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

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

PF_KCOMPACTD was introduced by commit ce6d9c1c2b5c ("NFS: fix
nfs_release_folio() to not deadlock via kcompactd writeback") so
nfs_release_folio() could detect kcompactd context and skip
writeback.  The flag is only consumed by current_is_kcompactd(),
whose sole caller is nfs_release_folio().

Replace the flag-based check with kthread_func(current) == kcompactd,
freeing the 0x00010000 PF flag bit.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
---
 include/linux/compaction.h               | 11 ++++++-----
 include/linux/sched.h                    |  2 +-
 mm/compaction.c                          |  9 ++++++---
 tools/sched_ext/include/scx/common.bpf.h |  1 -
 4 files changed, 13 insertions(+), 10 deletions(-)

diff --git a/include/linux/compaction.h b/include/linux/compaction.h
index 66a2f70e9e01..691c09f0a697 100644
--- a/include/linux/compaction.h
+++ b/include/linux/compaction.h
@@ -81,10 +81,6 @@ static inline unsigned long compact_gap(unsigned int order)
 	return min(2UL << order, COMPACT_CLUSTER_MAX);
 }
 
-static inline int current_is_kcompactd(void)
-{
-	return current->flags & PF_KCOMPACTD;
-}
 
 #ifdef CONFIG_COMPACTION
 
@@ -103,7 +99,7 @@ extern void compaction_defer_reset(struct zone *zone, int order,
 
 bool compaction_zonelist_suitable(struct alloc_context *ac, int order,
 					int alloc_flags, gfp_t gfp_mask);
-
+bool current_is_kcompactd(void);
 extern void __meminit kcompactd_run(int nid);
 extern void __meminit kcompactd_stop(int nid);
 extern void wakeup_kcompactd(pg_data_t *pgdat, int order, int highest_zoneidx);
@@ -120,6 +116,11 @@ static inline bool compaction_suitable(struct zone *zone, int order,
 	return false;
 }
 
+static inline bool current_is_kcompactd(void)
+{
+	return false;
+}
+
 static inline void kcompactd_run(int nid)
 {
 }
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 86394121b6c0..eb12ff4cea6c 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1808,7 +1808,7 @@ extern struct pid *cad_pid;
 #define PF_USED_MATH		0x00002000	/* If unset the fpu must be initialized before use */
 #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__HOLE__00010000	0x00010000
 #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() */
diff --git a/mm/compaction.c b/mm/compaction.c
index a049415512c6..4994e200bbec 100644
--- a/mm/compaction.c
+++ b/mm/compaction.c
@@ -3197,7 +3197,6 @@ static int kcompactd(void *p)
 	long default_timeout = msecs_to_jiffies(HPAGE_FRAG_CHECK_INTERVAL_MSEC);
 	long timeout = default_timeout;
 
-	current->flags |= PF_KCOMPACTD;
 	set_freezable();
 
 	pgdat->kcompactd_max_order = 0;
@@ -3254,11 +3253,15 @@ static int kcompactd(void *p)
 			pgdat->proactive_compact_trigger = false;
 	}
 
-	current->flags &= ~PF_KCOMPACTD;
-
 	return 0;
 }
 
+bool current_is_kcompactd(void)
+{
+	return kthread_func(current) == kcompactd;
+}
+EXPORT_SYMBOL_GPL(current_is_kcompactd);
+
 /*
  * This kcompactd start function will be called by init and node-hot-add.
  * On node-hot-add, kcompactd will moved to proper cpus if cpus are hot-added.
diff --git a/tools/sched_ext/include/scx/common.bpf.h b/tools/sched_ext/include/scx/common.bpf.h
index 3e095343ae84..807ef881ce2d 100644
--- a/tools/sched_ext/include/scx/common.bpf.h
+++ b/tools/sched_ext/include/scx/common.bpf.h
@@ -31,7 +31,6 @@
 #define PF_IDLE				0x00000002	/* I am an IDLE thread */
 #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_KTHREAD			0x00200000	/* I am a kernel thread */
 #define PF_EXITING			0x00000004
 #define CLOCK_MONOTONIC			1
-- 
2.55.0