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

[Gerald Schaefer <gerald.schaefer@linux.ibm.com>]

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.

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