2019-01-29 02:05:41

by Xiaoguang Wang

Subject: [PATCH] ext4: unlock unused_pages timely when doing writeback

In mpage_add_bh_to_extent(), when accumulated extents length is greater
than MAX_WRITEPAGES_EXTENT_LEN or buffer head's b_stat is not equal, we
will not continue to search unmapped area for this page, but note this
page is locked, and will only be unlocked in mpage_release_unused_pages()
after ext4_io_submit, if io also is throttled by blk-throttle or similar
io qos, we will hold this page locked for a while, it's unnecessary.

I think the best fix is to refactor mpage_add_bh_to_extent() to let it
return some hints whether to unlock this page, but given that we will
improve dioread_nolock later, we can let it done later, so currently
the simple fix would just call mpage_release_unused_pages() before
ext4_io_submit().

Signed-off-by: Xiaoguang Wang <[email protected]>
---
fs/ext4/inode.c | 9 +++++----
1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 34d7e0703cc6..83a0d7c76736 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -2836,12 +2836,12 @@ static int ext4_writepages(struct address_space *mapping,
goto unplug;
}
ret = mpage_prepare_extent_to_map(&mpd);
+ /* Unlock pages we didn't use */
+ mpage_release_unused_pages(&mpd, false);
/* Submit prepared bio */
ext4_io_submit(&mpd.io_submit);
ext4_put_io_end_defer(mpd.io_submit.io_end);
mpd.io_submit.io_end = NULL;
- /* Unlock pages we didn't use */
- mpage_release_unused_pages(&mpd, false);
if (ret < 0)
goto unplug;

@@ -2909,10 +2909,11 @@ static int ext4_writepages(struct address_space *mapping,
handle = NULL;
mpd.do_map = 0;
}
- /* Submit prepared bio */
- ext4_io_submit(&mpd.io_submit);
/* Unlock pages we didn't use */
mpage_release_unused_pages(&mpd, give_up_on_write);
+ /* Submit prepared bio */
+ ext4_io_submit(&mpd.io_submit);
+
/*
* Drop our io_end reference we got from init. We have
* to be careful and use deferred io_end finishing if
--
2.17.2

2019-02-11 04:54:37

by Theodore Ts'o

Subject: Re: [PATCH] ext4: unlock unused_pages timely when doing writeback

On Tue, Jan 29, 2019 at 10:05:31AM +0800, Xiaoguang Wang wrote:
> In mpage_add_bh_to_extent(), when accumulated extents length is greater
> than MAX_WRITEPAGES_EXTENT_LEN or buffer head's b_stat is not equal, we
> will not continue to search unmapped area for this page, but note this
> page is locked, and will only be unlocked in mpage_release_unused_pages()
> after ext4_io_submit, if io also is throttled by blk-throttle or similar
> io qos, we will hold this page locked for a while, it's unnecessary.
>
> I think the best fix is to refactor mpage_add_bh_to_extent() to let it
> return some hints whether to unlock this page, but given that we will
> improve dioread_nolock later, we can let it done later, so currently
> the simple fix would just call mpage_release_unused_pages() before
> ext4_io_submit().
>
> Signed-off-by: Xiaoguang Wang <[email protected]>

Thanks, applied.

- Ted