netdev - Re: [PATCH v2] vmalloc: Fix issues with flush flag

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Date:   Wed, 22 May 2019 22:40:02 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "davem@...emloft.net" <davem@...emloft.net>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "mroos@...ux.ee" <mroos@...ux.ee>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "namit@...are.com" <namit@...are.com>,
        "luto@...nel.org" <luto@...nel.org>, "bp@...en8.de" <bp@...en8.de>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>
Subject: Re: [PATCH v2] vmalloc: Fix issues with flush flag

On Wed, 2019-05-22 at 12:26 -0700, Rick Edgecombe wrote:
> On Wed, 2019-05-22 at 10:40 -0700, David Miller wrote:
> > From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
> > Date: Tue, 21 May 2019 01:59:54 +0000
> > 
> > > On Mon, 2019-05-20 at 18:43 -0700, David Miller wrote:
> > > > From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
> > > > Date: Tue, 21 May 2019 01:20:33 +0000
> > > > 
> > > > > Should it handle executing an unmapped page gracefully?
> > > > > Because
> > > > > this
> > > > > change is causing that to happen much earlier. If something
> > > > > was
> > > > > relying
> > > > > on a cached translation to execute something it could find
> > > > > the
> > > > > mapping
> > > > > disappear.
> > > > 
> > > > Does this work by not mapping any kernel mappings at the
> > > > beginning,
> > > > and then filling in the BPF mappings in response to faults?
> > > No, nothing too fancy. It just flushes the vm mapping immediatly
> > > in
> > > vfree for execute (and RO) mappings. The only thing that happens
> > > around
> > > allocation time is setting of a new flag to tell vmalloc to do
> > > the
> > > flush.
> > > 
> > > The problem before was that the pages would be freed before the
> > > execute
> > > mapping was flushed. So then when the pages got recycled, random,
> > > sometimes coming from userspace, data would be mapped as
> > > executable
> > > in
> > > the kernel by the un-flushed tlb entries.
> > 
> > If I am to understand things correctly, there was a case where
> > 'end'
> > could be smaller than 'start' when doing a range flush.  That would
> > definitely kill some of the sparc64 TLB flush routines.
> 
> Ok, thanks.
> 
> The patch at the beginning of this thread doesn't have that behavior
> though and it apparently still hung. I asked if Meelis could test
> with
> this feature disabled and DEBUG_PAGEALLOC on, since it flushes on
> every
> vfree and is not new logic, and also with a patch that logs exact TLB
> flush ranges and fault addresses on top of the kernel having this
> issue. Hopefully that will shed some light.
> 
> Sorry for all the noise and speculation on this. It has been
> difficult
> to debug remotely with a tester and developer in different time
> zones.
> 
> 
Ok, so with a patch to disable setting the new vmalloc flush flag on
architectures that have normal memory as executable (includes sparc),
boot succeeds.

With this disable patch and DEBUG_PAGEALLOC on, it hangs earlier than
before. Going from clues in other logs, it looks like it hangs right at
the first normal vfree.

Thanks for all the testing Meelis!

So it seems like other, not new, TLB flushes also trigger the hang.

From earlier logs provided, this vfree would be the first call to
flush_tlb_kernel_range(), and before any BPF allocations appear in the
logs. So I am suspecting some other cause than the bisected patch at
this point, but I guess it's not fully conclusive.

It could be informative to bisect upstream again with the
DEBUG_PAGEALLOC configs on, to see if it indeed points to an earlier
commit.