Re: [PATCH 6/7] writeback: separate starting of sync vs opportunistic writeback

Jan Kara
2009-09-14T13:33:08Z

On Mon 14-09-09 11:36:33, Jens Axboe wrote:
> bdi_start_writeback() is currently split into two paths, one for
> WB_SYNC_NONE and one for WB_SYNC_ALL. Add bdi_sync_writeback()
> for WB_SYNC_ALL writeback and let bdi_start_writeback() handle
> only WB_SYNC_NONE.
What I don't like about this patch is that if somebody sets up
writeback_control with WB_SYNC_ALL mode set and then submits it to disk via
bdi_start_writeback() it will just silently convert his writeback to an
asynchronous one.
So I'd maybe leave setting of sync_mode to the caller and just WARN_ON if
it does not match the purpose of the function...

Honza

>
> Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
> ---
> fs/fs-writeback.c | 51 ++++++++++++++++++++++++++++++++++-----------------
> mm/page-writeback.c | 1 -
> 2 files changed, 34 insertions(+), 18 deletions(-)
>
> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
> index 5cd8b3b..64ca471 100644
> --- a/fs/fs-writeback.c
> +++ b/fs/fs-writeback.c
> @@ -204,24 +204,42 @@ static void bdi_alloc_queue_work(struct backing_dev_info *bdi,
> }
> }
>
> -void bdi_start_writeback(struct writeback_control *wbc)
> +/**
> + * bdi_sync_writeback - start and wait for writeback
> + * @wbc: writeback parameters
> + *
> + * Description:
> + * This does WB_SYNC_ALL data integrity writeback and waits for the
> + * IO to complete. Callers must hold the sb s_umount semaphore for
> + * reading, to avoid having the super disappear before we are done.
> + */
> +static void bdi_sync_writeback(struct writeback_control *wbc)
> {
> - /*
> - * WB_SYNC_NONE is opportunistic writeback. If this allocation fails,
> - * bdi_queue_work() will wake up the thread and flush old data. This
> - * should ensure some amount of progress in freeing memory.
> - */
> - if (wbc->sync_mode != WB_SYNC_ALL)
> - bdi_alloc_queue_work(wbc->bdi, wbc);
> - else {
> - struct bdi_work work;
> + struct bdi_work work;
>
> - bdi_work_init(&work, wbc);
> - work.state |= WS_ONSTACK;
> + wbc->sync_mode = WB_SYNC_ALL;
>
> - bdi_queue_work(wbc->bdi, &work);
> - bdi_wait_on_work_clear(&work);
> - }
> + bdi_work_init(&work, wbc);
> + work.state |= WS_ONSTACK;
> +
> + bdi_queue_work(wbc->bdi, &work);
> + bdi_wait_on_work_clear(&work);
> +}
> +
> +/**
> + * bdi_start_writeback - start writeback
> + * @wbc: writeback parameters
> + *
> + * Description:
> + * This does WB_SYNC_NONE opportunistic writeback. The IO is only
> + * started when this function returns, we make no guarentees on
> + * completion. Caller need not hold sb s_umount semaphore.
> + *
> + */
> +void bdi_start_writeback(struct writeback_control *wbc)
> +{
> + wbc->sync_mode = WB_SYNC_NONE;
> + bdi_alloc_queue_work(wbc->bdi, wbc);
> }
>
> /*
> @@ -1119,14 +1137,13 @@ long sync_inodes_sb(struct super_block *sb)
> struct writeback_control wbc = {
> .sb = sb,
> .bdi = sb->s_bdi,
> - .sync_mode = WB_SYNC_ALL,
> .range_start = 0,
> .range_end = LLONG_MAX,
> };
> long nr_to_write = LONG_MAX; /* doesn't actually matter */
>
> wbc.nr_to_write = nr_to_write;
> - bdi_start_writeback(&wbc);
> + bdi_sync_writeback(&wbc);
> wait_sb_inodes(&wbc);
> return nr_to_write - wbc.nr_to_write;
> }
> diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> index a5f0f76..f61f0cc 100644
> --- a/mm/page-writeback.c
> +++ b/mm/page-writeback.c
> @@ -585,7 +585,6 @@ static void balance_dirty_pages(struct address_space *mapping)
> > background_thresh))) {
> struct writeback_control wbc = {
> .bdi = bdi,
> - .sync_mode = WB_SYNC_NONE,
> .nr_to_write = nr_writeback,
> };
>
> --
> 1.6.4.1.207.g68ea
>
--
Jan Kara <jack@suse.cz>
SUSE Labs, CR