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Date:   Sat, 9 May 2020 09:41:55 -0400 (EDT)
From:   Alan Stern <stern@rowland.harvard.edu>
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To:     Thomas Gleixner <tglx@linutronix.de>
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Subject: Re: WARNING in memtype_reserve
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On Sat, 9 May 2020, Thomas Gleixner wrote:

> Greg KH <gregkh@linuxfoundation.org> writes:
> > On Sat, May 09, 2020 at 12:20:14AM -0700, syzbot wrote:
> >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
> >> WARNING: CPU: 1 PID: 7025 at arch/x86/mm/pat/memtype.c:589 memtype_reserve+0x69f/0x820 arch/x86/mm/pat/memtype.c:589
> >
> > So should memtype_reserve() not do a WARN if given invalid parameters as
> > it can be triggered by userspace requests?
> >
> > A normal "invalid request" debug line is probably all that is needed,
> > right?
> 
> I disagree. The callsite espcially if user space triggerable should not
> attempt to ask for a reservation where start > end:
> 
>   >> memtype_reserve failed: [mem 0xffffffffff000-0x00008fff], req write-back
> 
> The real question is which part of the call chain is responsible for
> this. That needs to be fixed.

What about all the other ways in which a reservation request could be
invalid?  The MM core already checks for these; what point is there in
duplicating these checks in many places higher up the call chain?

Alan Stern