[PATCH] mm: document risk of PF_MEMALLOC_NORECLAIM

[Michal Hocko <mhocko@suse.com>]

Andrew, could you merge the following before PF_MEMALLOC_NORECLAIM can
be removed from the tree altogether please? For the full context the 
email thread starts here: https://lore.kernel.org/all/20240812090525.80299-1-laoar.shao@gmail.com/T/#u
--- 
From f17d36975ec343d9388aa6dbf9ca8d1b58ed09ce Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.com>
Date: Fri, 16 Aug 2024 10:10:00 +0200
Subject: [PATCH] mm: document risk of PF_MEMALLOC_NORECLAIM

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. Any
potential __GFP_NOFAIL request from withing PF_MEMALLOC_NORECLAIM
context would BUG_ON if the allocation would fail.

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

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
 include/linux/sched.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index f8d150343d42..0c9061d2a8bd 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1657,7 +1657,12 @@ 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_NORECLAIM	0x00800000	/* All allocation requests will clear __GFP_DIRECT_RECLAIM.
+						 * This is inherently unsafe unless the context fully controls
+						 * all allocations used. Any potential __GFP_NOFAIL nested allocation
+						 * could BUG_ON as the page allocator doesn't support non-sleeping
+						 * __GFP_NOFAIL requests.
+						 */
 #define PF_MEMALLOC_NOWARN	0x01000000	/* All allocation requests will inherit __GFP_NOWARN */
 #define PF__HOLE__02000000	0x02000000
 #define PF_NO_SETAFFINITY	0x04000000	/* Userland is not allowed to meddle with cpus_mask */
-- 
2.46.0

-- 
2.46.0