[PATCH 3/4] mm: replace PF_KSWAPD flag with kthread_func() check

[Kefeng Wang <wangkefeng.wang@huawei.com>]

The preceding commits removed the last consumer that propagated
PF_KSWAPD beyond kswapd itself (XFS btree split worker inheritance).
The only remaining setter of PF_KSWAPD is kswapd(), and every
current_is_kswapd() caller only needs to check whether the current
task *is* the kswapd thread, not whether it inherited the flag.

Replace the flag-based test with kthread_func(current) == kswapd,
freeing the 0x00020000 PF flag bit.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 include/linux/sched.h                    |  2 +-
 include/linux/swap.h                     |  7 +------
 mm/vmscan.c                              | 10 ++++++++--
 tools/sched_ext/include/scx/common.bpf.h |  1 -
 4 files changed, 10 insertions(+), 10 deletions(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 8b3d47a325cc..86394121b6c0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1809,7 +1809,7 @@ extern struct pid *cad_pid;
 #define PF_USER_WORKER		0x00004000	/* Kernel thread cloned from userspace thread */
 #define PF_NOFREEZE		0x00008000	/* This thread should not be frozen */
 #define PF_KCOMPACTD		0x00010000	/* I am kcompactd */
-#define PF_KSWAPD		0x00020000	/* I am kswapd */
+#define PF__HOLE__00020000	0x00020000
 #define PF_MEMALLOC_NOFS	0x00040000	/* All allocations inherit GFP_NOFS. See memalloc_nfs_save() */
 #define PF_MEMALLOC_NOIO	0x00080000	/* All allocations inherit GFP_NOIO. See memalloc_noio_save() */
 #define PF_LOCAL_THROTTLE	0x00100000	/* Throttle writes only against the bdi I write to,
diff --git a/include/linux/swap.h b/include/linux/swap.h
index a72ecf12c00d..fc290e29e4a9 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -25,12 +25,6 @@
 #define SWAP_FLAGS_VALID	(SWAP_FLAG_PRIO_MASK | SWAP_FLAG_PREFER | \
 				 SWAP_FLAG_DISCARD | SWAP_FLAG_DISCARD_ONCE | \
 				 SWAP_FLAG_DISCARD_PAGES)
-
-static inline int current_is_kswapd(void)
-{
-	return current->flags & PF_KSWAPD;
-}
-
 /*
  * MAX_SWAPFILES defines the maximum number of swaptypes: things which can
  * be swapped to.  The swap type and the offset into that swap type are
@@ -339,6 +333,7 @@ void check_move_unevictable_folios(struct folio_batch *fbatch);
 
 extern void __meminit kswapd_run(int nid);
 extern void __meminit kswapd_stop(int nid);
+bool current_is_kswapd(void);
 
 #ifdef CONFIG_SWAP
 int add_swap_extent(struct swap_info_struct *sis, unsigned long start_page,
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 07ba86634b84..775d3f71a9de 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -7468,7 +7468,7 @@ static int kswapd(void *p)
 	 * us from recursively trying to free more memory as we're
 	 * trying to free the first piece of memory in the first place).
 	 */
-	tsk->flags |= PF_MEMALLOC | PF_KSWAPD;
+	tsk->flags |= PF_MEMALLOC;
 	set_freezable();
 
 	WRITE_ONCE(pgdat->kswapd_order, 0);
@@ -7518,11 +7518,17 @@ static int kswapd(void *p)
 			goto kswapd_try_sleep;
 	}
 
-	tsk->flags &= ~(PF_MEMALLOC | PF_KSWAPD);
+	tsk->flags &= ~PF_MEMALLOC;
 
 	return 0;
 }
 
+bool current_is_kswapd(void)
+{
+	return kthread_func(current) == kswapd;
+}
+EXPORT_SYMBOL_GPL(current_is_kswapd);
+
 /*
  * A zone is low on free memory or too fragmented for high-order memory.  If
  * kswapd should reclaim (direct reclaim is deferred), wake it up for the zone's
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 76f5e025e107..3e095343ae84 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -32,7 +32,6 @@
 #define PF_IO_WORKER			0x00000010	/* Task is an IO worker */
 #define PF_WQ_WORKER			0x00000020	/* I'm a workqueue worker */
 #define PF_KCOMPACTD			0x00010000      /* I am kcompactd */
-#define PF_KSWAPD			0x00020000      /* I am kswapd */
 #define PF_KTHREAD			0x00200000	/* I am a kernel thread */
 #define PF_EXITING			0x00000004
 #define CLOCK_MONOTONIC			1
-- 
2.55.0