Date:   Mon, 6 Feb 2023 05:09:57 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Peter Xu <peterx@redhat.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-arm-kernel@lists.infradead.org, linux-s390@vger.kernel.org
Subject: Re: [PATCH 0/3] mm/arch: Fix a few collide definition on private use
 of VM_FAULT_*
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On Sun, Feb 05, 2023 at 10:18:30PM -0500, Peter Xu wrote:
> On Mon, Feb 06, 2023 at 02:51:18AM +0000, Matthew Wilcox wrote:
> > That wasn't what I meant.  I meant putting VM_FAULT_BADMAP and
> > VM_FAULT_SIGSEGV in mm_types.h.  Not having "Here is a range of reserved
> > arch private ones".
> 
> VM_FAULT_SIGSEGV is there already; I assume you meant adding them all
> directly into vm_fault_reason.
> 
> Then I don't think it's a good idea..
> 
> Currently vm_fault_reason is a clear interface for handle_mm_fault() for
> not only arch pffault handlers but also soft faults like GUP.
> 
> If handle_mm_fault() doesn't return VM_FAULT_BADMAP at all, I don't think
> we should have it as public API at all.  When arch1 people reading the
> VM_FAULT_ documents, it shouldn't care about some fault reason that only
> happens with arch2.  Gup shouldn't care about it either.
> 
> Logically a new page fault handler should handle all the retval of
> vm_fault_reason afaiu.  That shouldn't include e.g. VM_FAULT_BADMAP either.

Hmm, right.  Looking specifically at how s390 uses VM_FAULT_BADMAP,
it just seems to be a badly structured fault.c.  Seems to me that
do_fault_error() should take an extra si_code argument, and
instead of returning VM_FAULT_BADACCESS / VM_FAULT_BADMAP from
various functions, those functions should call do_fault_error()
directly, passing it VM_FAULT_SIGSEGV and the appropriate si_code.

But this is all on the s390 people to fix; I don't want to break their
arch by trying it myself.