Re: [PATCH 1/2] iommu/iova: Make the rcache depot scale better

From: Srivastava, Dheeraj Kumar
Date: Mon Aug 21 2023 - 04:12:31 EST

Hello Robin,

On 8/14/2023 11:23 PM, Robin Murphy wrote:
The algorithm in the original paper specifies the storage of full
magazines in the depot as an unbounded list rather than a fixed-size
array. It turns out to be pretty straightforward to do this in our
implementation with no significant loss of efficiency. This allows
the depot to scale up to the working set sizes of larger systems,
while also potentially saving some memory on smaller ones too.

Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
---
drivers/iommu/iova.c | 65 ++++++++++++++++++++++++--------------------
1 file changed, 36 insertions(+), 29 deletions(-)

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 10b964600948..d2de6fb0e9f4 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -625,10 +625,16 @@ EXPORT_SYMBOL_GPL(reserve_iova);
* will be wasted.
*/
#define IOVA_MAG_SIZE 127
-#define MAX_GLOBAL_MAGS 32 /* magazines per bin */
struct iova_magazine {
- unsigned long size;
+ /*
+ * Only full magazines are inserted into the depot, so we can avoid
+ * a separate list head and preserve maximum space-efficiency.
+ */
+ union {
+ unsigned long size;
+ struct iova_magazine *next;
+ };
unsigned long pfns[IOVA_MAG_SIZE];
};
@@ -640,8 +646,7 @@ struct iova_cpu_rcache {
struct iova_rcache {
spinlock_t lock;
- unsigned long depot_size;
- struct iova_magazine *depot[MAX_GLOBAL_MAGS];
+ struct iova_magazine *depot;
struct iova_cpu_rcache __percpu *cpu_rcaches;
};
@@ -717,6 +722,21 @@ static void iova_magazine_push(struct iova_magazine *mag, unsigned long pfn)
mag->pfns[mag->size++] = pfn;
}
+static struct iova_magazine *iova_depot_pop(struct iova_rcache *rcache)
+{
+ struct iova_magazine *mag = rcache->depot;
+
+ rcache->depot = mag->next;

While doing routine domain change test for a device ("unbind device from driver -> change domain of the device -> bind device back to the driver"), i ran into the following NULL pointer dereferencing issue.

[ 599.020261] BUG: kernel NULL pointer dereference, address: 0000000000000000
[ 599.020986] #PF: supervisor read access in kernel mode
[ 599.021588] #PF: error_code(0x0000) - not-present page
[ 599.022180] PGD 0 P4D 0
[ 599.022770] Oops: 0000 [#1] PREEMPT SMP NOPTI
[ 599.023365] CPU: 68 PID: 3122 Comm: avocado Not tainted 6.5.0-rc6-ChngDomainIssue #16
[ 599.023970] Hardware name: Dell Inc. PowerEdge R6515/07PXPY, BIOS 2.3.6 07/06/2021
[ 599.024571] RIP: 0010:free_iova_rcaches+0x9c/0x110
[ 599.025170] Code: d1 ff 39 05 36 d2 bc 01 48 89 c3 77 b4 49 8b 7f 10 e8 b8 69 93 ff 49 8d 7f 20 e8 6f e4 6b ff eb 05 e8 48 ba 93 ff 49 8b 7f 08 <48> 8b 07 49 89 47 08 48 c7 07 7f 00 00 00 41 83 6f 04 01 48 85 ff
[ 599.026436] RSP: 0018:ffffb78b4c9f7c68 EFLAGS: 00010296
[ 599.027075] RAX: ffffffff9fa8c100 RBX: 0000000000000080 RCX: 0000000000000005
[ 599.027719] RDX: 0000000000000000 RSI: 000000007fffffff RDI: 0000000000000000
[ 599.028359] RBP: ffffb78b4c9f7c98 R08: 0000000000000000 R09: 0000000000000006
[ 599.028995] R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
[ 599.029636] R13: ffff93910d9c6008 R14: ffffd78b3bde1000 R15: ffff9391144ebc00
[ 599.030283] FS: 00007fa5c9e5c000(0000) GS:ffff93cf72700000(0000) knlGS:0000000000000000
[ 599.030941] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 599.031588] CR2: 0000000000000000 CR3: 000000013f526006 CR4: 0000000000770ee0
[ 599.032237] PKRU: 55555554
[ 599.032878] Call Trace:
[ 599.033512] <TASK>
[ 599.034140] ? show_regs+0x6e/0x80
[ 599.034769] ? __die+0x29/0x70
[ 599.035393] ? page_fault_oops+0x154/0x4a0
[ 599.036021] ? __x86_return_thunk+0x9/0x10
[ 599.036647] ? do_user_addr_fault+0x318/0x6b0
[ 599.037258] ? __x86_return_thunk+0x9/0x10
[ 599.037866] ? __slab_free+0xc7/0x320
[ 599.038472] ? exc_page_fault+0x7d/0x190
[ 599.039074] ? asm_exc_page_fault+0x2b/0x30
[ 599.039683] ? free_iova_rcaches+0x9c/0x110
[ 599.040286] ? free_iova_rcaches+0x91/0x110
[ 599.040875] ? __x86_return_thunk+0x9/0x10
[ 599.041460] put_iova_domain+0x32/0xa0
[ 599.042041] iommu_put_dma_cookie+0x177/0x1b0
[ 599.042620] iommu_domain_free+0x1f/0x50
[ 599.043194] iommu_setup_default_domain+0x2fb/0x420
[ 599.043774] iommu_group_store_type+0xb6/0x210
[ 599.044362] iommu_group_attr_store+0x21/0x40
[ 599.044938] sysfs_kf_write+0x42/0x50
[ 599.045511] kernfs_fop_write_iter+0x143/0x1d0
[ 599.046084] vfs_write+0x2c2/0x3f0
[ 599.046653] ksys_write+0x6b/0xf0
[ 599.047219] __x64_sys_write+0x1d/0x30
[ 599.047782] do_syscall_64+0x60/0x90
[ 599.048346] ? syscall_exit_to_user_mode+0x2a/0x50
[ 599.048911] ? __x64_sys_lseek+0x1c/0x30
[ 599.049465] ? __x86_return_thunk+0x9/0x10
[ 599.050013] ? do_syscall_64+0x6d/0x90
[ 599.050562] ? do_syscall_64+0x6d/0x90
[ 599.051098] ? do_syscall_64+0x6d/0x90
[ 599.051625] ? __x86_return_thunk+0x9/0x10
[ 599.052149] ? exc_page_fault+0x8e/0x190
[ 599.052665] entry_SYSCALL_64_after_hwframe+0x73/0xdd
[ 599.053180] RIP: 0033:0x7fa5c9d14a6f
[ 599.053670] Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 19 c0 f7 ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 5c c0 f7 ff 48
[ 599.054672] RSP: 002b:00007ffdeb05f540 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
[ 599.055182] RAX: ffffffffffffffda RBX: 0000557de3275a80 RCX: 00007fa5c9d14a6f
[ 599.055692] RDX: 0000000000000003 RSI: 0000557de418ff60 RDI: 0000000000000011
[ 599.056203] RBP: 0000557de39a25e0 R08: 0000000000000000 R09: 0000000000000000
[ 599.056718] R10: 0000000000000000 R11: 0000000000000293 R12: 0000000000000003
[ 599.057227] R13: 00007fa5c9e5bf80 R14: 0000000000000011 R15: 0000557de418ff60
[ 599.057738] </TASK>
[ 599.058227] Modules linked in: xt_CHECKSUM xt_MASQUERADE xt_conntrack ipt_REJECT nf_reject_ipv4 xt_tcpudp nft_compat nft_chain_nat nf_nat nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 nf_tables nfnetlink bridge stp llc ipmi_ssif binfmt_misc nls_iso8859_1 intel_rapl_msr intel_rapl_common amd64_edac edac_mce_amd kvm_amd kvm dell_smbios dcdbas rapl dell_wmi_descriptor wmi_bmof joydev input_leds ccp ptdma k10temp acpi_ipmi ipmi_si acpi_power_meter mac_hid sch_fq_codel dm_multipath scsi_dh_rdac ipmi_devintf scsi_dh_emc ipmi_msghandler scsi_dh_alua msr ramoops reed_solomon pstore_blk pstore_zone efi_pstore ip_tables x_tables autofs4 hid_generic usbhid hid btrfs blake2b_generic raid10 raid456 async_raid6_recov async_memcpy async_pq async_xor async_tx xor raid6_pq libcrc32c raid1 raid0 multipath linear mgag200 i2c_algo_bit drm_shmem_helper drm_kms_helper crct10dif_pclmul crc32_pclmul ghash_clmulni_intel sha512_ssse3 aesni_intel crypto_simd cryptd nvme drm mpt3sas tg3 ahci nvme_core libahci xhci_pci raid_class i2c_piix4
[ 599.058423] xhci_pci_renesas scsi_transport_sas wmi
[ 599.064210] CR2: 0000000000000000
[ 599.064841] ---[ end trace 0000000000000000 ]---
--

Looking at the RIP: free_iova_rcaches+0x9c/0x110 pointed me to the above line leading me to believe we are popping element from an empty stack. Following hunk fixed the issue for me:

diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
index 76a7d694708e..899f1c2ba62a 100644
--- a/drivers/iommu/iova.c
+++ b/drivers/iommu/iova.c
@@ -732,6 +732,9 @@ static struct iova_magazine *iova_depot_pop(struct iova_rcache *rcache)
{
struct iova_magazine *mag = rcache->depot;

+ if (!mag)
+ return NULL;
+
rcache->depot = mag->next;
mag->size = IOVA_MAG_SIZE;
rcache->depot_size--;
--

+ mag->size = IOVA_MAG_SIZE;
+ return mag;
+}
+

--
Thanks and Regards
Dheeraj Kumar Srivastava