[PATCH mmotm] mm/fs: delete PF_SWAPWRITE

[Hugh Dickins <hughd@google.com>]

PF_SWAPWRITE has been redundant since v3.2 commit ee72886d8ed5
("mm: vmscan: do not writeback filesystem pages in direct reclaim").

Coincidentally, NeilBrown's current patch "remove inode_congested()"
deletes may_write_to_inode(), which appeared to be the one function
which took notice of PF_SWAPWRITE.  But if you study the old logic,
and the conditions under which may_write_to_inode() was called, you
discover that flag and function have been pointless for a decade.

Signed-off-by: Hugh Dickins <hughd@google.com>
---

 fs/fs-writeback.c         | 3 ---
 fs/xfs/libxfs/xfs_btree.c | 2 +-
 include/linux/sched.h     | 1 -
 mm/migrate.c              | 7 -------
 mm/vmscan.c               | 8 ++------
 5 files changed, 3 insertions(+), 18 deletions(-)

--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2196,7 +2196,6 @@ void wb_workfn(struct work_struct *work)
 	long pages_written;
 
 	set_worker_desc("flush-%s", bdi_dev_name(wb->bdi));
-	current->flags |= PF_SWAPWRITE;
 
 	if (likely(!current_is_workqueue_rescuer() ||
 		   !test_bit(WB_registered, &wb->state))) {
@@ -2225,8 +2224,6 @@ void wb_workfn(struct work_struct *work)
 		wb_wakeup(wb);
 	else if (wb_has_dirty_io(wb) && dirty_writeback_interval)
 		wb_wakeup_delayed(wb);
-
-	current->flags &= ~PF_SWAPWRITE;
 }
 
 /*
--- a/fs/xfs/libxfs/xfs_btree.c
+++ b/fs/xfs/libxfs/xfs_btree.c
@@ -2818,7 +2818,7 @@ struct xfs_btree_split_args {
 	 * in any way.
 	 */
 	if (args->kswapd)
-		new_pflags |= PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD;
+		new_pflags |= PF_MEMALLOC | PF_KSWAPD;
 
 	current_set_flags_nested(&pflags, new_pflags);
 	xfs_trans_set_context(args->cur->bc_tp);
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1700,7 +1700,6 @@ static inline int is_global_init(struct task_struct *tsk)
 						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
 #define PF_RANDOMIZE		0x00400000	/* Randomize virtual address space */
-#define PF_SWAPWRITE		0x00800000	/* Allowed to write to swap */
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
 #define PF_MCE_EARLY		0x08000000      /* Early kill for mce process policy */
 #define PF_MEMALLOC_PIN		0x10000000	/* Allocation context constrained to zones which allow long term pinning. */
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1350,7 +1350,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	bool is_thp = false;
 	struct page *page;
 	struct page *page2;
-	int swapwrite = current->flags & PF_SWAPWRITE;
 	int rc, nr_subpages;
 	LIST_HEAD(ret_pages);
 	LIST_HEAD(thp_split_pages);
@@ -1359,9 +1358,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 
 	trace_mm_migrate_pages_start(mode, reason);
 
-	if (!swapwrite)
-		current->flags |= PF_SWAPWRITE;
-
 thp_subpage_migration:
 	for (pass = 0; pass < 10 && (retry || thp_retry); pass++) {
 		retry = 0;
@@ -1516,9 +1512,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 	trace_mm_migrate_pages(nr_succeeded, nr_failed_pages, nr_thp_succeeded,
 			       nr_thp_failed, nr_thp_split, mode, reason);
 
-	if (!swapwrite)
-		current->flags &= ~PF_SWAPWRITE;
-
 	if (ret_succeeded)
 		*ret_succeeded = nr_succeeded;
 
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -4468,7 +4468,7 @@ 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_SWAPWRITE | PF_KSWAPD;
+	tsk->flags |= PF_MEMALLOC | PF_KSWAPD;
 	set_freezable();
 
 	WRITE_ONCE(pgdat->kswapd_order, 0);
@@ -4519,7 +4519,7 @@ static int kswapd(void *p)
 			goto kswapd_try_sleep;
 	}
 
-	tsk->flags &= ~(PF_MEMALLOC | PF_SWAPWRITE | PF_KSWAPD);
+	tsk->flags &= ~(PF_MEMALLOC | PF_KSWAPD);
 
 	return 0;
 }
@@ -4760,11 +4760,8 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	fs_reclaim_acquire(sc.gfp_mask);
 	/*
 	 * We need to be able to allocate from the reserves for RECLAIM_UNMAP
-	 * and we also need to be able to write out pages for RECLAIM_WRITE
-	 * and RECLAIM_UNMAP.
 	 */
 	noreclaim_flag = memalloc_noreclaim_save();
-	p->flags |= PF_SWAPWRITE;
 	set_task_reclaim_state(p, &sc.reclaim_state);
 
 	if (node_pagecache_reclaimable(pgdat) > pgdat->min_unmapped_pages) {
@@ -4778,7 +4775,6 @@ static int __node_reclaim(struct pglist_data *pgdat, gfp_t gfp_mask, unsigned in
 	}
 
 	set_task_reclaim_state(p, NULL);
-	current->flags &= ~PF_SWAPWRITE;
 	memalloc_noreclaim_restore(noreclaim_flag);
 	fs_reclaim_release(sc.gfp_mask);
 	psi_memstall_leave(&pflags);