linux-kernel - RE: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios

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Message-ID: <ecd1c723930ed436a0b85652c95d453254664af3.camel@ibm.com>
Date: Mon, 8 Sep 2025 18:40:38 +0000
From: Viacheslav Dubeyko <Slava.Dubeyko@....com>
To: "max.kellermann@...os.com" <max.kellermann@...os.com>
CC: Alex Markuze <amarkuze@...hat.com>,
        "idryomov@...il.com"
	<idryomov@...il.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>,
        "ceph-devel@...r.kernel.org" <ceph-devel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Xiubo Li
	<xiubli@...hat.com>
Subject: RE: [PATCH] fs/ceph/addr: always call ceph_shift_unused_folios_left()

On Fri, 2025-09-05 at 20:35 +0200, Max Kellermann wrote:
> On Fri, Sep 5, 2025 at 7:11 PM Viacheslav Dubeyko <Slava.Dubeyko@....com> wrote:
> > 
> > On Fri, 2025-09-05 at 05:41 +0200, Max Kellermann wrote:
> > > Thanks, I'm glad you could verify the bug and my fix. In case this
> > > wasn't clear: you saw just a warning, but this is usually a kernel
> > > crash due to NULL pointer dereference. If you only got a warning but
> > > no crash, it means your test VM does not use transparent huge pages
> > > (no huge_zero_folio allocated yet). In a real workload, the kernel
> > > would have crashed.
> > 
> > I would like to reproduce the crash. But you've share only these steps.
> > And it looks like that it's not the complete recipe. So, something was missing.
> > If you could share more precise explanation of steps, it will be great.
> 
> The email you just cited explains the circumstances that are necessary
> for the crash to occur.
> 
> Let me repeat it for you: you have to ensure that huge_zero_folio gets
> allocated (or else the code that dereferences the NULL pointer and
> crashes gets skipped).
> 
> Got it now?

Not yet. I would like to see the clear explanation of steps.
Otherwise, the probability to reproduce the crash is equal to zero.

Thanks,
Slava.