or if the page
> + * is being written back. So grab it first before we start
> + * the transaction handle. This also allows us to allocate
> + * the page (if needed) without using GFP_NOFS.
> + */
> +retry_grab:
> + page = grab_cache_page_write_begin(mapping, index, flags);
> + if (!page)
> + return -ENOMEM;
> + unlock_page(page);
> +
> +retry_journal:
> handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, needed_blocks);
> if (IS_ERR(handle)) {
> - ret = PTR_ERR(handle);
> - goto out;
> + page_cache_release(page);
> + return PTR_ERR(handle);
> }
>
> - /* 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);
> - if (!page) {
> + lock_page(page);
> + if (page->mapping != mapping) {
> + /* The page got truncated from under us */
> + unlock_page(page);
> + page_cache_release(page);
> ext4_journal_stop(handle);
> - ret = -ENOMEM;
> - goto out;
> + goto retry_grab;
> }
> -
> - *pagep = page;
> + wait_on_page_writeback(page);
>
> if (ext4_should_dioread_nolock(inode))
> ret = __block_write_begin(page, pos, len, ext4_get_block_write);
> @@ -946,7 +954,6 @@ retry:
>
> if (ret) {
> unlock_page(page);
> - page_cache_release(page);
> /*
> * __block_write_begin may have instantiated a few blocks
> * outside i_size. Trim these off again. Don't need
> @@ -970,11 +977,14 @@ retry:
> if (inode->i_nlink)
> ext4_orphan_del(NULL, inode);
> }
> - }
>
> - if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> - goto retry;
> -out:
> + if (ret == -ENOSPC &&
> + ext4_should_retry_alloc(inode->i_sb, &retries))
> + goto retry_journal;
> + page_cache_release(page);
> + return ret;
> + }
> + *pagep = page;
> return ret;
> }
>
> @@ -2524,42 +2534,52 @@ static int ext4_da_write_begin(struct file *file, struct address_space *mapping,
> pos, len, flags,
> pagep, fsdata);
> if (ret < 0)
> - goto out;
> - if (ret == 1) {
> - ret = 0;
> - goto out;
> - }
> + return ret;
> + if (ret == 1)
> + return 0;
> }
>
> -retry:
> + /*
> + * grab_cache_page_write_begin() can take a long time if the
> + * system is thrashing due to memory pressure, or if the page
> + * is being written back. So grab it first before we start
> + * the transaction handle. This also allows us to allocate
> + * the page (if needed) without using GFP_NOFS.
> + */
> +retry_grab:
> + page = grab_cache_page_write_begin(mapping, index, flags);
> + if (!page)
> + return -ENOMEM;
> + unlock_page(page);
> +
> /*
> * With delayed allocation, we don't log the i_disksize update
> * if there is delayed block allocation. But we still need
> * to journalling the i_disksize update if writes to the end
> * of file which has an already mapped buffer.
> */
> +retry_journal:
> handle = ext4_journal_start(inode, EXT4_HT_WRITE_PAGE, 1);
> if (IS_ERR(handle)) {
> - ret = PTR_ERR(handle);
> - goto out;
> + page_cache_release(page);
> + return PTR_ERR(handle);
> }
> - /* 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);
> - if (!page) {
> + lock_page(page);
> + if (page->mapping != mapping) {
> + /* The page got truncated from under us */
> + unlock_page(page);
> + page_cache_release(page);
> ext4_journal_stop(handle);
> - ret = -ENOMEM;
> - goto out;
> + goto retry_grab;
> }
> - *pagep = page;
> + /* In case writeback began while the page was unlocked */
> + wait_on_page_writeback(page);
>
> ret = __block_write_begin(page, pos, len, ext4_da_get_block_prep);
> if (ret < 0) {
> unlock_page(page);
> ext4_journal_stop(handle);
> - page_cache_release(page);
> /*
> * block_write_begin may have instantiated a few blocks
> * outside i_size. Trim these off again. Don't need
> @@ -2567,11 +2587,16 @@ retry:
> */
> if (pos + len > inode->i_size)
> ext4_truncate_failed_write(inode);
> +
> + if (ret == -ENOSPC &&
> + ext4_should_retry_alloc(inode->i_sb, &retries))
> + goto retry_journal;
> +
> + page_cache_release(page);
> + return ret;
> }
>
> - if (ret == -ENOSPC && ext4_should_retry_alloc(inode->i_sb, &retries))
> - goto retry;
> -out:
> + *pagep = page;
> return ret;
> }
>
> --
> 1.7.12.rc0.22.gcdd159b
>
> --
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--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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