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 07:36:26 -0700
From:   Dave Hansen <dave.hansen@...el.com>
To:     "zhangfei.gao@...mail.com" <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 07:20, zhangfei.gao@...mail.com wrote:
>> Is there nothing before this call trace?  Usually there will be at least
>> some warning text.
> I added dump_stack() in ioasid_free.

Hold on a sec, though...

What's the *problem* here?  Did something break or are you just saying
that something looks weird to _you_?

For instance, if we have:

	nginx: ioasid_alloc()==1
	       fork(); // spawn the daemon
	       exit();
	       ioasid_free(1);

and then a moment later:

	lynx:  ioasid_alloc()==1
	       fork();
	       exit();
	       ioasid_free(1);

There's no problem.  The original nginx process with ioasid==1 exited.
The fact that *some* process called nginx is still running doesn't mean
that it still has a reference to asid==1.

Are you sure the nginx process that allocated the ASID is the same
process you see in ps?