linux-kernel - Re: [PATCH] vmalloc: Don't use flush flag when no exec perm

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Message-ID: <67241836-621c-6933-1278-f04aedcefcb3@linux.ee>
Date:   Thu, 30 May 2019 10:44:47 +0300
From:   Meelis Roos <mroos@...ux.ee>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Cc:     "daniel@...earbox.net" <daniel@...earbox.net>,
        "jeyu@...nel.org" <jeyu@...nel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "ard.biesheuvel@....com" <ard.biesheuvel@....com>,
        "will.deacon@....com" <will.deacon@....com>,
        "namit@...are.com" <namit@...are.com>,
        "luto@...nel.org" <luto@...nel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>
Subject: Re: [PATCH] vmalloc: Don't use flush flag when no exec perm

>> The addition of VM_FLUSH_RESET_PERMS for BPF JIT allocations was
>> bisected to prevent boot on an UltraSparc III machine. It was found
>> that
>> sometime shortly after the TLB flush this flag does on vfree of the
>> BPF
>> program, the machine hung. Further investigation showed that before
>> any of
>> the changes for this flag were introduced, with
>> CONFIG_DEBUG_PAGEALLOC
>> configured (which does a similar TLB flush of the vmalloc range on
>> every vfree), this machine also hung shortly after the first vmalloc
>> unmap/free.
>>
>> So the evidence points to there being some existing issue with the
>> vmalloc TLB flushes, but it's still unknown exactly why these hangs
>> are
>> happening on sparc. It is also unknown when someone with this
>> hardware
>> could resolve this, and in the meantime using this flag on it turns a
>> lurking behavior into something that prevents boot.
> 
> The sparc TLB flush issue has been bisected and is being worked on now,
> so hopefully we won't need this patch:
> https://marc.info/?l=linux-sparc&m=155915694304118&w=2

And the sparc64 patch that fixes CONFIG_DEBUG_PAGEALLOC also fixes booting
of the latest git kernel on Sun V445 where my problem initially happened.

-- 
Meelis Roos <mroos@...ux.ee>