Re: a old issue of ext4 on lts 3.10

[Dmitry Monakhov <dmonakhov@openvz.org>]

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Alex Shi <alex.shi@linaro.org> writes:

> Hi Dmitry&Theodore,
>
> Someone said without the following patch on lts 3.10 kernel (which used
> as android base kernel). the write maybe very very slow, needs 1 or 2
> seconds to finish.
In fact this was an optimization.
wait_for_stable_page() is actually and optimized wait_on_page_writeback()

see:
void wait_for_stable_page(struct page *page)
{
        struct address_space *mapping = page_mapping(page);
                struct backing_dev_info *bdi =
                mapping->backing_dev_info;

        if (!bdi_cap_stable_pages_required(bdi))
                        return;

        wait_on_page_writeback(page);
}
It is very unlikely the patch provokes such huge slowdown.
Can you please repeat your measurements and double check your evidence.
>
> I quick looked this patch, seems it's no harm for a normal fs function.
> but still don't know why it is helpful. So do you remember why you
> commit this change at that time?
>
> Thanks
> Alex
>
> ommit 7afe5aa59ed3da7b6161617e7f157c7c680dc41e
> Author: Dmitry Monakhov <dmonakhov@openvz.org>
> Date:   Wed Aug 28 14:30:47 2013 -0400
>
>     ext4: convert write_begin methods to stable_page_writes semantics
>
>     Use wait_for_stable_page() instead of wait_on_page_writeback()
>
>     Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
>     Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
>     Reviewed-by: Jan Kara <jack@suse.cz>
>
> diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
> index fc4051e..47c8e46 100644
> --- a/fs/ext4/inode.c
> +++ b/fs/ext4/inode.c
> @@ -969,7 +969,8 @@ retry_journal:
>                 ext4_journal_stop(handle);
>                 goto retry_grab;
>         }
> -       wait_on_page_writeback(page);
> +       /* In case writeback began while the page was unlocked */
> +       wait_for_stable_page(page);
>
>         if (ext4_should_dioread_nolock(inode))
>                 ret = __block_write_begin(page, pos, len,
> ext4_get_block_write);
> @@ -2678,7 +2679,7 @@ retry_journal:
>                 goto retry_grab;
>         }
>         /* In case writeback began while the page was unlocked */
> -       wait_on_page_writeback(page);
> +       wait_for_stable_page(page);
>
>         ret = __block_write_begin(page, pos, len, ext4_da_get_block_prep);
>         if (ret < 0) {
> ~
>
> -- 
> Thanks
>     Alex

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