linux-kernel - Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

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Date:   Mon, 11 Apr 2022 07:10:43 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     Zhangfei Gao <zhangfei.gao@...mail.com>,
        Joerg Roedel <joro@...tes.org>,
        Fenghua Yu <fenghua.yu@...el.com>,
        jean-philippe <jean-philippe@...aro.org>
Cc:     Ravi V Shankar <ravi.v.shankar@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        x86 <x86@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v4 05/11] iommu/sva: Assign a PASID to mm on PASID
 allocation and free it on mm exit

On 4/11/22 07:00, Zhangfei Gao wrote:
> with this patchset, each time after  sbin/nginx, ioasid is freed
> immediately. lynx test will alloc the same ioasid=1.

That doesn't seem right.  Isn't 'sbin/nginx' still running when lynx
runs?  How can they get the same ioasid?

This sounds like a refcounting problem, like that the ioasid wasn't
properly refcounted as nginx forked into the background.

> To verify, hack comment mm_pasid_drop in __mmput will make the issue
> disappear.
> 
> log: after sbin/nginx.
> [   96.526730] Call trace:
> [   96.526732]  dump_backtrace+0xe4/0xf0
> [   96.526741]  show_stack+0x20/0x70
> [   96.526744]  dump_stack_lvl+0x8c/0xb8
> [   96.526751]  dump_stack+0x18/0x34
> [   96.526754]  ioasid_free+0xdc/0xfc
> [   96.526757]  mmput+0x138/0x160
> [   96.526760]  do_exit+0x284/0x9d0
> [   96.526765]  do_group_exit+0x3c/0xa8
> [   96.526767]  __wake_up_parent+0x0/0x38
> [   96.526770]  invoke_syscall+0x4c/0x110
> [   96.526775]  el0_svc_common.constprop.0+0x68/0x128
> [   96.526778]  do_el0_svc+0x2c/0x90
> [   96.526781]  el0_svc+0x30/0x98
> [   96.526783]  el0t_64_sync_handler+0xb0/0xb8
> [   96.526785]  el0t_64_sync+0x18c/0x190

Is there nothing before this call trace?  Usually there will be at least
some warning text.