On Wed, 12 Feb 2025 15:26:59 +0900 Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx> wrote:
> Concurrent modifications of meta table entries is now handled
> by per-entry spin-lock. This has a number of shortcomings.
>
> First, this imposes atomic requirements on compression backends.
> zram can call both zcomp_compress() and zcomp_decompress() under
> entry spin-lock, which implies that we can use only compression
> algorithms that don't schedule/sleep/wait during compression and
> decompression. This, for instance, makes it impossible to use
> some of the ASYNC compression algorithms (H/W compression, etc.)
> implementations.
>
> Second, this can potentially trigger watchdogs. For example,
> entry re-compression with secondary algorithms is performed
> under entry spin-lock. Given that we chain secondary
> compression algorithms and that some of them can be configured
> for best compression ratio (and worst compression speed) zram
> can stay under spin-lock for quite some time.
>
> Having a per-entry mutex (or, for instance, a rw-semaphore)
> significantly increases sizeof() of each entry and hence the
> meta table. Therefore entry locking returns back to bit
> locking, as before, however, this time also preempt-rt friendly,
> because if waits-on-bit instead of spinning-on-bit. Lock owners
> are also now permitted to schedule, which is a first step on the
> path of making zram non-atomic.
>
> ...
>
> -static int zram_slot_trylock(struct zram *zram, u32 index)
> +static void zram_slot_lock_init(struct zram *zram, u32 index)
> {
> - return spin_trylock(&zram->table[index].lock);
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + lockdep_init_map(&zram->table[index].lockdep_map, "zram-entry->lock",
> + &zram->table_lockdep_key, 0);
> +#endif
> +}
> +
>
> ...
>
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC
> + lockdep_register_key(&zram->table_lockdep_key);
> +#endif
> +
Please check whether all the ifdefs are needed - some of these things
have CONFIG_LOCKDEP=n stubs.
