linux-kernel - Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings

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Date:   Tue, 16 Aug 2022 22:43:37 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Gerald Schaefer <gerald.schaefer@...ux.ibm.com>
Cc:     Mike Kravetz <mike.kravetz@...cle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, stable <stable@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <hca@...ux.ibm.com>
Subject: Re: [PATCH v2 2/2] mm/hugetlb: support write-faults in shared mappings

Hi Gerald,

>
> Thanks, we were also trying to reproduce on x86, w/o success so far. But
> I guess that matches David latest observations wrt to our exception handling
> code on s390.
>
> Good news is that the problem goes away when I add this simple patch, which
> should result in proper VM_WRITE check for vma flags, before triggering a
> FAULT_FLAG_WRITE fault:
>
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -379,7 +379,9 @@ static inline vm_fault_t do_exception(struct pt_regs *regs, int access)
>         flags = FAULT_FLAG_DEFAULT;
>         if (user_mode(regs))
>                 flags |= FAULT_FLAG_USER;
> -       if (access == VM_WRITE || is_write)
> +       if (is_write)
> +               access = VM_WRITE;
> +       if (access == VM_WRITE)
>                 flags |= FAULT_FLAG_WRITE;
>         mmap_read_lock(mm);

That's what I had in mind, good.

>
> Still find it a bit hard to believe that this > 10 years old logic really
> is/was broken all the time. I guess it simply did not matter for normal
> PTE faults, probably because the common fault handling code later would
> check itself via maybe_mkwrite(). And for hugetlb PTEs, it might not have
> mattered before commit bcd51a3c679d.

It is akward, but maybe we never really noticed for hugetlb (not sure
how common read-only mappings are after all).

>
> >
> > bcd51a3c679d eliminates the copying of page tables at fork for non-anon
> > hugetlb vmas.  So, in these tests you would likely see more pte_none()
> > faults.
>
> Yes, makes sense, assuming now that it actually is related to s390
> exception handling code, not checking for VM_WRITE before triggering a
> write fault for pte_none().
>
> Thanks for checking! And Thanks a lot to David for finding that issue
> in s390 exception handling code!

Thanks! Looks like adding the WARN_ON_ONCE was the right decision.