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

[David Hildenbrand <david@redhat.com>]

On Mon, Aug 15, 2022 at 3:36 PM Gerald Schaefer
<gerald.schaefer@linux.ibm.com> wrote:
>
> On Thu, 11 Aug 2022 11:59:09 -0700
> Mike Kravetz <mike.kravetz@oracle.com> wrote:
>
> > On 08/11/22 12:34, David Hildenbrand wrote:
> > > If we ever get a write-fault on a write-protected page in a shared mapping,
> > > we'd be in trouble (again). Instead, we can simply map the page writable.
> > >
> > <snip>
> > >
> > > Reason is that uffd-wp doesn't clear the uffd-wp PTE bit when
> > > unregistering and consequently keeps the PTE writeprotected. Reason for
> > > this is to avoid the additional overhead when unregistering. Note
> > > that this is the case also for !hugetlb and that we will end up with
> > > writable PTEs that still have the uffd-wp PTE bit set once we return
> > > from hugetlb_wp(). I'm not touching the uffd-wp PTE bit for now, because it
> > > seems to be a generic thing -- wp_page_reuse() also doesn't clear it.
> > >
> > > VM_MAYSHARE handling in hugetlb_fault() for FAULT_FLAG_WRITE
> > > indicates that MAP_SHARED handling was at least envisioned, but could never
> > > have worked as expected.
> > >
> > > While at it, make sure that we never end up in hugetlb_wp() on write
> > > faults without VM_WRITE, because we don't support maybe_mkwrite()
> > > semantics as commonly used in the !hugetlb case -- for example, in
> > > wp_page_reuse().
> >
> > Nit,
> > to me 'make sure that we never end up in hugetlb_wp()' implies that
> > we would check for condition in callers as opposed to first thing in
> > hugetlb_wp().  However, I am OK with description as it.
>

Hi Gerald,

> Is that new WARN_ON_ONCE() in hugetlb_wp() meant to indicate a real bug?

Most probably, unless I am missing something important.

Something triggers FAULT_FLAG_WRITE on a VMA without VM_WRITE and
hugetlb_wp() would map the pte writable.
Consequently, we'd have a writable pte inside a VMA that does not have
write permissions, which is dubious. My check prevents that and bails
out.

Ordinary (!hugetlb) faults have maybe_mkwrite() (e.g., for FOLL_FORCE
or breaking COW) semantics such that we won't be mapping PTEs writable
if the VMA does not have write permissions.

I suspect that either

a) Some write fault misses a protection check and ends up triggering a
FAULT_FLAG_WRITE where we should actually fail early.

b) The write fault is valid and some VMA misses proper flags (VM_WRITE).

c) The write fault is valid (e.g., for breaking COW or FOLL_FORCE) and
we'd actually want maybe_mkwrite semantics.

> It is triggered by libhugetlbfs testcase "HUGETLB_ELFMAP=R linkhuge_rw"
> (at least on s390), and crashes our CI, because it runs with panic_on_warn
> enabled.
>
> Not sure if this means that we have bug elsewhere, allowing us to
> get to the WARN in hugetlb_wp().

That's what I suspect. Do you have a backtrace?

Note that I'm on vacation this week and might not reply as fast as usual.