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    Subject[RFC] fs: Use slab constructor to initialize conn objects in fsnotify
    Date
    While reading the famous slab paper [1], I noticed that the conn->lock
    spinlock and conn->list hlist in fsnotify code is being initialized
    during every object allocation. This seems a good fit for the
    constructor within the slab to take advantage of the slab design. Move
    the initializtion to that.

    spin_lock_init(&conn->lock);
    INIT_HLIST_HEAD(&conn->list);

    [1] https://pdfs.semanticscholar.org/1acc/3a14da69dd240f2fbc11d00e09610263bdbd.pdf

    Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
    ---
    I've only build-tested. I am not very familiar with this code, so I
    kindly request the maintainers/reviewers to take a look and see if it
    makes sense. From what I see, the ->list is always empty during
    allocation. The spinlock allocation also seems unnecesary.


    fs/notify/fsnotify.c | 15 +++++++++++++--
    fs/notify/mark.c | 2 --
    2 files changed, 13 insertions(+), 4 deletions(-)

    diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
    index 46f2255800091..55ed450da3295 100644
    --- a/fs/notify/fsnotify.c
    +++ b/fs/notify/fsnotify.c
    @@ -385,6 +385,14 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
    }
    EXPORT_SYMBOL_GPL(fsnotify);

    +static void fsnotify_ctor(void *addr)
    +{
    + struct fsnotify_mark_connector *conn = addr;
    +
    + spin_lock_init(&conn->lock);
    + INIT_HLIST_HEAD(&conn->list);
    +}
    +
    static __init int fsnotify_init(void)
    {
    int ret;
    @@ -395,8 +403,11 @@ static __init int fsnotify_init(void)
    if (ret)
    panic("initializing fsnotify_mark_srcu");

    - fsnotify_mark_connector_cachep = KMEM_CACHE(fsnotify_mark_connector,
    - SLAB_PANIC);
    + fsnotify_mark_connector_cachep =
    + kmem_cache_create("fsnotify_mark_connector",
    + sizeof(struct fsnotify_mark_connector),
    + __alignof__(struct fsnotify_mark_connector),
    + SLAB_PANIC, fsnotify_ctor);

    return 0;
    }
    diff --git a/fs/notify/mark.c b/fs/notify/mark.c
    index 1d96216dffd19..d388e7f45e2ea 100644
    --- a/fs/notify/mark.c
    +++ b/fs/notify/mark.c
    @@ -479,8 +479,6 @@ static int fsnotify_attach_connector_to_object(fsnotify_connp_t *connp,
    conn = kmem_cache_alloc(fsnotify_mark_connector_cachep, GFP_KERNEL);
    if (!conn)
    return -ENOMEM;
    - spin_lock_init(&conn->lock);
    - INIT_HLIST_HEAD(&conn->list);
    conn->type = type;
    conn->obj = connp;
    /* Cache fsid of filesystem containing the object */
    --
    2.26.1.301.g55bc3eb7cb9-goog
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