[PATCH net-next] page_pool: always add GFP_NOWARN for ATOMIC allocations

[Jakub Kicinski <kuba@kernel.org>]

Driver authors often forget to add GFP_NOWARN for page allocation
from the datapath. This is annoying to operators as OOMs are a fact
of life, and we pretty much expect network Rx to hit page allocation
failures during OOM. Make page pool add GFP_NOWARN for ATOMIC allocations
by default.

Don't compare to GFP_ATOMIC because it's a mask with 2 bits set.
We want a single bit so that the compiler can do an unconditional
mask and shift. clang builds the condition as:

    1c31: 89 e8                        	movl	%ebp, %eax
    1c33: 83 e0 20                     	andl	$0x20, %eax
    1c36: c1 e0 0d                     	shll	$0xd, %eax
    1c39: 09 e8                        	orl	%ebp, %eax

so there seems to be no need any more to use the old flag multiplication
tricks which is less readable. Pick the lowest bit out of GFP_ATOMIC
to limit the size of the instructions.

The specific change which makes me propose this is that bnxt, after
commit cd1fafe7da1f ("eth: bnxt: add support rx side device memory TCP"),
lost the GFP_NOWARN, again. It used to allocate with page_pool_dev_alloc_*
which added the NOWARN unconditionally. While switching to
__bnxt_alloc_rx_netmem() authors forgot to add NOWARN in the explicitly
specified flags.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
---
CC: hawk@kernel.org
CC: ilias.apalodimas@linaro.org
CC: nathan@kernel.org
CC: nick.desaulniers+lkml@gmail.com
CC: morbo@google.com
CC: justinstitt@google.com
CC: llvm@lists.linux.dev
---
 net/core/page_pool.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/net/core/page_pool.c b/net/core/page_pool.c
index ba70569bd4b0..6ffce0e821e4 100644
--- a/net/core/page_pool.c
+++ b/net/core/page_pool.c
@@ -555,6 +555,13 @@ static noinline netmem_ref __page_pool_alloc_netmems_slow(struct page_pool *pool
 	netmem_ref netmem;
 	int i, nr_pages;
 
+	/* Unconditionally set NOWARN if allocating from the datapath.
+	 * Use a single bit from the ATOMIC mask to help compiler optimize.
+	 */
+	BUILD_BUG_ON(!(GFP_ATOMIC & __GFP_HIGH));
+	if (gfp & __GFP_HIGH)
+		gfp |= __GFP_NOWARN;
+
 	/* Don't support bulk alloc for high-order pages */
 	if (unlikely(pp_order))
 		return page_to_netmem(__page_pool_alloc_page_order(pool, gfp));
-- 
2.51.0