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Message-ID: <20130116020000.GA25806@quack.suse.cz>
Date: Wed, 16 Jan 2013 03:00:00 +0100
From: Jan Kara <jack@...e.cz>
To: "Darrick J. Wong" <darrick.wong@...cle.com>
Cc: axboe@...nel.dk, lucho@...kov.net, jack@...e.cz, ericvh@...il.com,
tytso@....edu, viro@...iv.linux.org.uk, rminnich@...dia.gov,
martin.petersen@...cle.com, neilb@...e.de, david@...morbit.com,
gnehzuil.liu@...il.com, linux-kernel@...r.kernel.org,
hch@...radead.org, linux-fsdevel@...r.kernel.org,
Andy Lutomirski <luto@...capital.net>,
adilger.kernel@...ger.ca, bharrosh@...asas.com, jlayton@...ba.org,
linux-ext4@...r.kernel.org, hirofumi@...l.parknet.co.jp
Subject: Re: [PATCH 4/6] block: Optionally snapshot page contents to
provide stable pages during write
On Mon 14-01-13 21:43:05, Darrick J. Wong wrote:
> This provides a band-aid to provide stable page writes on jbd without needing
> to backport the fixed locking and page writeback bit handling schemes of jbd2.
> The band-aid works by using bounce buffers to snapshot page contents instead of
> waiting.
>
> For those wondering about the ext3 bandage -- fixing the jbd locking (which was
> done as part of ext4dev years ago) is a lot of surgery, and setting
> PG_writeback on data pages when we actually hold the page lock dropped ext3
> performance by nearly an order of magnitude. If we're going to migrate iscsi
> and raid to use stable page writes, the complaints about high latency will
> likely return. We might as well centralize their page snapshotting thing to
> one place.
>
> Tested-by: Andy Lutomirski <luto@...capital.net>
> Signed-off-by: Darrick J. Wong <darrick.wong@...cle.com>
> ---
> arch/tile/Kconfig | 6 ------
> block/blk-core.c | 8 +++++---
> fs/ext3/super.c | 1 +
> include/uapi/linux/fs.h | 1 +
> mm/Kconfig | 13 +++++++++++++
> mm/bounce.c | 47 +++++++++++++++++++++++++++++++++++++++++++----
> mm/page-writeback.c | 4 ++++
> 7 files changed, 67 insertions(+), 13 deletions(-)
>
>
> diff --git a/arch/tile/Kconfig b/arch/tile/Kconfig
> index 875d008..c671fda 100644
> --- a/arch/tile/Kconfig
> +++ b/arch/tile/Kconfig
> @@ -410,12 +410,6 @@ config TILE_USB
> Provides USB host adapter support for the built-in EHCI and OHCI
> interfaces on TILE-Gx chips.
>
> -# USB OHCI needs the bounce pool since tilegx will often have more
> -# than 4GB of memory, but we don't currently use the IOTLB to present
> -# a 32-bit address to OHCI. So we need to use a bounce pool instead.
> -config NEED_BOUNCE_POOL
> - def_bool USB_OHCI_HCD
> -
> source "drivers/pci/hotplug/Kconfig"
>
> endmenu
> diff --git a/block/blk-core.c b/block/blk-core.c
> index c973249..277134c 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1474,6 +1474,11 @@ void blk_queue_bio(struct request_queue *q, struct bio *bio)
> */
> blk_queue_bounce(q, &bio);
>
> + if (bio_integrity_enabled(bio) && bio_integrity_prep(bio)) {
> + bio_endio(bio, -EIO);
> + return;
> + }
> +
> if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
> spin_lock_irq(q->queue_lock);
> where = ELEVATOR_INSERT_FLUSH;
> @@ -1714,9 +1719,6 @@ generic_make_request_checks(struct bio *bio)
> */
> blk_partition_remap(bio);
>
> - if (bio_integrity_enabled(bio) && bio_integrity_prep(bio))
> - goto end_io;
> -
Umm, why did you move this hunk?
> if (bio_check_eod(bio, nr_sectors))
> goto end_io;
> diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
> index 780d4c6..0144fbb 100644
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
> @@ -69,6 +69,7 @@ struct inodes_stat_t {
> #define MS_REMOUNT 32 /* Alter flags of a mounted FS */
> #define MS_MANDLOCK 64 /* Allow mandatory locks on an FS */
> #define MS_DIRSYNC 128 /* Directory modifications are synchronous */
> +#define MS_SNAP_STABLE 256 /* Snapshot pages during writeback, if needed */
> #define MS_NOATIME 1024 /* Do not update access times. */
> #define MS_NODIRATIME 2048 /* Do not update directory access times */
> #define MS_BIND 4096
Please don't mix MS_SNAP_STABLE flag among flags passed by mount(2)
syscall. I think putting it at 1 << 27 might be acceptable. I remember
Al Viro saying something along the lines that kernel internal superblock
flags should be separated from those passed from userspace into a special
superblock entry but that's a different story I guess.
> diff --git a/mm/Kconfig b/mm/Kconfig
> index 278e3ab..7901d83 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -258,6 +258,19 @@ config BOUNCE
> def_bool y
> depends on BLOCK && MMU && (ZONE_DMA || HIGHMEM)
>
> +# On the 'tile' arch, USB OHCI needs the bounce pool since tilegx will often
> +# have more than 4GB of memory, but we don't currently use the IOTLB to present
> +# a 32-bit address to OHCI. So we need to use a bounce pool instead.
> +#
> +# We also use the bounce pool to provide stable page writes for jbd. jbd
> +# initiates buffer writeback without locking the page or setting PG_writeback,
> +# and fixing that behavior (a second time; jbd2 doesn't have this problem) is
> +# a major rework effort. Instead, use the bounce buffer to snapshot pages
> +# (until jbd goes away). The only jbd user is ext3.
> +config NEED_BOUNCE_POOL
> + bool
> + default y if (TILE && USB_OHCI_HCD) || (BLK_DEV_INTEGRITY && JBD)
> +
> config NR_QUICK
> int
> depends on QUICKLIST
> diff --git a/mm/bounce.c b/mm/bounce.c
> index 0420867..a5b30f9 100644
> --- a/mm/bounce.c
> +++ b/mm/bounce.c
> @@ -178,8 +178,44 @@ static void bounce_end_io_read_isa(struct bio *bio, int err)
> __bounce_end_io_read(bio, isa_page_pool, err);
> }
>
> +#ifdef CONFIG_NEED_BOUNCE_POOL
> +static int must_snapshot_stable_pages(struct bio *bio)
> +{
> + struct page *page;
> + struct backing_dev_info *bdi;
> + struct address_space *mapping;
> + struct bio_vec *from;
> + int i;
> +
> + if (bio_data_dir(bio) != WRITE)
> + return 0;
> +
> + /*
> + * Based on the first page that has a valid mapping, decide whether or
> + * not we have to employ bounce buffering to guarantee stable pages.
> + */
> + bio_for_each_segment(from, bio, i) {
> + page = from->bv_page;
> + mapping = page_mapping(page);
> + if (!mapping)
> + continue;
> + bdi = mapping->backing_dev_info;
> + if (!bdi_cap_stable_pages_required(bdi))
> + return 0;
> + return mapping->host->i_sb->s_flags & MS_SNAP_STABLE;
> + }
> +
> + return 0;
> +}
How about using q->backing_dev_info for the
bdi_cap_stable_pages_required() check? It will be a fast path and this check
will be faster..
Honza
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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