[PATCH] mm/page_alloc: drop flag-conversion "optimisation"

[Brendan Jackman <jackmanb@google.com>]

This code uses flag equivalences to try to optimise conversion from
GFP_ to ALLOC_ but there's no clear reason to believe it makes things
faster. Even if it gets rid of conditional branches, it just trades them
for a data dependency.

CPUs are pretty good at conditional branches. But, in my GCC x86 build
it doesn't look like there are any branches anyway, the compiler found
some conditional instruction tricks. (Caveat: This was extracted &
annotated by Gemini AI, I did not actually read the disasm myself)

Old code:

        ae50:    8b 04 24                 mov    (%rsp),%eax           # Load gfp_mask
        ...
        ae5d:    41 89 c4                 mov    %eax,%r12d
        ae64:    41 81 e4 20 08 00 00     and    $0x820,%r12d          # Mask both flags at once
        ...
        ae6f:    44 89 e1                 mov    %r12d,%ecx
        ae77:    83 c9 40                 or     $0x40,%ecx            # OR with ALLOC_CPUSET (0x40)
        ae7a:    89 4c 24 60              mov    %ecx,0x60(%rsp)       # Store to alloc_flags

New code:

  For  __GFP_HIGH  ( 0x20 ):
  It uses the Carry Flag (via  sbb ) to conditionally add  0x20  to the base  0x40  ( ALLOC_CPUSET ) flag:

        ae63:    83 e0 20                 and    $0x20,%eax            # Test __GFP_HIGH
        ...
        ae6a:    83 f8 01                 cmp    $0x1,%eax             # Set carry flag if 0
        ae6f:    45 19 e4                 sbb    %r12d,%r12d           # %r12d = (gfp & 0x20) ? 0 : -1
        ae80:    41 83 e4 e0              and    $0xffffffe0,%r12d     # %r12d = (gfp & 0x20) ? 0 : -32
        ae87:    41 83 c4 60              add    $0x60,%r12d           # %r12d = (gfp & 0x20) ? 0x60 : 0x40

  For  __GFP_KSWAPD_RECLAIM  ( 0x800 ):
  It uses a conditional move ( cmov ) later in the function to set the  ALLOC_KSWAPD  ( 0x800 ) bit:

        ae72:    25 00 08 00 00           and    $0x800,%eax           # Test __GFP_KSWAPD_RECLAIM
        ae77:    89 44 24 30              mov    %eax,0x30(%rsp)       # Store result
        ...
        af2c:    80 cf 08                 or     $0x8,%bh              # Set ALLOC_KSWAPD (0x800) in temp reg
        af2f:    45 85 c9                 test   %r9d,%r9d             # Check if __GFP_KSWAPD_RECLAIM was set
        af32:    0f 44 d8                 cmove  %eax,%ebx             # If not, revert to flags without it

Testing with a modified version[0] of lib/free_pages_test.c (adding
printks with timing)...

[0] https://github.com/bjackman/aethelred/blob/2ccdc84ef087c2a631914f58e106e99e19bd3b98/page-alloc-test/page-alloc-test.c

Old results from a Sapphire Rapids consumer CPU:

[   67.157118] page_alloc_test: Testing with GFP_KERNEL
[   67.157122] page_alloc_test: Starting 1,000,000 allocations...
[   70.704446] page_alloc_test: Completed. Time: 3543002 us (Avg: 3543.00 ns per alloc+free loop)
[   70.704456] page_alloc_test: Testing with GFP_KERNEL | __GFP_COMP
[   70.704460] page_alloc_test: Starting 1,000,000 allocations...
[   70.944672] page_alloc_test: Completed. Time: 239980 us (Avg: 239.98 ns per alloc+free loop)
[   70.944675] page_alloc_test: Test completed

New results:

[   70.079015] page_alloc_test: Testing with GFP_KERNEL
[   70.079020] page_alloc_test: Starting 1,000,000 allocations...
[   73.669396] page_alloc_test: Completed. Time: 3586954 us (Avg: 3586.95 ns per alloc+free loop)
[   73.669402] page_alloc_test: Testing with GFP_KERNEL | __GFP_COMP
[   73.669405] page_alloc_test: Starting 1,000,000 allocations...
[   73.905084] page_alloc_test: Completed. Time: 235496 us (Avg: 235.49 ns per alloc+free loop)
[   73.905086] page_alloc_test: Test completed

Seems like a wash.

So, drop the flag value coupling here and let the compiler and CPU do
their job. Superscalar CPUs are pretty neat after all.

(Used AI for the disasm but the rest is all manual).

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 mm/page_alloc.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ee902a468c2f5..9e1949ea13a6d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -4478,22 +4478,16 @@ gfp_to_alloc_flags(gfp_t gfp_mask, unsigned int order)
 {
 	unsigned int alloc_flags = ALLOC_WMARK_MIN | ALLOC_CPUSET;
 
-	/*
-	 * __GFP_HIGH is assumed to be the same as ALLOC_MIN_RESERVE
-	 * and __GFP_KSWAPD_RECLAIM is assumed to be the same as ALLOC_KSWAPD
-	 * to save two branches.
-	 */
-	BUILD_BUG_ON(__GFP_HIGH != (__force gfp_t) ALLOC_MIN_RESERVE);
-	BUILD_BUG_ON(__GFP_KSWAPD_RECLAIM != (__force gfp_t) ALLOC_KSWAPD);
-
 	/*
 	 * The caller may dip into page reserves a bit more if the caller
 	 * cannot run direct reclaim, or if the caller has realtime scheduling
 	 * policy or is asking for __GFP_HIGH memory.  GFP_ATOMIC requests will
 	 * set both ALLOC_NON_BLOCK and ALLOC_MIN_RESERVE(__GFP_HIGH).
 	 */
-	alloc_flags |= (__force int)
-		(gfp_mask & (__GFP_HIGH | __GFP_KSWAPD_RECLAIM));
+	if (gfp_mask & __GFP_HIGH)
+		alloc_flags |= ALLOC_MIN_RESERVE;
+	if (gfp_mask & __GFP_KSWAPD_RECLAIM)
+		alloc_flags |= ALLOC_KSWAPD;
 
 	if (!(gfp_mask & __GFP_DIRECT_RECLAIM)) {
 		/*

---
base-commit: ca2351ac6da277a470d4fcf122b53267e02b2716
change-id: 20260612-gfp-pessimisation-b258a4bb5ebd

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