linux-kernel - Re: [PATCHv4 14/17] zsmalloc: make zspage lock preemptible

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Message-ID: <mgvallgt7a65q6comfgz7kreksbes2dxipmkez3a6b6tgqznz4@3txnoxq2tbkr>
Date: Thu, 6 Feb 2025 12:28:42 +0900
From: Sergey Senozhatsky <senozhatsky@...omium.org>
To: Yosry Ahmed <yosry.ahmed@...ux.dev>
Cc: Andrew Morton <akpm@...ux-foundation.org>, 
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, 
	Sergey Senozhatsky <senozhatsky@...omium.org>
Subject: Re: [PATCHv4 14/17] zsmalloc: make zspage lock preemptible

On (25/02/06 12:05), Sergey Senozhatsky wrote:
> 
> Sure.  That's what it currently looks like (can always improve)
> 

- added must-check
- added preemptible() check  // just in case
- added locking rules list

Oh, and also switched to acquire/release semantics, like you suggested
a couple of days ago.

---

/*
 * zspage locking rules:
 *
 * 1) writer-lock is exclusive
 *
 * 2) writer-lock owner cannot sleep
 *
 * 3) writer-lock owner cannot spin waiting for the lock
 *   - caller (e.g. compaction and migration) must check return value and
 *     handle locking failures
 *   - there is only TRY variant of writer-lock function
 *
 * 4) reader-lock owners (multiple) can sleep
 *
 * 5) reader-lock owners can spin waiting for the lock, in any context
 *   - existing readers (even preempted ones) don't block new readers
 *   - writer-lock owners never sleep, always unlock at some point
 */
static void zspage_read_lock(struct zspage *zspage)
{
        atomic_t *lock = &zspage->lock;
        int old = atomic_read_acquire(lock);

        do {
                if (old == ZS_PAGE_WRLOCKED) {
                        cpu_relax();
                        old = atomic_read_acquire(lock);
                        continue;
                }
        } while (!atomic_try_cmpxchg_acquire(lock, &old, old + 1));

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        rwsem_acquire_read(&zspage->lockdep_map, 0, 0, _RET_IP_);
#endif
}

static void zspage_read_unlock(struct zspage *zspage)
{
        atomic_dec_return_release(&zspage->lock);

#ifdef CONFIG_DEBUG_LOCK_ALLOC
        rwsem_release(&zspage->lockdep_map, _RET_IP_);
#endif
}

static __must_check bool zspage_try_write_lock(struct zspage *zspage)
{
        atomic_t *lock = &zspage->lock;
        int old = ZS_PAGE_UNLOCKED;

        WARN_ON_ONCE(preemptible());

        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_);
#endif
                return true;
        }

        preempt_enable();
        return false;
}

static void zspage_write_unlock(struct zspage *zspage)
{
        atomic_set_release(&zspage->lock, ZS_PAGE_UNLOCKED);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
        rwsem_release(&zspage->lockdep_map, _RET_IP_);
#endif
        preempt_enable();
}