What am I doing wrong? submit_bio() suddenly stops working...

What am I doing wrong? submit_bio() suddenly stops working...

[Theodore Ts'o]

Hey Jens,

I've been trying to figure out what I'm doing wrong.  I've been trying
to convert the data writeback bath to use the bio layer.  It mostly
works --- until all of sudden all calls to block_bio_queue(), either via
submit_bh() or via submit_bio(), start turning into no-ops.

I'm sure I'm doing something wrong, but the bio layer isn't terribly
well documented, so I'm not sure what it might be.  The patch which
causes the problem can be found be found here:

http://userweb.kernel.org/~tytso/ext4-bio-patches/0006-Ext4-Use-bio-layer-instead-of-buffer-layer-in-mpage_.patch

Here is an except from an ftrace I've been taking to get to the bottom
of it.  It's a combination of some trace_printk's, blktrace, and the
block_bio_queue tracepoint.   The full log can be found at:

http://userweb.kernel.org/~tytso/ext4-bio-patches/kvm-console

It shows all of the blktrace events that shows up after block_bio_queue
tracepoint, but at some point, after jbd2 or ext4 calls submit_bh() or
submit_bio(), after the block_bio_queue tracepoint, we stop seeing the
blktrace events, and it looks like the block I/O layer stops answering
the phone.  No complaints in dmesg, no BUG_ON's, no errors....

If I back out the ext4 bio patches, things work correctly, and as I
said, I'm pretty sure the bug is in my code.  But the failure is
happening deep in the block I/O stack, and I can't figure out why it's
failing.

I'm hoping this rings a bell, and perhaps we should consider some of the
debugging trace_printk's as possible new tracepoints?

Any help you could give me would be greatly appreciated.  Ideally, you
or someone can tell me what stupid thing I'm doing.  :-)

Thanks,

					- Ted

[  215.663499] jbd2/sdb-2126    0..... 6976406us : submit_bh: <5>dev sdb blk 131072, ret_pc c0211ecb (__sync_dirty_buffer) pid 2126
[  215.663499] jbd2/sdb-2126    0d.... 6976415us :   8,16   Q  WS 1048576 + 8 [jbd2/sdb-8]
[  215.663499] jbd2/sdb-2126    0..... 6976419us : block_bio_queue: 8,16 WS 1048576 + 8 [jbd2/sdb-8]
[  215.663499] jbd2/sdb-2126    0d.... 6976439us :   8,16   m   N cfq2126 alloced
[  215.663499] jbd2/sdb-2126    0d.... 6976440us :   8,16   G  WS 1048576 + 8 [jbd2/sdb-8]
[  215.663499] jbd2/sdb-2126    0d.... 6976445us :   8,16   I   W 1048576 + 8 [jbd2/sdb-8]
[  215.663499] jbd2/sdb-2126    0d.... 6976446us :   8,16   m   N cfq2126 insert_request
[  215.663499] jbd2/sdb-2126    0d.... 6976448us :   8,16   m   N cfq2126 add_to_rr
[  215.663499] jbd2/sdb-2126    0d.... 6976451us :   8,16   m   N cfq2126 preempt
[  215.663499] jbd2/sdb-2126    0d.... 6976452us :   8,16   m   N cfq2993 slice expired t=1
[  215.663499] jbd2/sdb-2126    0d.... 6976453us :   8,16   m   N cfq2993 resid=-331
[  215.663499] jbd2/sdb-2126    0d.... 6976456us :   8,16   m   N cfq2993 sl_used=12 disp=2 charge=12 iops=0 sect=296
[  215.663499] jbd2/sdb-2126    0d.... 6976788us :   8,16   m   N cfq workload slice:45
[  215.663499] jbd2/sdb-2126    0d.... 6976791us :   8,16   m   N cfq2126 set_active wl_prio:0 wl_type:2
[  215.663499] jbd2/sdb-2126    0..... 6976797us : __wait_on_buffer: <5>dev sdb blk 131072, ret_pc c0210e83 (wait_on_buffer) pid 2126
[  215.663499] jbd2/sdb-2126    0d.... 6976801us :   8,16   U   N [jbd2/sdb-8] 10
[  215.663499] jbd2/sda-1439    0..... 10966709us : submit_bh: <5>dev sda blk 128401, ret_pc c02128fb (__block_write_full_page) pid 1439
[  215.663499] jbd2/sda-1439    0..... 10966719us : block_bio_queue: 8,0 WS 1027208 + 8 [jbd2/sda-8]
[  215.663499] jbd2/sda-1439    0..... 10966750us : submit_bh: <5>dev sda blk 128402, ret_pc c02128fb (__block_write_full_page) pid 1439
...

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-21 04:00, Theodore Ts'o wrote:
> Hey Jens,
> 
> I've been trying to figure out what I'm doing wrong.  I've been trying
> to convert the data writeback bath to use the bio layer.  It mostly
> works --- until all of sudden all calls to block_bio_queue(), either via
> submit_bh() or via submit_bio(), start turning into no-ops.
> 
> I'm sure I'm doing something wrong, but the bio layer isn't terribly
> well documented, so I'm not sure what it might be.  The patch which
> causes the problem can be found be found here:
> 
> http://userweb.kernel.org/~tytso/ext4-bio-patches/0006-Ext4-Use-bio-layer-instead-of-buffer-layer-in-mpage_.patch
> 
> Here is an except from an ftrace I've been taking to get to the bottom
> of it.  It's a combination of some trace_printk's, blktrace, and the
> block_bio_queue tracepoint.   The full log can be found at:
> 
> http://userweb.kernel.org/~tytso/ext4-bio-patches/kvm-console
> 
> It shows all of the blktrace events that shows up after block_bio_queue
> tracepoint, but at some point, after jbd2 or ext4 calls submit_bh() or
> submit_bio(), after the block_bio_queue tracepoint, we stop seeing the
> blktrace events, and it looks like the block I/O layer stops answering
> the phone.  No complaints in dmesg, no BUG_ON's, no errors....
> 
> If I back out the ext4 bio patches, things work correctly, and as I
> said, I'm pretty sure the bug is in my code.  But the failure is
> happening deep in the block I/O stack, and I can't figure out why it's
> failing.
> 
> I'm hoping this rings a bell, and perhaps we should consider some of the
> debugging trace_printk's as possible new tracepoints?
> 
> Any help you could give me would be greatly appreciated.  Ideally, you
> or someone can tell me what stupid thing I'm doing.  :-)

I don't see anything immediately wrong with your approach. I suspect
we'll need to see sysrq-t traces of the relevant processes to make a
more educated guess!

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Ted Ts'o]

On Thu, Oct 21, 2010 at 08:59:46AM +0200, Jens Axboe wrote:
> 
> I don't see anything immediately wrong with your approach. I suspect
> we'll need to see sysrq-t traces of the relevant processes to make a
> more educated guess!

I've uploaded a trace output that includes the sysrq-t trace, but I
don't think it shows anything interesting.  We're not hanging on any
kind of loack as near as I can tell.  It looks like
__generic_make_request() is calling q->make_request_fn(), and this is
returning without actually doing anything.

http://userweb.kernel.org/~tytso/ext4-bio-patches/kvm-console-2

In this trace, I added a patch to prove that __generic_make_request()
is calling __make_request (I wasn't sure what q->make_request_fn was
indirecting to, so I added a brute force lookup to make sure I
understood what was going on), but at one point, it just starts
queuing the request, and it enters cfq, but the request never gets
dispatched out.  Maybe this is a failure of the plugging/unplugging
mechanisms?

I guess I can start putting in more brute-force printk's inside
__make_request and inside the cfq scheduler to try to understand what
is going on, but I'm really guessing at this point.

If you have any suggestions about more elegant ways of figuring what
is happening, please do let me know....

						- Ted

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Boaz Harrosh]

On 10/21/2010 06:55 PM, Ted Ts'o wrote:
> From be341f01064389dd7d82ed0379765656a816aa8a Mon Sep 17 00:00:00 2001
> From: Theodore Ts'o <tytso@mit.edu>
> Date: Wed, 20 Oct 2010 21:27:13 -0400
> Subject: [PATCH 6/6] Ext4: Use bio layer instead of buffer layer in mpage_da_submit_io
> 
> Rough draft; not done yet
> ---
>  fs/buffer.c           |   36 +++++
>  fs/ext4/Makefile      |    2 +-
>  fs/ext4/ext4.h        |   33 ++++-
>  fs/ext4/extents.c     |    4 +-
>  fs/ext4/inode.c       |  118 ++-------------
>  fs/ext4/mballoc.c     |    8 +-
>  fs/ext4/page-io.c     |  390 +++++++++++++++++++++++++++++++++++++++++++++++++
>  fs/ext4/super.c       |    8 +-
>  fs/jbd2/commit.c      |   17 ++
>  fs/jbd2/transaction.c |    5 +
>  mm/filemap.c          |    7 +
>  11 files changed, 518 insertions(+), 110 deletions(-)
>  create mode 100644 fs/ext4/page-io.c
> 
> diff --git a/fs/buffer.c b/fs/buffer.c
> index 3e7dca2..8789975 100644
> --- a/fs/buffer.c
> +++ b/fs/buffer.c
<snip>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index f379ceb..2ceb1e8 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -2016,8 +2016,10 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  	struct buffer_head *bh, *page_bufs = NULL;
>  	int journal_data = ext4_should_journal_data(inode);
>  	sector_t pblock, cur_logical;
> +	struct ext4_io_submit io_submit;
>  
>  	BUG_ON(mpd->next_page <= mpd->first_page);
> +	memset(&io_submit, 0, sizeof(io_submit));
>  	/*
>  	 * We need to start from the first_page to the next_page - 1
>  	 * to make sure we also write the mapped dirty buffer_heads.
> @@ -2109,16 +2111,16 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  				/* mark the buffer_heads as dirty & uptodate */
>  				block_commit_write(page, 0, len);
>  
> -			if (journal_data && PageChecked(page))
> +			/*
> +			 * Delalloc doesn't support data journalling,
> +			 * but eventually maybe we'll lift this
> +			 * restriction.
> +			 */
> +			if (unlikely(journal_data && PageChecked(page)))
>  				err = __ext4_journalled_writepage(page, len);
> -			else if (buffer_uninit(page_bufs)) {
> -				ext4_set_bh_endio(page_bufs, inode);
> -				err = block_write_full_page_endio(page,
> -					noalloc_get_block_write,
> -					mpd->wbc, ext4_end_io_buffer_write);
> -			} else
> -				err = block_write_full_page(page,
> -					    noalloc_get_block_write, mpd->wbc);
> +			else
> +				err = ext4_bio_write_page(&io_submit, page,
> +							  len, mpd->wbc);
>  
>  			if (!err)
>  				mpd->pages_written++;
> @@ -2131,6 +2133,7 @@ static int mpage_da_submit_io(struct mpage_da_data *mpd,
>  		}
>  		pagevec_release(&pvec);
>  	}
> +	ext4_io_submit(&io_submit);
>  	return ret;
>  }
>  
<snip>
> diff --git a/fs/ext4/page-io.c b/fs/ext4/page-io.c
> new file mode 100644
> index 0000000..08a6b2a
> --- /dev/null
> +++ b/fs/ext4/page-io.c
> @@ -0,0 +1,390 @@
> +/*
> + * linux/fs/ext4/page-io.c
> + *
> + * This contains the new page_io functions for ext4
> + *
> + * Written by Theodore Ts'o, 2010.
> + */
> +
> +#include <linux/module.h>
> +#include <linux/fs.h>
> +#include <linux/time.h>
> +#include <linux/jbd2.h>
> +#include <linux/highuid.h>
> +#include <linux/pagemap.h>
> +#include <linux/quotaops.h>
> +#include <linux/string.h>
> +#include <linux/buffer_head.h>
> +#include <linux/writeback.h>
> +#include <linux/pagevec.h>
> +#include <linux/mpage.h>
> +#include <linux/namei.h>
> +#include <linux/uio.h>
> +#include <linux/bio.h>
> +#include <linux/workqueue.h>
> +#include <linux/kernel.h>
> +#include <linux/slab.h>
> +
> +#include "ext4_jbd2.h"
> +#include "xattr.h"
> +#include "acl.h"
> +#include "ext4_extents.h"
> +
> +#define PDEBUG
> +
> +static struct kmem_cache *io_page_cachep;
> +
> +int __init init_ext4_pageio(void)
> +{
> +	io_page_cachep =
> +		kmem_cache_create("ext4_io_page",
> +				     sizeof(struct ext4_io_page),
> +				     0, SLAB_RECLAIM_ACCOUNT, NULL);
> +	if (io_page_cachep == NULL)
> +		return -ENOMEM;
> +
> +	return 0;
> +}
> +
> +void exit_ext4_pageio(void)
> +{
> +	kmem_cache_destroy(io_page_cachep);
> +}
> +
> +void ext4_free_io_end(ext4_io_end_t *io)
> +{
> +	struct ext4_io_page *io_page, *next;
> +
> +	BUG_ON(!io);
> +	if (io->page)
> +		put_page(io->page);
> +	iput(io->inode);
> +	for (io_page = io->io_page; io_page; io_page = next) {
> +		put_page(io_page->p_page);
> +		next = io_page->p_next;
> +		kmem_cache_free(io_page_cachep, io_page);
> +	}
> +	kfree(io);
> +}
> +
> +/*
> + * check a range of space and convert unwritten extents to written.
> + */
> +int ext4_end_io_nolock(ext4_io_end_t *io)
> +{
> +	struct inode *inode = io->inode;
> +	loff_t offset = io->offset;
> +	ssize_t size = io->size;
> +	int ret = 0;
> +
> +	ext4_debug("ext4_end_io_nolock: io 0x%p from inode %lu,list->next 0x%p,"
> +		   "list->prev 0x%p\n",
> +	           io, inode->i_ino, io->list.next, io->list.prev);
> +
> +	if (list_empty(&io->list))
> +		return ret;
> +
> +	if (!(io->flag & EXT4_IO_END_UNWRITTEN))
> +		return ret;
> +
> +	ret = ext4_convert_unwritten_extents(inode, offset, size);
> +	if (ret < 0) {
> +		printk(KERN_EMERG "%s: failed to convert unwritten"
> +			"extents to written extents, error is %d"
> +			" io is still on inode %lu aio dio list\n",
> +                       __func__, ret, inode->i_ino);
> +		return ret;
> +	}
> +
> +	if (io->iocb)
> +		aio_complete(io->iocb, io->result, 0);
> +	/* clear the DIO AIO unwritten flag */
> +	io->flag &= ~EXT4_IO_END_UNWRITTEN;
> +	return ret;
> +}
> +
> +/*
> + * work on completed aio dio IO, to convert unwritten extents to extents
> + */
> +static void ext4_end_io_work(struct work_struct *work)
> +{
> +	ext4_io_end_t		*io = container_of(work, ext4_io_end_t, work);
> +	struct inode		*inode = io->inode;
> +	struct ext4_inode_info	*ei = EXT4_I(inode);
> +	unsigned long		flags;
> +	int			ret;
> +
> +	mutex_lock(&inode->i_mutex);
> +	ret = ext4_end_io_nolock(io);
> +	if (ret < 0) {
> +		mutex_unlock(&inode->i_mutex);
> +		return;
> +	}
> +
> +	spin_lock_irqsave(&ei->i_completed_io_lock, flags);
> +	if (!list_empty(&io->list))
> +		list_del_init(&io->list);
> +	spin_unlock_irqrestore(&ei->i_completed_io_lock, flags);
> +	mutex_unlock(&inode->i_mutex);
> +	ext4_free_io_end(io);
> +}
> +
> +ext4_io_end_t *ext4_init_io_end(struct inode *inode, gfp_t flags)
> +{
> +	ext4_io_end_t *io = NULL;
> +
> +	io = kmalloc(sizeof(*io), flags);
> +
> +	if (io) {
> +		memset(io, 0, sizeof(*io));
> +		igrab(inode);
> +		io->inode = inode;
> +		INIT_WORK(&io->work, ext4_end_io_work);
> +		INIT_LIST_HEAD(&io->list);
> +	}
> +
> +	return io;
> +}
> +
> +static void ext4_end_bio(struct bio *bio, int error)
> +{
> +	ext4_io_end_t *io_end = bio->bi_private;
> +	struct workqueue_struct *wq;
> +	struct inode *inode;
> +	unsigned long flags;
> +	struct ext4_io_page *io_page;
> +
> +	inode = io_end->inode;
> +#ifdef PDEBUG
> +	trace_printk("%s: enter: ino %lu io_end=%p", inode->i_sb->s_id,
> +		     inode->i_ino, io_end);
> +#endif
> +	bio->bi_private = NULL;
> +	bio->bi_end_io = NULL;
> +	bio_put(bio);
> +
> +	if (!io_end)
> +		return;
> +
> +	if (!(inode->i_sb->s_flags & MS_ACTIVE)) {
> +		printk("sb umounted, discard end_io request for inode %lu\n",
> +			io_end->inode->i_ino);
> +		ext4_free_io_end(io_end);
> +		return;
> +	}
> +
> +	if (test_bit(BIO_UPTODATE, &bio->bi_flags))
> +		error = 0;
> +	if (error)
> +		io_end->flag |= EXT4_IO_END_ERROR;
> +
> +	for (io_page = io_end->io_page; io_page; io_page = io_page->p_next) {
> +		struct page *page = io_page->p_page;
> +		struct buffer_head *bh, *head = page_buffers(io_page->p_page);
> +
> +		if (error)
> +			SetPageError(page);
> +		BUG_ON(!head);
> +		if (head->b_size == PAGE_CACHE_SIZE)
> +			clear_buffer_dirty(head);
> +		else {
> +			/*
> +			 * If block_size < page_size we need to worry
> +			 * about partial writes.
> +			 */
> +			loff_t offset;
> +			loff_t io_end_offset = io_end->offset + io_end->size;
> +			int partial = 0;
> +
> +			offset = (sector_t) page->index << PAGE_CACHE_SHIFT;
> +			bh = head;
> +			do {
> +				if ((offset >= io_end->offset) &&
> +				    (offset+bh->b_size <= io_end_offset))
> +					clear_buffer_dirty(bh);
> +				if (buffer_dirty(bh))
> +					partial = 1;
> +				offset += bh->b_size;
> +				bh = bh->b_this_page;
> +			} while (bh != head);
> +			if (partial)
> +				continue;
> +		}
> +#ifdef PDEBUG
> +		trace_printk("%s: end_page_writeback for %lu:%lu\n",
> +			     inode->i_sb->s_id, inode->i_ino,
> +			     (unsigned long) page->index);
> +#endif
> +		end_page_writeback(page);
> +	}
> +
> +	/* Add the io_end to per-inode completed io list*/
> +	spin_lock_irqsave(&EXT4_I(inode)->i_completed_io_lock, flags);
> +	list_add_tail(&io_end->list, &EXT4_I(inode)->i_completed_io_list);
> +	spin_unlock_irqrestore(&EXT4_I(inode)->i_completed_io_lock, flags);
> +
> +	wq = EXT4_SB(inode->i_sb)->dio_unwritten_wq;
> +	/* queue the work to convert unwritten extents to written */
> +	queue_work(wq, &io_end->work);
> +#ifdef PDEBUG
> +	trace_printk("%s: exit: ino %lu", inode->i_sb->s_id,
> +		     io_end->inode->i_ino);
> +#endif
> +}
> +
> +void ext4_io_submit(struct ext4_io_submit *io)
> +{
> +	if (!io->io_bio)
> +		return;
> +#ifdef PDEBUG
> +	trace_printk("%s: io submitted io_end %p\n", 
> +		     io->io_end->inode->i_sb->s_id, io->io_end);
> +#endif
> +	submit_bio(io->io_op, io->io_bio);
> +	ASSERT(!bio_flagged(bio, BIO_EOPNOTSUPP));
> +	bio_put(io->io_bio);

The extra get/put is only done for the duration of the ASSERT above, right?
I'd put a comment. And why don't you just call the _get here just before
submit_bio instead of down at io_submit_init.

> +	io->io_bio = 0;
> +	io->io_op = 0;
> +	io->io_end = 0;
> +}
> +
> +static void io_submit_init(struct ext4_io_submit *io,
> +			   struct inode *inode,
> +			   struct writeback_control *wbc,
> +			   struct buffer_head *bh)
> +{
> +	ext4_io_end_t *io_end;
> +	struct page *page = bh->b_page;
> +	int nvecs = bio_get_nr_vecs(bh->b_bdev);
> +	struct bio *bio;
> +
> +	do {
> +		bio = bio_alloc(GFP_NOIO, nvecs);
> +		nvecs >>= 1;
> +	} while (bio == NULL);

This is surly bad. bio_alloc must be allowed to fail
(Specially with GFP_NOIO). You should only loop down to
1 and then prepare to return -ENOMEM from this function
and handle it properly in callers. (Or schedule and wait
like below)

> +	bio->bi_sector = bh->b_blocknr * (bh->b_size >> 9);
> +	bio->bi_bdev = bh->b_bdev;
> +	bio_get(bio);
> +
> +retry_alloc:
> +	io_end = ext4_init_io_end(inode, GFP_NOFS);

Or you could move the bio allocation to ext4_init_io_end()

> +	if (!io_end) {
> +		if (printk_ratelimit())
> +			ext4_msg(inode->i_sb, KERN_WARNING,
> +				 "%s: allocation fail\n", __func__);
> +		schedule();
> +		goto retry_alloc;
> +	}
> +	io_end->inode = inode;
> +	io_end->offset = (sector_t)page->index << PAGE_CACHE_SHIFT;
> +	bio->bi_private = io->io_end = io_end;
> +	bio->bi_end_io = ext4_end_bio;
> +
> +	io->io_bio = bio;
> +	io->io_op = (wbc->sync_mode == WB_SYNC_ALL ?
> +			WRITE_SYNC_PLUG : WRITE);
> +	io->io_next_block = bh->b_blocknr;
> +#ifdef PDEBUG
> +	trace_printk("%s: io_submit_init for ino %lu, nvecs = %d",
> +		     inode->i_sb->s_id, inode->i_ino, nvecs);
> +#endif
> +}
> +
> +static noinline_for_stack 
> +void io_submit_add_bh(struct ext4_io_submit *io,
> +		      struct inode *inode,
> +		      struct writeback_control *wbc,
> +		      struct buffer_head *bh)
> +{
> +	struct ext4_io_page *new_io_page;
> +	int ret;
> +
> +#ifdef PDEBUG
> +	trace_printk("%s enter: ino %lu blk %lu", inode->i_sb->s_id,
> +		     inode->i_ino, (unsigned long) bh->b_blocknr);
> +#endif
> +	if (buffer_new(bh)) {
> +		clear_buffer_new(bh);
> +		unmap_underlying_metadata(bh->b_bdev, bh->b_blocknr);
> +	}
> +
> +retry:
> +	if (io->io_bio && bh->b_blocknr != io->io_next_block)
> +		ext4_io_submit(io);
> +	if (io->io_bio == NULL)
> +		io_submit_init(io, inode, wbc, bh);
> +	if (buffer_uninit(bh))
> +		io->io_end->flag |= EXT4_IO_END_UNWRITTEN;
> +	io->io_end->size += bh->b_size;
> +	io->io_next_block++;
> +	ret = bio_add_page(io->io_bio, bh->b_page, bh->b_size, bh_offset(bh));
> +	if (ret != bh->b_size) {
> +	submit_and_retry:
> +#ifdef PDEBUG
> +		trace_printk("%s: submit and retry (ret = %d, size=%d, "
> +			     "offset=%lu)\n", inode->i_sb->s_id, ret,
> +			     bh->b_size, bh_offset(bh));
> +#endif
> +		ext4_io_submit(io);
> +		goto retry;
> +	}
> +	if (!io->io_end->io_page || io->io_end->io_page->p_page != bh->b_page) {
> +		new_io_page = kmem_cache_alloc(io_page_cachep, GFP_NOFS);
> +		if (!new_io_page)
> +			goto submit_and_retry;
> +		get_page(bh->b_page);
> +		new_io_page->p_page = bh->b_page;
> +		new_io_page->p_next = io->io_end->io_page;
> +		io->io_end->io_page = new_io_page;
> +	}
> +#ifdef PDEBUG
> +	trace_printk("%s: exit: ino %lu", inode->i_sb->s_id, inode->i_ino);
> +#endif
> +}
> +
> +int ext4_bio_write_page(struct ext4_io_submit *io,
> +			struct page *page,
> +			int len,
> +			struct writeback_control *wbc)
> +{
> +	struct inode *inode = page->mapping->host;
> +	unsigned block_start, block_end;
> +	unsigned blocksize;
> +	struct buffer_head *bh, *head;
> +
> +#ifdef PDEBUG
> +	trace_printk("%s: enter: ino %lu page %lu len %d", inode->i_sb->s_id,
> +		     inode->i_ino, page->index, len);
> +#endif
> +	blocksize = 1 << inode->i_blkbits;
> +
> +	BUG_ON(PageWriteback(page));
> +	set_page_writeback(page);
> +	ClearPageError(page);
> +
> +	for (bh = head = page_buffers(page), block_start = 0;
> +	     bh != head || !block_start;
> +	     block_start=block_end, bh = bh->b_this_page) {
> +		block_end = block_start + blocksize;
> +		if (block_start >= len) {
> +			clear_buffer_dirty(bh);
> +			set_buffer_uptodate(bh);
> +			continue;
> +		}
> +		io_submit_add_bh(io, inode, wbc, bh);
> +	}
> +	unlock_page(page);
> +	/*
> +	 * If the page was truncated before we could do the writeback,
> +	 * we won't have submitted any pages for I/O.  In that case we
> +	 * need to make sure we've cleared the PageWriteback bit from
> +	 * the page to prevent the system from wedging later on.
> +	 */
> +	if (len == 0)
> +		end_page_writeback(page);
> +#ifdef PDEBUG
> +	trace_printk("%s: exit: for ino %lu", inode->i_sb->s_id,
> +		     inode->i_ino);
> +#endif
> +	return 0;
> +}
> diff --git a/fs/ext4/super.c b/fs/ext4/super.c
> index 16002ec..9f602c2 100644
> --- a/fs/ext4/super.c
> +++ b/fs/ext4/super.c
> @@ -4768,9 +4768,12 @@ static int __init init_ext4_fs(void)
>  	int err;
>  
>  	ext4_check_flag_values();
> -	err = init_ext4_system_zone();
> +	err = init_ext4_pageio();
>  	if (err)
>  		return err;
> +	err = init_ext4_system_zone();
> +	if (err)
> +		goto out5;
>  	ext4_kset = kset_create_and_add("ext4", NULL, fs_kobj);
>  	if (!ext4_kset)
>  		goto out4;
> @@ -4811,6 +4814,8 @@ out3:
>  	kset_unregister(ext4_kset);
>  out4:
>  	exit_ext4_system_zone();
> +out5:
> +	exit_ext4_pageio();
>  	return err;
>  }
>  
> @@ -4826,6 +4831,7 @@ static void __exit exit_ext4_fs(void)
>  	remove_proc_entry("fs/ext4", NULL);
>  	kset_unregister(ext4_kset);
>  	exit_ext4_system_zone();
> +	exit_ext4_pageio();
>  }
>  
>  MODULE_AUTHOR("Remy Card, Stephen Tweedie, Andrew Morton, Andreas Dilger, Theodore Ts'o and others");
> diff --git a/fs/jbd2/commit.c b/fs/jbd2/commit.c
> index 6494c81..256008b 100644
> --- a/fs/jbd2/commit.c
> +++ b/fs/jbd2/commit.c
> @@ -631,6 +631,9 @@ void jbd2_journal_commit_transaction(journal_t *journal)
>  		 * (which is of type BJ_IO)
>  		 */
>  		JBUFFER_TRACE(jh, "ph3: write metadata");
> +#if 1				/* PDEBUG */
> +		trace_printk("@635 %s block %llu\n", journal->j_devname, blocknr);
> +#endif
>  		flags = jbd2_journal_write_metadata_buffer(commit_transaction,
>  						      jh, &new_jh, blocknr);
>  		if (flags < 0) {
> @@ -693,6 +696,11 @@ start_journal_io:
>  				clear_buffer_dirty(bh);
>  				set_buffer_uptodate(bh);
>  				bh->b_end_io = journal_end_buffer_io_sync;
> +#if 1				/* PDEBUG */
> +				trace_printk("@700 %s block %llu\n",
> +					     journal->j_devname,
> +					     bh->b_blocknr);
> +#endif
>  				submit_bh(write_op, bh);
>  			}
>  			cond_resched();
> @@ -762,6 +770,10 @@ wait_for_iobuf:
>  		jh = commit_transaction->t_iobuf_list->b_tprev;
>  		bh = jh2bh(jh);
>  		if (buffer_locked(bh)) {
> +#if 1				/* PDEBUG */
> +			trace_printk("jbd wait_on_buffer@765: %lu\n",
> +				     (unsigned long) bh->b_blocknr);
> +#endif
>  			wait_on_buffer(bh);
>  			goto wait_for_iobuf;
>  		}
> @@ -818,6 +830,11 @@ wait_for_iobuf:
>  		jh = commit_transaction->t_log_list->b_tprev;
>  		bh = jh2bh(jh);
>  		if (buffer_locked(bh)) {
> +#if 1				/* PDEBUG */
> +			trace_printk("%s: jbd wait_on_buffer@823: %lu\n",
> +				     journal->j_devname,
> +				     (unsigned long) bh->b_blocknr);
> +#endif
>  			wait_on_buffer(bh);
>  			goto wait_for_ctlbuf;
>  		}
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index 6bf0a24..8873caa 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -701,6 +701,11 @@ repeat:
>  			for ( ; ; ) {
>  				prepare_to_wait(wqh, &wait.wait,
>  						TASK_UNINTERRUPTIBLE);
> +#if 1				/* PDEBUG */
> +				trace_printk("%s: BJ shadow waiting on %lu\n",
> +				       journal->j_devname,
> +				       (unsigned long) bh->b_blocknr);
> +#endif
>  				if (jh->b_jlist != BJ_Shadow)
>  					break;
>  				schedule();
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 3d4df44..dfdbcb1 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -295,6 +295,13 @@ int filemap_fdatawait_range(struct address_space *mapping, loff_t start_byte,
>  			if (page->index > end)
>  				continue;
>  
> +#if 1				/* PDEBUG */
> +			if (PageWriteback(page))
> +				trace_printk(KERN_NOTICE "pid %d waiting on %lu:%lu\n", 
> +					     task_pid_nr(current),
> +					     mapping->host->i_ino,
> +					     (unsigned long) page->index);
> +#endif
>  			wait_on_page_writeback(page);
>  			if (PageError(page))
>  				ret = -EIO;
> -- 
> 1.7.1
> 

Other then that I can't see anything obvious
Boaz

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-21 19:41, Boaz Harrosh wrote:
>> +void ext4_io_submit(struct ext4_io_submit *io)
>> +{
>> +	if (!io->io_bio)
>> +		return;
>> +#ifdef PDEBUG
>> +	trace_printk("%s: io submitted io_end %p\n", 
>> +		     io->io_end->inode->i_sb->s_id, io->io_end);
>> +#endif
>> +	submit_bio(io->io_op, io->io_bio);
>> +	ASSERT(!bio_flagged(bio, BIO_EOPNOTSUPP));
>> +	bio_put(io->io_bio);
> 
> The extra get/put is only done for the duration of the ASSERT above, right?
> I'd put a comment. And why don't you just call the _get here just before
> submit_bio instead of down at io_submit_init.

Ted is using it to signal the reference from io:

>> +	io->io_bio = 0;

which makes sense (even if you don't need it, as long as you don't deref
bio after calling submit_bio()).

>> +	do {
>> +		bio = bio_alloc(GFP_NOIO, nvecs);
>> +		nvecs >>= 1;
>> +	} while (bio == NULL);
> 
> This is surly bad. bio_alloc must be allowed to fail
> (Specially with GFP_NOIO). You should only loop down to
> 1 and then prepare to return -ENOMEM from this function
> and handle it properly in callers. (Or schedule and wait
> like below)

Since __GFP_WAIT is set, it'll never return NULL. And as long as you
don't allocate more than 1 before doing you submit_bio(), it should be
OK in this case.

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Peter Zijlstra]

On Thu, 2010-10-21 at 20:07 +0200, Jens Axboe wrote:
> 
> >> +    do {
> >> +            bio = bio_alloc(GFP_NOIO, nvecs);
> >> +            nvecs >>= 1;
> >> +    } while (bio == NULL);
> > 
> > This is surly bad. bio_alloc must be allowed to fail
> > (Specially with GFP_NOIO). You should only loop down to
> > 1 and then prepare to return -ENOMEM from this function
> > and handle it properly in callers. (Or schedule and wait
> > like below)
> 
> Since __GFP_WAIT is set, it'll never return NULL. And as long as you
> don't allocate more than 1 before doing you submit_bio(), it should be
> OK in this case. 

__GFP_WAIT can return NULL, on OOM and when the size is over a magic
threshold.

I find it bad form to rely on any allocation not failing.

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-22 15:04, Peter Zijlstra wrote:
> On Thu, 2010-10-21 at 20:07 +0200, Jens Axboe wrote:
>>
>>>> +    do {
>>>> +            bio = bio_alloc(GFP_NOIO, nvecs);
>>>> +            nvecs >>= 1;
>>>> +    } while (bio == NULL);
>>>
>>> This is surly bad. bio_alloc must be allowed to fail
>>> (Specially with GFP_NOIO). You should only loop down to
>>> 1 and then prepare to return -ENOMEM from this function
>>> and handle it properly in callers. (Or schedule and wait
>>> like below)
>>
>> Since __GFP_WAIT is set, it'll never return NULL. And as long as you
>> don't allocate more than 1 before doing you submit_bio(), it should be
>> OK in this case. 
> 
> __GFP_WAIT can return NULL, on OOM and when the size is over a magic
> threshold.

The memory allocator, yes, but a mempool backed allocation with
__GFP_WAIT cannot. That's the difference.

> I find it bad form to rely on any allocation not failing.

On that we can agree, but it's not always easily doable. I bet you would
not like the block layer tossing IOs if it can't get a bio or request
structure :-)

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Ted Ts'o]

On Thu, Oct 21, 2010 at 07:41:54PM +0200, Boaz Harrosh wrote:
> > +#ifdef PDEBUG
> > +	trace_printk("%s: io submitted io_end %p\n", 
> > +		     io->io_end->inode->i_sb->s_id, io->io_end);
> > +#endif
> > +	submit_bio(io->io_op, io->io_bio);
> > +	ASSERT(!bio_flagged(bio, BIO_EOPNOTSUPP));
> > +	bio_put(io->io_bio);
> 
> The extra get/put is only done for the duration of the ASSERT above, right?
> I'd put a comment. And why don't you just call the _get here just before
> submit_bio instead of down at io_submit_init.

The bio layer isn't well documented, so a lot of this was written by
cargo cult programming.  In this case, I was following what XFS did
when I originally wrote it.  It's only in the past week when I've been
diving deep into the source and was able to conclude that yes, it's
only needed for the ASSERT.

One of the frustrating things about the bio layer is that all of the
different file systems use very different patterns about how they
access the bio layer --- which is perhaps caused by the lack of
documentation.  But I finally did notice that the JFS code dodn't do a
bio_get() after calling bio_alloc(), and it was only when I did a lot
of source code tracing that I realized that it strictly speaking the
ASSERT isn't really even all that necessary, since we're not calling
using a discard or barrier op, which is the only thing that could
possibly set the EOPNOTSUPP flag anyway....

> > +	do {
> > +		bio = bio_alloc(GFP_NOIO, nvecs);
> > +		nvecs >>= 1;
> > +	} while (bio == NULL);
> 
> This is surly bad. bio_alloc must be allowed to fail
> (Specially with GFP_NOIO). You should only loop down to
> 1 and then prepare to return -ENOMEM from this function
> and handle it properly in callers. (Or schedule and wait
> like below)

Copied exactly from the XFS code.  I was under the (possibly mistaken)
assumption that the XFS code was a good place to copy from, since
everyone sings the praises of XFS.....

I do agree that bio_alloc() should be allowed to fail, although life
gets really tough deep in the writeback stack to throw a failure all
the way back out.  This was on my todo list to fix, after I got things
basically working.

							- Ted

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-21 20:14, Ted Ts'o wrote:
> On Thu, Oct 21, 2010 at 07:41:54PM +0200, Boaz Harrosh wrote:
>>> +#ifdef PDEBUG
>>> +	trace_printk("%s: io submitted io_end %p\n", 
>>> +		     io->io_end->inode->i_sb->s_id, io->io_end);
>>> +#endif
>>> +	submit_bio(io->io_op, io->io_bio);
>>> +	ASSERT(!bio_flagged(bio, BIO_EOPNOTSUPP));
>>> +	bio_put(io->io_bio);
>>
>> The extra get/put is only done for the duration of the ASSERT above, right?
>> I'd put a comment. And why don't you just call the _get here just before
>> submit_bio instead of down at io_submit_init.
> 
> The bio layer isn't well documented, so a lot of this was written by
> cargo cult programming.  In this case, I was following what XFS did
> when I originally wrote it.  It's only in the past week when I've been
> diving deep into the source and was able to conclude that yes, it's
> only needed for the ASSERT.

Come on Ted, you are laying it on pretty thick now. It's ordinary
reference counting, and the use case you have here is exactly the one
that is described in the comment for bio_get(). You allocate the bio, it
has a single reference. The end_io function you define drops the
reference you had to this bio. Since IO can complete before submit_bio()
has returned, you need to grab an extra reference if you depend on
dereferencing the bio after submission.

> One of the frustrating things about the bio layer is that all of the
> different file systems use very different patterns about how they
> access the bio layer --- which is perhaps caused by the lack of
> documentation.  But I finally did notice that the JFS code dodn't do a
> bio_get() after calling bio_alloc(), and it was only when I did a lot
> of source code tracing that I realized that it strictly speaking the
> ASSERT isn't really even all that necessary, since we're not calling
> using a discard or barrier op, which is the only thing that could
> possibly set the EOPNOTSUPP flag anyway....
> 
>>> +	do {
>>> +		bio = bio_alloc(GFP_NOIO, nvecs);
>>> +		nvecs >>= 1;
>>> +	} while (bio == NULL);
>>
>> This is surly bad. bio_alloc must be allowed to fail
>> (Specially with GFP_NOIO). You should only loop down to
>> 1 and then prepare to return -ENOMEM from this function
>> and handle it properly in callers. (Or schedule and wait
>> like below)
> 
> Copied exactly from the XFS code.  I was under the (possibly mistaken)
> assumption that the XFS code was a good place to copy from, since
> everyone sings the praises of XFS.....

And it'll work.

> I do agree that bio_alloc() should be allowed to fail, although life
> gets really tough deep in the writeback stack to throw a failure all
> the way back out.  This was on my todo list to fix, after I got things
> basically working.

So don't use __GFP_WAIT, then it returns NULL.

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-21 18:55, Ted Ts'o wrote:
> On Thu, Oct 21, 2010 at 08:59:46AM +0200, Jens Axboe wrote:
>>
>> I don't see anything immediately wrong with your approach. I suspect
>> we'll need to see sysrq-t traces of the relevant processes to make a
>> more educated guess!
> 
> I've uploaded a trace output that includes the sysrq-t trace, but I
> don't think it shows anything interesting.  We're not hanging on any
> kind of loack as near as I can tell.  It looks like
> __generic_make_request() is calling q->make_request_fn(), and this is
> returning without actually doing anything.
> 
> http://userweb.kernel.org/~tytso/ext4-bio-patches/kvm-console-2
> 
> In this trace, I added a patch to prove that __generic_make_request()
> is calling __make_request (I wasn't sure what q->make_request_fn was
> indirecting to, so I added a brute force lookup to make sure I
> understood what was going on), but at one point, it just starts
> queuing the request, and it enters cfq, but the request never gets
> dispatched out.  Maybe this is a failure of the plugging/unplugging
> mechanisms?
> 
> I guess I can start putting in more brute-force printk's inside
> __make_request and inside the cfq scheduler to try to understand what
> is going on, but I'm really guessing at this point.
> 
> If you have any suggestions about more elegant ways of figuring what
> is happening, please do let me know....

I will take a look at the traces.

By the sound of things, if I were you I'd turn on the mem and slab
debugging to catch use-before-init and use-after-free. Mysterious hangs
in the IO sub system are usually caused by such bugs. And the regular
debugging aids, just to see if that produces anything of interest.

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Ted Ts'o]

On Thu, Oct 21, 2010 at 07:46:15PM +0200, Jens Axboe wrote:
> By the sound of things, if I were you I'd turn on the mem and slab
> debugging to catch use-before-init and use-after-free. Mysterious hangs
> in the IO sub system are usually caused by such bugs. And the regular
> debugging aids, just to see if that produces anything of interest.

I've tried turning on slub_debug=FZP, and that didn't turn up
anything.

Looking at the traces, it looks like the queue gets plugged near the
end, but then I don't see an unplug timer event.  I'm not sure what
could cuase that, though...

						- Ted

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Ted Ts'o]

On Thu, Oct 21, 2010 at 07:46:15PM +0200, Jens Axboe wrote:
> By the sound of things, if I were you I'd turn on the mem and slab
> debugging to catch use-before-init and use-after-free. Mysterious hangs
> in the IO sub system are usually caused by such bugs. And the regular
> debugging aids, just to see if that produces anything of interest.

It looks like it was a use-after-free bug in my code.  I'm running a
full set of set of tests now, but so far, it's gotten a lot further
than it went before, so I think I've figured it out.

I'm not sure why it caused the weird behaviour that it did (I got as
far as figuring out that somehow we lost the unplug timer, so after
the queue got plugged it never got unplugged), but I'm not going to
ask too many questions.  :-)

Maybe later on I'll try to figure out if there's any way to add some
kind of sanity checking so that screw ups in in the bio code's caller
cause a clearer failure (such as a BUG_ON), but that'll have to wait
for when I have some free time.

	    					- Ted

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Jens Axboe]

On 2010-10-22 05:34, Ted Ts'o wrote:
> On Thu, Oct 21, 2010 at 07:46:15PM +0200, Jens Axboe wrote:
>> By the sound of things, if I were you I'd turn on the mem and slab
>> debugging to catch use-before-init and use-after-free. Mysterious hangs
>> in the IO sub system are usually caused by such bugs. And the regular
>> debugging aids, just to see if that produces anything of interest.
> 
> It looks like it was a use-after-free bug in my code.  I'm running a
> full set of set of tests now, but so far, it's gotten a lot further
> than it went before, so I think I've figured it out.
> 
> I'm not sure why it caused the weird behaviour that it did (I got as
> far as figuring out that somehow we lost the unplug timer, so after
> the queue got plugged it never got unplugged), but I'm not going to
> ask too many questions.  :-)
> 
> Maybe later on I'll try to figure out if there's any way to add some
> kind of sanity checking so that screw ups in in the bio code's caller
> cause a clearer failure (such as a BUG_ON), but that'll have to wait
> for when I have some free time.

In my experience, use-after-free bugs are best caught using the
allocator poisoning along with lock checking. I guess that didn't
trigger for you?

Out of curiousity, what was the exact bug?

-- 
Jens Axboe

Re: What am I doing wrong? submit_bio() suddenly stops working...

[Ted Ts'o]

On Fri, Oct 22, 2010 at 09:19:37AM +0200, Jens Axboe wrote:
> 
> In my experience, use-after-free bugs are best caught using the
> allocator poisoning along with lock checking. I guess that didn't
> trigger for you?

No, I tried adding slab_debug=FP, but the structure got reused too
quickly.  And I've always had lock checking enabled.

> Out of curiousity, what was the exact bug?

To be honest, I don't know why it was causing the problem.  I had a 

       if (test_bit(BIO_UPTODATE, &bio->bi_flags))
               error = 0;

After the bio_put, which was clearly wrong, so I fixed it.  But
test_bit is a read-only instruction, and if error should have been
zero since I'm testing using kvm and there haven't been any errors.
(I need to test to make sure my error handling paths work correctly;
later).

But it's now working, and at this point, given that I have something
that works, although I'm curious what was causing the mysterious
failures, I may be willing to just let it be a mystery...

	    	   	      	   - Ted