Revert "mm: introduce PF_MEMALLOC_NORECLAIM, PF_MEMALLOC_NOWARN"

This reverts commit eab0af90.

There is no existing user of those flags.  PF_MEMALLOC_NOWARN is dangerous
because a nested allocation context can use GFP_NOFAIL which could cause
unexpected failure.  Such a code would be hard to maintain because it
could be deeper in the call chain.

PF_MEMALLOC_NORECLAIM has been added even when it was pointed out [1] that
such a allocation contex is inherently unsafe if the context doesn't fully
control all allocations called from this context.

While PF_MEMALLOC_NOWARN is not dangerous the way PF_MEMALLOC_NORECLAIM is
it doesn't have any user and as Matthew has pointed out we are running out
of those flags so better reclaim it without any real users.

[1] https://lore.kernel.org/all/ZcM0xtlKbAOFjv5n@tiehlicka/

Link: https://lkml.kernel.org/r/20240926172940.167084-3-mhocko@kernel.orgSigned-off-by: Michal Hocko <mhocko@suse.com>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: James Morris <jmorris@namei.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Paul Moore <paul@paul-moore.com>
Cc: Serge E. Hallyn <serge@hallyn.com>
Cc: Yafang Shao <laoar.shao@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/sched.h

@@ -1681,8 +1681,8 @@ extern struct pid *cad_pid;
 						 * 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_MEMALLOC_NORECLAIM	0x00800000	/* All allocation requests will clear __GFP_DIRECT_RECLAIM */
-#define PF_MEMALLOC_NOWARN	0x01000000	/* All allocation requests will inherit __GFP_NOWARN */
+#define PF__HOLE__00800000	0x00800000
+#define PF__HOLE__01000000	0x01000000
 #define PF__HOLE__02000000	0x02000000
 #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 */

include/linux/sched/mm.h

@@ -251,25 +251,16 @@ static inline gfp_t current_gfp_context(gfp_t flags)
 {
 	unsigned int pflags = READ_ONCE(current->flags);
 
-	if (unlikely(pflags & (PF_MEMALLOC_NOIO |
-			       PF_MEMALLOC_NOFS |
-			       PF_MEMALLOC_NORECLAIM |
-			       PF_MEMALLOC_NOWARN |
-			       PF_MEMALLOC_PIN))) {
+	if (unlikely(pflags & (PF_MEMALLOC_NOIO | PF_MEMALLOC_NOFS | PF_MEMALLOC_PIN))) {
 		/*
-		 * Stronger flags before weaker flags:
-		 * NORECLAIM implies NOIO, which in turn implies NOFS
+		 * NOIO implies both NOIO and NOFS and it is a weaker context
+		 * so always make sure it makes precedence
 		 */
-		if (pflags & PF_MEMALLOC_NORECLAIM)
-			flags &= ~__GFP_DIRECT_RECLAIM;
-		else if (pflags & PF_MEMALLOC_NOIO)
+		if (pflags & PF_MEMALLOC_NOIO)
 			flags &= ~(__GFP_IO | __GFP_FS);
 		else if (pflags & PF_MEMALLOC_NOFS)
 			flags &= ~__GFP_FS;
 
-		if (pflags & PF_MEMALLOC_NOWARN)
-			flags |= __GFP_NOWARN;
-
 		if (pflags & PF_MEMALLOC_PIN)
 			flags &= ~__GFP_MOVABLE;
 	}