Re: [PATCH 6/6] xfs: stop using the page cache to back the buffer cache

Alex Elder
2011-03-25T20:59:49Z

On Wed, 2011-03-23 at 17:14 +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Now that the buffer cache has it's own LRU, we do not need to use
> the page cache to provide persistent caching and reclaim
> infrastructure. Convert the buffer cache to use alloc_pages()
> instead of the page cache. This will remove all the overhead of page
> cache management from setup and teardown of the buffers, as well as
> needing to mark pages accessed as we find buffers in the buffer
> cache.
>
> By avoiding the page cache, we also remove the need to keep state in
> the page_private(page) field for persistant storage across buffer
> free/buffer rebuild and so all that code can be removed. This also
> fixes the long-standing problem of not having enough bits in the
> page_private field to track all the state needed for a 512
> sector/64k page setup.
>
> It also removes the need for page locking during reads as the pages
> are unique to the buffer and nobody else will be attempting to
> access them.
>
> Finally, it removes the buftarg address space lock as a point of
> global contention on workloads that allocate and free buffers
> quickly such as when creating or removing large numbers of inodes in
> parallel. This remove the 16TB limit on filesystem size on 32 bit
> machines as the page index (32 bit) is no longer used for lookups
> of metadata buffers - the buffer cache is now solely indexed by disk
> address which is stored in a 64 bit field in the buffer.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>

This is really a great change, a long time coming.

I have two comments below, one of which I think is
a real (but simple) problem.

I've been using this series all week without problems.
This patch cleans things up so nicely I *would* like
to include it in 2.6.39 if you can update it and turn
it around with a pull request for me.

If so, I'll do some sanity testing and push it to
oss.sgi.com, then send a pull request to Linus with
it early next week.

Reviewed-by: Alex Elder <aelder@sgi.com>

PS I'm sorry it took so long to get back to you on
this stuff. I've had a hard time getting my brain
re-engaged this week after coming back from vacation
for some reason...

> ---
> fs/xfs/linux-2.6/xfs_buf.c | 337 ++++++++++----------------------------------
> fs/xfs/linux-2.6/xfs_buf.h | 40 +-----
> 2 files changed, 81 insertions(+), 296 deletions(-)
>
> diff --git a/fs/xfs/linux-2.6/xfs_buf.c b/fs/xfs/linux-2.6/xfs_buf.c
> index fe51e09..19b0769 100644
> --- a/fs/xfs/linux-2.6/xfs_buf.c
> +++ b/fs/xfs/linux-2.6/xfs_buf.c

. . .

> @@ -719,7 +659,7 @@ xfs_buf_readahead(
> {
> struct backing_dev_info *bdi;
>
> - bdi = target->bt_mapping->backing_dev_info;
> + bdi = blk_get_backing_dev_info(target->bt_bdev);
> if (bdi_read_congested(bdi))
> return;

Why not just target->bt_bdi here? In which case, just skip
the local variable and call:

if (bdi_read_congested(target->bt_bdi))
return;

. . .

> @@ -1728,12 +1546,11 @@ xfs_alloc_buftarg(
> btp->bt_mount = mp;
> btp->bt_dev = bdev->bd_dev;
> btp->bt_bdev = bdev;
> + btp->bt_bdi = blk_get_backing_dev_info(bdev);

I think you need to check for a null return value here.

if (!btp->bt_bdi)
goto error;

> INIT_LIST_HEAD(&btp->bt_lru);
> spin_lock_init(&btp->bt_lru_lock);
> if (xfs_setsize_buftarg_early(btp, bdev))
> goto error;
> - if (xfs_mapping_buftarg(btp, bdev))
> - goto error;
> if (xfs_alloc_delwrite_queue(btp, fsname))
> goto error;
> btp->bt_shrinker.shrink = xfs_buftarg_shrink;

. . .