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    Subject[PATCH 5.15 098/530] ext4: place buffer head allocation before handle start
    Date
    From: Jinke Han <hanjinke.666@bytedance.com>

    commit d1052d236eddf6aa851434db1897b942e8db9921 upstream.

    In our product environment, we encounter some jbd hung waiting handles to
    stop while several writters were doing memory reclaim for buffer head
    allocation in delay alloc write path. Ext4 do buffer head allocation with
    holding transaction handle which may be blocked too long if the reclaim
    works not so smooth. According to our bcc trace, the reclaim time in
    buffer head allocation can reach 258s and the jbd transaction commit also
    take almost the same time meanwhile. Except for these extreme cases,
    we often see several seconds delays for cgroup memory reclaim on our
    servers. This is more likely to happen considering docker environment.

    One thing to note, the allocation of buffer heads is as often as page
    allocation or more often when blocksize less than page size. Just like
    page cache allocation, we should also place the buffer head allocation
    before startting the handle.

    Cc: stable@kernel.org
    Signed-off-by: Jinke Han <hanjinke.666@bytedance.com>
    Link: https://lore.kernel.org/r/20220903012429.22555-1-hanjinke.666@bytedance.com
    Signed-off-by: Theodore Ts'o <tytso@mit.edu>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    fs/ext4/inode.c | 7 +++++++
    1 file changed, 7 insertions(+)

    --- a/fs/ext4/inode.c
    +++ b/fs/ext4/inode.c
    @@ -1177,6 +1177,13 @@ retry_grab:
    page = grab_cache_page_write_begin(mapping, index, flags);
    if (!page)
    return -ENOMEM;
    + /*
    + * The same as page allocation, we prealloc buffer heads before
    + * starting the handle.
    + */
    + if (!page_has_buffers(page))
    + create_empty_buffers(page, inode->i_sb->s_blocksize, 0);
    +
    unlock_page(page);

    retry_journal:

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