Re: [RFC][PATCH] uprobe: support for private hugetlb mappings

[David Hildenbrand <david@redhat.com>]

On 26.04.24 02:09, Guillaume Morin wrote:
> On 25 Apr 21:56, David Hildenbrand wrote:
>>
>> On 25.04.24 17:19, Guillaume Morin wrote:
>>> On 24 Apr 23:00, David Hildenbrand wrote:
>>>>> One issue here is that FOLL_FORCE|FOLL_WRITE is not implemented for
>>>>> hugetlb mappings. However this was also on my TODO and I have a draft
>>>>> patch that implements it.
>>>>
>>>> Yes, I documented it back then and added sanity checks in GUP code to fence
>>>> it off. Shouldn't be too hard to implement (famous last words) and would be
>>>> the cleaner thing to use here once I manage to switch over to
>>>> FOLL_WRITE|FOLL_FORCE to break COW.
>>>
>>> Yes, my patch seems to be working. The hugetlb code is pretty simple.
>>> And it allows ptrace and the proc pid mem file to work on the executable
>>> private hugetlb mappings.
>>>
>>> There is one thing I am unclear about though. hugetlb enforces that
>>> huge_pte_write() is true on FOLL_WRITE in both the fault and
>>> follow_page_mask paths. I am not sure if we can simply assume in the
>>> hugetlb code that if the pte is not writable and this is a write fault
>>> then we're in the FOLL_FORCE|FOLL_WRITE case.  Or do we want to keep the
>>> checks simply not enforce it for FOLL_FORCE|FOLL_WRITE?
>>>
>>> The latter is more complicated in the fault path because there is no
>>> FAULT_FLAG_FORCE flag.
>>>
>>
>> I just pushed something to
>> 	https://github.com/davidhildenbrand/linux/tree/uprobes_cow
>>
>> Only very lightly tested so far. Expect the worst :)
> 
> 
> I'll try it out and send you the hugetlb bits
> 
>>
>> I still detest having the zapping logic there, but to get it all right I
>> don't see a clean way around that.
>>
>>
>> For hugetlb, we'd primarily have to implement the
>> mm_walk_ops->hugetlb_entry() callback (well, and FOLL_FORCE).
> 
> For FOLL_FORCE, heer is my draft. Let me know if this is what you had in
> mind.
> 
> 
> diff --git a/mm/gup.c b/mm/gup.c
> index 1611e73b1121..ac60e0ae64e8 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -1056,9 +1056,6 @@ static int check_vma_flags(struct vm_area_struct *vma, unsigned long gup_flags)
>   		if (!(vm_flags & VM_WRITE) || (vm_flags & VM_SHADOW_STACK)) {
>   			if (!(gup_flags & FOLL_FORCE))
>   				return -EFAULT;
> -			/* hugetlb does not support FOLL_FORCE|FOLL_WRITE. */
> -			if (is_vm_hugetlb_page(vma))
> -				return -EFAULT;
>   			/*
>   			 * We used to let the write,force case do COW in a
>   			 * VM_MAYWRITE VM_SHARED !VM_WRITE vma, so ptrace could
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index 3548eae42cf9..73f86eddf888 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -5941,7 +5941,8 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   		       struct folio *pagecache_folio, spinlock_t *ptl,
>   		       struct vm_fault *vmf)
>   {
> -	const bool unshare = flags & FAULT_FLAG_UNSHARE;
> +	const bool make_writable = !(flags & FAULT_FLAG_UNSHARE) &&
> +		(vma->vm_flags & VM_WRITE);
>   	pte_t pte = huge_ptep_get(ptep);
>   	struct hstate *h = hstate_vma(vma);
>   	struct folio *old_folio;
> @@ -5959,16 +5960,9 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   	 * can trigger this, because hugetlb_fault() will always resolve
>   	 * uffd-wp bit first.
>   	 */
> -	if (!unshare && huge_pte_uffd_wp(pte))
> +	if (make_writable && huge_pte_uffd_wp(pte))
>   		return 0;
>   
> -	/*
> -	 * hugetlb does not support FOLL_FORCE-style write faults that keep the
> -	 * PTE mapped R/O such as maybe_mkwrite() would do.
> -	 */
> -	if (WARN_ON_ONCE(!unshare && !(vma->vm_flags & VM_WRITE)))
> -		return VM_FAULT_SIGSEGV;
> -
>   	/* Let's take out MAP_SHARED mappings first. */
>   	if (vma->vm_flags & VM_MAYSHARE) {
>   		set_huge_ptep_writable(vma, haddr, ptep);
> @@ -5989,7 +5983,7 @@ static vm_fault_t hugetlb_wp(struct mm_struct *mm, struct vm_area_struct *vma,
>   			folio_move_anon_rmap(old_folio, vma);
>   			SetPageAnonExclusive(&old_folio->page);
>   		}
> -		if (likely(!unshare))
> +		if (likely(make_writable))
>   			set_huge_ptep_writable(vma, haddr, ptep);

Maybe we want to refactor that similarly into a 
set_huge_ptep_maybe_writable, and handle the VM_WRITE check internally.

Then, here you'd do

if (unshare)
	set_huge_ptep(vma, haddr, ptep);
else
	set_huge_ptep_maybe_writable(vma, haddr, ptep);

Something like that.



>   		/* Break COW or unshare */
>   		huge_ptep_clear_flush(vma, haddr, ptep);
> @@ -6883,6 +6878,17 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
>   }
>   #endif /* CONFIG_USERFAULTFD */
>   
> +static bool is_force_follow(struct vm_area_struct* vma, unsigned int flags,
> +			     struct page* page) {
> +	if (vma->vm_flags & VM_WRITE)
> +		return false;
> +
> +	if (!(flags & FOLL_FORCE))
> +		return false;
> +
> +	return page && PageAnon(page) && page_mapcount(page) == 1;
> +}

A couple of points:

a) Don't use page_mapcount(). Either folio_mapcount(), but likely you 
want to check PageAnonExclusive.

b) If you're not following the can_follow_write_pte/_pmd model, you are 
doing something wrong :)

c) The code was heavily changed in mm/mm-unstable. It was merged with t
the common code.

Likely, in mm/mm-unstable, the existing can_follow_write_pte and 
can_follow_write_pmd checks will already cover what you want in most cases.

We'd need a can_follow_write_pud() to cover follow_huge_pud() and 
(unfortunately) something to handle follow_hugepd() as well similarly.

Copy-pasting what we do in can_follow_write_pte() and adjusting for 
different PTE types is the right thing to do. Maybe now it's time to 
factor out the common checks into a separate helper.


-- 
Cheers,

David / dhildenb