On Tue, May 2, 2023 at 5:50 AM Petr Tesařík [off-list ref] wrote:

On Mon,  1 May 2023 09:54:19 -0700
Suren Baghdasaryan [off-list ref] wrote:

quoted

Introduce __GFP_NO_OBJ_EXT flag in order to prevent recursive allocations
when allocating slabobj_ext on a slab.

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 include/linux/gfp_types.h | 12 ++++++++++--
 1 file changed, 10 insertions(+), 2 deletions(-)

diff --git a/include/linux/gfp_types.h b/include/linux/gfp_types.h
index 6583a58670c5..aab1959130f9 100644
--- a/include/linux/gfp_types.h
+++ b/include/linux/gfp_types.h

@@ -53,8 +53,13 @@ typedef unsigned int __bitwise gfp_t;
 #define ___GFP_SKIP_ZERO     0
 #define ___GFP_SKIP_KASAN    0
 #endif
+#ifdef CONFIG_SLAB_OBJ_EXT
+#define ___GFP_NO_OBJ_EXT       0x4000000u
+#else
+#define ___GFP_NO_OBJ_EXT       0
+#endif
 #ifdef CONFIG_LOCKDEP
-#define ___GFP_NOLOCKDEP     0x4000000u
+#define ___GFP_NOLOCKDEP     0x8000000u

So now we have two flags that depend on config options, but the first
one is always allocated in fact. I wonder if you could use an enum to
let the compiler allocate bits. Something similar to what Muchun Song
did with section flags.

See commit ed7802dd48f7a507213cbb95bb4c6f1fe134eb5d for reference.

Thanks for the reference. I'll take a closer look and will try to clean it up.

quoted

 #else
 #define ___GFP_NOLOCKDEP     0
 #endif

@@ -99,12 +104,15 @@ typedef unsigned int __bitwise gfp_t;
  * node with no fallbacks or placement policy enforcements.
  *
  * %__GFP_ACCOUNT causes the allocation to be accounted to kmemcg.
+ *
+ * %__GFP_NO_OBJ_EXT causes slab allocation to have no object

extension. */
 #define __GFP_RECLAIMABLE ((__force gfp_t)___GFP_RECLAIMABLE)
 #define __GFP_WRITE  ((__force gfp_t)___GFP_WRITE)
 #define __GFP_HARDWALL   ((__force gfp_t)___GFP_HARDWALL)
 #define __GFP_THISNODE       ((__force gfp_t)___GFP_THISNODE)
 #define __GFP_ACCOUNT        ((__force gfp_t)___GFP_ACCOUNT)
+#define __GFP_NO_OBJ_EXT   ((__force gfp_t)___GFP_NO_OBJ_EXT)

 /**
  * DOC: Watermark modifiers

@@ -249,7 +257,7 @@ typedef unsigned int __bitwise gfp_t;
 #define __GFP_NOLOCKDEP ((__force gfp_t)___GFP_NOLOCKDEP)

 /* Room for N __GFP_FOO bits */
-#define __GFP_BITS_SHIFT (26 + IS_ENABLED(CONFIG_LOCKDEP))
+#define __GFP_BITS_SHIFT (27 + IS_ENABLED(CONFIG_LOCKDEP))

If the above suggestion is implemented, this could be changed to
something like __GFP_LAST_BIT (the enum's last identifier).

Ack.

Thanks for reviewing!
Suren.

Petr T