mm: document memalloc_noreclaim_save() and memalloc_pin_save()

The memalloc_noreclaim_save() function currently has no documentation
comment, so the implications of its usage are not obvious.  Namely that it
not only prevents entering reclaim (as the name suggests), but also allows
using all memory reserves and thus should be only used in contexts that
are allocating memory to free memory.  This may lead to new improper
usages being added.

Thus add a documenting comment, based on the description of
__GFP_MEMALLOC.  While at it, also document memalloc_pin_save() so that
all the memalloc_ scopes are documented.  For those already documented,
add missing Return: descriptions, and mark Context: description per
kernel-docs style guide.

In the comments describing the relevant PF_MEMALLOC flags, refer to their
scope setting functions.

[vbabka@suse.cz: fix issues that Mike pointed out]
  Link: https://lkml.kernel.org/r/20240215095827.13756-2-vbabka@suse.cz
Link: https://lkml.kernel.org/r/20240212182950.32730-2-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reviewed-by: Mike Rapoport (IBM) <rppt@kernel.org>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 5 insertions, 4 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b9ce285d8c9c..998861865b84 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1624,15 +1624,15 @@ extern struct pid *cad_pid;
 #define PF_SUPERPRIV		0x00000100	/* Used super-user privileges */
 #define PF_DUMPCORE		0x00000200	/* Dumped core */
 #define PF_SIGNALED		0x00000400	/* Killed by a signal */
-#define PF_MEMALLOC		0x00000800	/* Allocating memory */
+#define PF_MEMALLOC		0x00000800	/* Allocating memory to free memory. See memalloc_noreclaim_save() */
 #define PF_NPROC_EXCEEDED	0x00001000	/* set_user() noticed that RLIMIT_NPROC was exceeded */
 #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__HOLE__00010000	0x00010000
 #define PF_KSWAPD		0x00020000	/* I am kswapd */
-#define PF_MEMALLOC_NOFS	0x00040000	/* All allocation requests will inherit GFP_NOFS */
-#define PF_MEMALLOC_NOIO	0x00080000	/* All allocation requests will inherit GFP_NOIO */
+#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,
 						 * I am cleaning dirty pages from some other bdi. */
 #define PF_KTHREAD		0x00200000	/* I am a kernel thread */
@@ -1642,7 +1642,8 @@ extern struct pid *cad_pid;
 #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 */
-#define PF_MEMALLOC_PIN		0x10000000	/* Allocation context constrained to zones which allow long term pinning. */
+#define PF_MEMALLOC_PIN		0x10000000	/* Allocations constrained to zones which allow long term pinning.
+						 * See memalloc_pin_save() */
 #define PF__HOLE__20000000	0x20000000
 #define PF__HOLE__40000000	0x40000000
 #define PF_SUSPEND_TASK		0x80000000      /* This thread called freeze_processes() and should not be frozen */