[PATCH v2] mm: clarify GFP_ATOMIC/GFP_NOWAIT doc-comment

[Brendan Jackman <jackmanb@google.com>]

The current description of contexts where it's invalid to make
GFP_ATOMIC and GFP_NOWAIT calls is rather vague.

Replace this with a direct description of the actual contexts of concern
and refer to the RT docs where this is explained more discursively.

While rejigging this prose, also move the documentation of GFP_NOWAIT to
the GFP_NOWAIT section.

Link: https://lore.kernel.org/all/d912480a-5229-4efe-9336-b31acded30f5@suse.cz/
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
Changes in v2:
- Specified section in docs reference
- Link to v1: https://lore.kernel.org/all/DF0J58HOVLL4.2E16Q87D2UXRW@google.com/
---
 include/linux/gfp_types.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 3de43b12209ee79f84e7c9c919729303338433ff..814bb2892f99b829fde9b6dce1a317b3f29d01dc 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h
@@ -309,8 +309,10 @@ enum {
  *
  * %GFP_ATOMIC users can not sleep and need the allocation to succeed. A lower
  * watermark is applied to allow access to "atomic reserves".
- * The current implementation doesn't support NMI and few other strict
- * non-preemptive contexts (e.g. raw_spin_lock). The same applies to %GFP_NOWAIT.
+ * The current implementation doesn't support NMI, nor contexts that disable
+ * preemption under PREEMPT_RT. This includes raw_spin_lock() and plain
+ * preempt_disable() - see "Memory allocation" in
+ * Documentation/core-api/real-time/differences.rst for more info.
  *
  * %GFP_KERNEL is typical for kernel-internal allocations. The caller requires
  * %ZONE_NORMAL or a lower zone for direct access but can direct reclaim.
@@ -321,6 +323,7 @@ enum {
  * %GFP_NOWAIT is for kernel allocations that should not stall for direct
  * reclaim, start physical IO or use any filesystem callback.  It is very
  * likely to fail to allocate memory, even for very small allocations.
+ * The same restrictions on calling contexts apply as for %GFP_ATOMIC.
  *
  * %GFP_NOIO will use direct reclaim to discard clean pages or slab pages
  * that do not require the starting of any physical IO.

---
base-commit: fc2af7a8060126c3a99fc1dfd7c5abd71a4dc81d
change-id: 20251219-b4-gfp_atomic-comment-3e5950152139

Best regards,
-- 
Brendan Jackman <jackmanb@google.com>