linux-kernel - Re: [PATCHv4 02/17] zram: do not use per-CPU compression streams

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Message-ID: <hvnfal244vw42valllltcfvopsyp2yqle5kdyji3flfypgwhyb@c46zib6seixb>
Date: Sun, 9 Feb 2025 16:42:32 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Sergey Senozhatsky <senozhatsky@...omium.org>
Cc: Yosry Ahmed <yosry.ahmed@...ux.dev>, Kairui Song <ryncsn@...il.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4 02/17] zram: do not use per-CPU compression streams

On (25/02/09 15:22), Sergey Senozhatsky wrote:
> On (25/02/09 01:20), Sergey Senozhatsky wrote:
> > So I moved lockdep class keys to per-zram device and per-zsmalloc pool
> > to separate the lockdep chains.  Looks like that did the trick.
> 
> Also need to indicate "try lock":
> 
> drivers/block/zram/zram_drv.c
> @@ -86,7 +86,7 @@ static __must_check bool zram_slot_try_lock(struct zram *zram, u32 index)
>  
>         if (!test_and_set_bit_lock(ZRAM_ENTRY_LOCK, lock)) {
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> -               mutex_acquire(&zram->table[index].lockdep_map, 0, 0, _RET_IP_);
> +               mutex_acquire(&zram->table[index].lockdep_map, 0, 1, _RET_IP_);
>  #endif
>                 return true;
>         }
> 
> and
> 
> mm/zsmalloc.c
> @@ -388,7 +388,7 @@ static __must_check bool zspage_try_write_lock(struct zspage *zspage)
>         preempt_disable();
>         if (atomic_try_cmpxchg_acquire(lock, &old, ZS_PAGE_WRLOCKED)) {
>  #ifdef CONFIG_DEBUG_LOCK_ALLOC
> -               rwsem_acquire(&zspage->lockdep_map, 0, 0, _RET_IP_);
> +               rwsem_acquire(&zspage->lockdep_map, 0, 1, _RET_IP_);
>  #endif
>                 return true;
>         }

I guess this was the point lockdep was making.

lockdep knows about strm->lock -> shrink_folio_list which goes to ptllock
via folio_referenced and cluter_into lock via try_to_unmap.  Then lockdep
knows about zram entry->lock -> strm->lock and, most importantly, lockdep
knows about cluter_into lock -> zram_slot_free_notify() -> zram-entry->lock.
What lockdep doesn't know is that zram_slot_free_notify() is a try lock,
we don't re-enter zram unconditionally.