| Message ID | 20241003035333.49261-1-mhklinux@outlook.com (mailing list archive) |
|---|---|
| Headers | show |
| Series | hyper-v: Don't assume cpu_possible_mask is dense | expand |
On Wed, Oct 02, 2024 at 08:53:28PM -0700, mhkelley58@gmail.com wrote: > > Michael Kelley (5): > x86/hyperv: Don't assume cpu_possible_mask is dense > Drivers: hv: Don't assume cpu_possible_mask is dense > iommu/hyper-v: Don't assume cpu_possible_mask is dense > scsi: storvsc: Don't assume cpu_possible_mask is dense > hv_netvsc: Don't assume cpu_possible_mask is dense Thanks, these look sane. Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Hello: This series was applied to netdev/net-next.git (main) by Jakub Kicinski <kuba@kernel.org>: On Wed, 2 Oct 2024 20:53:28 -0700 you wrote: > From: Michael Kelley <mhklinux@outlook.com> > > Code specific to Hyper-V guests currently assumes the cpu_possible_mask > is "dense" -- i.e., all bit positions 0 thru (nr_cpu_ids - 1) are set, > with no "holes". Therefore, num_possible_cpus() is assumed to be equal > to nr_cpu_ids. > > [...] Here is the summary with links: - [1/5] x86/hyperv: Don't assume cpu_possible_mask is dense (no matching commit) - [2/5] Drivers: hv: Don't assume cpu_possible_mask is dense (no matching commit) - [3/5] iommu/hyper-v: Don't assume cpu_possible_mask is dense (no matching commit) - [4/5] scsi: storvsc: Don't assume cpu_possible_mask is dense (no matching commit) - [net-next,5/5] hv_netvsc: Don't assume cpu_possible_mask is dense https://git.kernel.org/netdev/net-next/c/c86ab60b92d1 You are awesome, thank you!
On Fri, 04 Oct 2024 23:20:40 +0000 patchwork-bot+netdevbpf@kernel.org wrote: > - [net-next,5/5] hv_netvsc: Don't assume cpu_possible_mask is dense > https://git.kernel.org/netdev/net-next/c/c86ab60b92d1 On reflection I probably should have asked you for a fixes tag and applied it to net :S Oh well.
From: Jakub Kicinski <kuba@kernel.org> Sent: Friday, October 4, 2024 4:26 PM > > On Fri, 04 Oct 2024 23:20:40 +0000 patchwork-bot+netdevbpf@kernel.org > wrote: > > - [net-next,5/5] hv_netvsc: Don't assume cpu_possible_mask is dense > > > https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=c86ab60b92d1 > > On reflection I probably should have asked you for a fixes tag and > applied it to net :S Oh well. I thought about including a Fixes: tag, but decided that since current and past builds don't exhibit a failure due to this issue, it's not necessary to backport. The patches are more for robustness against future changes. I've heard differing views on whether changes that aren't fixing an actual failure should be backported .... Also, patchwork bot email says that the *series* was applied to net-next. But appears that's not the case. Only the network-related patch from the series was applied. Still need the SCSI maintainer to pick up the SCSI patch, and the Hyper-V maintainer to pick up the others. Thanks, Michael
From: Michael Kelley <mhklinux@outlook.com> Code specific to Hyper-V guests currently assumes the cpu_possible_mask is "dense" -- i.e., all bit positions 0 thru (nr_cpu_ids - 1) are set, with no "holes". Therefore, num_possible_cpus() is assumed to be equal to nr_cpu_ids. Per a separate discussion[1], this assumption is not valid in the general case. For example, the function setup_nr_cpu_ids() in kernel/smp.c is coded to assume cpu_possible_mask may be sparse, and other patches have been made in the past to correctly handle the sparseness. See bc75e99983df1efd ("rcu: Correctly handle sparse possible cpu") as noted by Mark Rutland. The general case notwithstanding, the configurations that Hyper-V provides to guest VMs on x86 and ARM64 hardware, in combination with the algorithms currently used by architecture specific code to assign Linux CPU numbers, *does* always produce a dense cpu_possible_mask. So the invalid assumption is not currently causing failures. But in the interest of correctness, and robustness against future changes in the code that populates cpu_possible_mask, update the Hyper-V code to no longer assume denseness. The typical code pattern with the invalid assumption is as follows: array = kcalloc(num_possible_cpus(), sizeof(<some struct>), GFP_KERNEL); .... index into "array" with smp_processor_id() In such as case, the array might be indexed by a value beyond the size of the array. The correct approach is to allocate the array with size "nr_cpu_ids". While this will probably leave unused any array entries corresponding to holes in cpu_possible_mask, the holes are assumed to be minimal and hence the amount of memory wasted by unused entries is minimal. Removing the assumption in Hyper-V code is done in several patches because they touch different kernel subsystems: Patch 1: Hyper-V x86 initialization of hv_vp_assist_page (there's no hv_vp_assist_page on ARM64) Patch 2: Hyper-V common init of hv_vp_index Patch 3: Hyper-V IOMMU driver Patch 4: storvsc driver Patch 5: netvsc driver I tested the changes by hacking the construction of cpu_possible_mask to include a hole on x86. With a configuration set to demonstrate the problem, a Hyper-V guest kernel eventually crashes due to memory corruption. After the patches in this series, the crash does not occur. [1] https://lore.kernel.org/lkml/SN6PR02MB4157210CC36B2593F8572E5ED4692@SN6PR02MB4157.namprd02.prod.outlook.com/ Michael Kelley (5): x86/hyperv: Don't assume cpu_possible_mask is dense Drivers: hv: Don't assume cpu_possible_mask is dense iommu/hyper-v: Don't assume cpu_possible_mask is dense scsi: storvsc: Don't assume cpu_possible_mask is dense hv_netvsc: Don't assume cpu_possible_mask is dense arch/x86/hyperv/hv_init.c | 2 +- drivers/hv/hv_common.c | 4 ++-- drivers/iommu/hyperv-iommu.c | 4 ++-- drivers/net/hyperv/netvsc_drv.c | 2 +- drivers/scsi/storvsc_drv.c | 13 ++++++------- 5 files changed, 12 insertions(+), 13 deletions(-)