netdev - [PATCH net] net: allow small head cache usage with large MAX_SKB

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Message-ID: <6bf54579233038bc0e76056c5ea459872ce362ab.1739375933.git.pabeni@redhat.com>
Date: Wed, 12 Feb 2025 18:42:32 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Simon Horman <horms@...nel.org>,
	Sabrina Dubroca <sd@...asysnail.net>
Subject: [PATCH net] net: allow small head cache usage with large MAX_SKB_FRAGS values

Sabrina reported the following splat:

    WARNING: CPU: 0 PID: 1 at net/core/dev.c:6935 netif_napi_add_weight_locked+0x8f2/0xba0
    Modules linked in:
    CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.14.0-rc1-net-00092-g011b03359038 #996
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Arch Linux 1.16.3-1-1 04/01/2014
    RIP: 0010:netif_napi_add_weight_locked+0x8f2/0xba0
    Code: e8 c3 e6 6a fe 48 83 c4 28 5b 5d 41 5c 41 5d 41 5e 41 5f c3 cc cc cc cc c7 44 24 10 ff ff ff ff e9 8f fb ff ff e8 9e e6 6a fe <0f> 0b e9 d3 fe ff ff e8 92 e6 6a fe 48 8b 04 24 be ff ff ff ff 48
    RSP: 0000:ffffc9000001fc60 EFLAGS: 00010293
    RAX: 0000000000000000 RBX: ffff88806ce48128 RCX: 1ffff11001664b9e
    RDX: ffff888008f00040 RSI: ffffffff8317ca42 RDI: ffff88800b325cb6
    RBP: ffff88800b325c40 R08: 0000000000000001 R09: ffffed100167502c
    R10: ffff88800b3a8163 R11: 0000000000000000 R12: ffff88800ac1c168
    R13: ffff88800ac1c168 R14: ffff88800ac1c168 R15: 0000000000000007
    FS:  0000000000000000(0000) GS:ffff88806ce00000(0000) knlGS:0000000000000000
    CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
    CR2: ffff888008201000 CR3: 0000000004c94001 CR4: 0000000000370ef0
    DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
    DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
    Call Trace:
    <TASK>
    gro_cells_init+0x1ba/0x270
    xfrm_input_init+0x4b/0x2a0
    xfrm_init+0x38/0x50
    ip_rt_init+0x2d7/0x350
    ip_init+0xf/0x20
    inet_init+0x406/0x590
    do_one_initcall+0x9d/0x2e0
    do_initcalls+0x23b/0x280
    kernel_init_freeable+0x445/0x490
    kernel_init+0x20/0x1d0
    ret_from_fork+0x46/0x80
    ret_from_fork_asm+0x1a/0x30
    </TASK>
    irq event stamp: 584330
    hardirqs last  enabled at (584338): [<ffffffff8168bf87>] __up_console_sem+0x77/0xb0
    hardirqs last disabled at (584345): [<ffffffff8168bf6c>] __up_console_sem+0x5c/0xb0
    softirqs last  enabled at (583242): [<ffffffff833ee96d>] netlink_insert+0x14d/0x470
    softirqs last disabled at (583754): [<ffffffff8317c8cd>] netif_napi_add_weight_locked+0x77d/0xba0

on kernel built with MAX_SKB_FRAGS=45.

In such built, SKB_WITH_OVERHEAD(1024) is smaller than GRO_MAX_HEAD, and
thus after commit 011b03359038 ("Revert "net: skb: introduce and use a
single page frag cache"") napi_get_frags() ends up using the page frag
allocator, triggering the splat.

Address the issue updating napi_alloc_skb() and __netdev_alloc_skb() to
allow kmalloc() usage for GRO_MAX_HEAD allocation, regardless of the
configured MAX_SKB_FRAGS value.

Note that even before the mentioned revert, __netdev_alloc_skb() was
not using the small head cache for GRO_MAX_HEAD-size allocation for
MAX_SKB_FRAGS=45 kernel build, which is sort of unexpected.

Reported-by: Sabrina Dubroca <sd@...asysnail.net>
Fixes: 011b03359038 ("Revert "net: skb: introduce and use a single page frag cache"")
Fixes: 3948b05950fd ("net: introduce a config option to tweak MAX_SKB_FRAGS")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
 net/core/skbuff.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 6a99c453397f..6afd99a2736d 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -661,7 +661,7 @@ struct sk_buff *__netdev_alloc_skb(struct net_device *dev, unsigned int len,
 	/* If requested length is either too small or too big,
 	 * we use kmalloc() for skb->head allocation.
 	 */
-	if (len <= SKB_WITH_OVERHEAD(1024) ||
+	if (len <= SKB_WITH_OVERHEAD(max(1024, SKB_SMALL_HEAD_CACHE_SIZE)) ||
 	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
 	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
 		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
@@ -739,7 +739,7 @@ struct sk_buff *napi_alloc_skb(struct napi_struct *napi, unsigned int len)
 	/* If requested length is either too small or too big,
 	 * we use kmalloc() for skb->head allocation.
 	 */
-	if (len <= SKB_WITH_OVERHEAD(1024) ||
+	if (len <= SKB_WITH_OVERHEAD(max(1024, SKB_SMALL_HEAD_CACHE_SIZE)) ||
 	    len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
 	    (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
 		skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX | SKB_ALLOC_NAPI,
-- 
2.48.1