[+ Jann]
On Mon, Aug 26, 2024 at 10:41:07AM GMT, Christian Brauner wrote:
> I've managed to shrink struct file to 192 bytes aka three cachelines.
> What I wasn't aware of is that SLAB_TYPESAFE_BY_RCU silently grows the
> struct by another cacheline to accomodate the freelist pointer as
> pointed out by Vlastimil.
>
> It's not too bad because we mostly care about "runtime" size but it's
> still unfortunate that we're wasting a whole additional cacheline for no
> good reason.
>
> So I would like to find a way to avoid that. Below you see a hack
> because adding a parameter to kmem_cache_create() seemed to require me
> to fiddle with mm/slub.c and I'd rather not unless I'm told how.
>
> For the POC I'm using the offset into the union at the end of struct
> file so that SLUB places the freelist pointer into the union during
> kmem_cache_free().
>
> I'm just looking for ideas from mm to figure out an API where we can say
> "use that as the freelist pointer in the struct" or at least a reason
> why that's just not possible.
>
> Sketched-by: Christian Brauner <brauner@xxxxxxxxxx>
> ---
> fs/file_table.c | 3 ++-
> include/linux/fs.h | 2 ++
> 2 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/fs/file_table.c b/fs/file_table.c
> index 694199a1a966..7a5fac7d4bbd 100644
> --- a/fs/file_table.c
> +++ b/fs/file_table.c
> @@ -514,7 +514,8 @@ EXPORT_SYMBOL(__fput_sync);
>
> void __init files_init(void)
> {
> - filp_cachep = kmem_cache_create("filp", sizeof(struct file), 0,
> + filp_cachep = kmem_cache_create("filp",
> + offsetof(struct file, __f_slab_rcu), 0,
> SLAB_TYPESAFE_BY_RCU | SLAB_HWCACHE_ALIGN |
> SLAB_PANIC | SLAB_ACCOUNT, NULL);
> percpu_counter_init(&nr_files, 0, GFP_KERNEL);
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 36bb8456eb38..6c3664d624a0 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1010,6 +1010,7 @@ static inline int ra_has_index(struct file_ra_state *ra, pgoff_t index)
> * @f_task_work: task work entry point
> * @f_llist: work queue entrypoint
> * @f_ra: file's readahead state
> + * @f_slab_rcu: freelist pointer for SLAB_TYPESAFE_BY_RCU
> */
> struct file {
> atomic_long_t f_count;
> @@ -1041,6 +1042,7 @@ struct file {
> struct callback_head f_task_work;
> struct llist_node f_llist;
> struct file_ra_state f_ra;
> + void *__f_slab_rcu;
> };
> /* --- cacheline 3 boundary (192 bytes) --- */
> } __randomize_layout
> --
> 2.45.2
