Re: [PATCH v1 2/2] mm/hugetlb: support write-faults in shared mappings

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 05, 2022 at 01:03:29PM +0200, David Hildenbrand wrote:
> Let's add a safety net if we ever get (again) a write-fault on a R/O-mapped
> page in a shared mapping, in which case we simply have to map the
> page writable.
> 
> VM_MAYSHARE handling in hugetlb_fault() for FAULT_FLAG_WRITE
> indicates that this was at least envisioned, but could never have worked
> as expected. This theoretically paves the way for softdirty tracking
> support in hugetlb.
> 
> Tested without the fix for softdirty tracking.
> 
> Note that there is no need to do any kind of reservation in hugetlb_fault()
> in this case ... because we already have a hugetlb page mapped R/O
> that we will simply map writable and we are not dealing with COW/unsharing.
> 
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  mm/hugetlb.c | 21 ++++++++++++++-------
>  1 file changed, 14 insertions(+), 7 deletions(-)
> 
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index a18c071c294e..bbab7aa9d8f8 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5233,6 +5233,16 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>  	VM_BUG_ON(unshare && (flags & FOLL_WRITE));
>  	VM_BUG_ON(!unshare && !(flags & FOLL_WRITE));
>  
> +	/* Let's take out shared mappings first, this should be a rare event. */
> +	if (unlikely(vma->vm_flags & VM_MAYSHARE)) {

Should we check VM_SHARED instead?

> +		if (unshare)
> +			return 0;

Curious when will this happen especially if we switch to VM_SHARED above.
Shouldn't "unshare" not happen at all on a shared region?

> +		if (WARN_ON_ONCE(!(vma->vm_flags & VM_WRITE)))
> +			return VM_FAULT_SIGSEGV;

I had a feeling that you just want to double check we have write
permission, but IIUC this should be checked far earlier or we'll have
problem.  No strong opinion if so, but I'd suggest dropping this one,
otherwise we could add tons of WARN_ON_ONCE() in anywhere in the page fault
stack and they mostly won't trigger at all.

> +		set_huge_ptep_writable(vma, haddr, ptep);

Do we wanna set dirty bits too?

> +		return 0;
> +	}

-- 
Peter Xu