Skip dbuf_evict_one() from dbuf_evict_notify() for reclaim thread

Avoid calling dbuf_evict_one() from memory reclaim contexts (e.g. Linux
kswapd, FreeBSD pagedaemon). This prevents deadlock caused by reclaim
threads waiting for the dbuf hash lock in the call sequence:
dbuf_evict_one -> dbuf_destroy -> arc_buf_destroy

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <alexander.motin@TrueNAS.com>
Signed-off-by: Kaitlin Hoang <kthoang@amazon.com>
Closes #17561

include/os/freebsd/spl/sys/misc.h

@@ -55,4 +55,9 @@ struct opensolaris_utsname {
 #define	task_io_account_read(n)
 #define	task_io_account_write(n)
 
+/*
+ * Check if the current thread is a memory reclaim thread.
+ */
+extern int current_is_reclaim_thread(void);
+
 #endif	/* _OPENSOLARIS_SYS_MISC_H_ */

include/os/linux/spl/sys/misc.h

@@ -23,7 +23,13 @@
 #define	_OS_LINUX_SPL_MISC_H
 
 #include <linux/kobject.h>
+#include <linux/swap.h>
 
 extern void spl_signal_kobj_evt(struct block_device *bdev);
 
+/*
+ * Check if the current thread is a memory reclaim thread.
+ */
+extern int current_is_reclaim_thread(void);
+
 #endif

include/sys/zfs_context.h

@@ -235,6 +235,11 @@ typedef pthread_t	kthread_t;
 #define	thread_join(t)	pthread_join((pthread_t)(t), NULL)
 
 #define	newproc(f, a, cid, pri, ctp, pid)	(ENOSYS)
+/*
+ * Check if the current thread is a memory reclaim thread.
+ * Always returns false in userspace (no memory reclaim thread).
+ */
+#define	current_is_reclaim_thread()	(0)
 
 /* in libzpool, p0 exists only to have its address taken */
 typedef struct proc {

module/os/freebsd/spl/spl_misc.c

@@ -100,6 +100,15 @@ spl_panic(const char *file, const char *func, int line, const char *fmt, ...)
 	va_end(ap);
 }
 
+/*
+ * Check if the current thread is a memory reclaim thread.
+ * Returns true if curproc is pageproc (FreeBSD's page daemon).
+ */
+int
+current_is_reclaim_thread(void)
+{
+	return (curproc == pageproc);
+}
 
 SYSINIT(opensolaris_utsname_init, SI_SUB_TUNABLES, SI_ORDER_ANY,
     opensolaris_utsname_init, NULL);

module/os/linux/spl/spl-thread.c

@@ -27,6 +27,7 @@
 #include <sys/kmem.h>
 #include <sys/tsd.h>
 #include <sys/string.h>
+#include <sys/misc.h>
 
 /*
  * Thread interfaces
@@ -196,3 +197,14 @@ issig(void)
 }
 
 EXPORT_SYMBOL(issig);
+
+/*
+ * Check if the current thread is a memory reclaim thread.
+ * Returns true if current thread is kswapd.
+ */
+int
+current_is_reclaim_thread(void)
+{
+	return (current_is_kswapd());
+}
+EXPORT_SYMBOL(current_is_reclaim_thread);

module/zfs/dbuf.c

@@ -841,8 +841,16 @@ dbuf_evict_notify(uint64_t size)
 	 * and grabbing the lock results in massive lock contention.
 	 */
 	if (size > dbuf_cache_target_bytes()) {
-		if (size > dbuf_cache_hiwater_bytes())
+		/*
+		 * Avoid calling dbuf_evict_one() from memory reclaim context
+		 * (e.g. Linux kswapd, FreeBSD pagedaemon) to prevent deadlocks.
+		 * Memory reclaim threads can get stuck waiting for the dbuf
+		 * hash lock.
+		 */
+		if (size > dbuf_cache_hiwater_bytes() &&
+		    !current_is_reclaim_thread()) {
 			dbuf_evict_one();
+		}
 		cv_signal(&dbuf_evict_cv);
 	}
 }