Re: [PATCH 07/19] mm/fork: Accept huge pfnmap entries

[David Hildenbrand <david@redhat.com>]

On 09.08.24 19:15, Peter Xu wrote:
> On Fri, Aug 09, 2024 at 06:32:44PM +0200, David Hildenbrand wrote:
>> On 09.08.24 18:08, Peter Xu wrote:
>>> Teach the fork code to properly copy pfnmaps for pmd/pud levels.  Pud is
>>> much easier, the write bit needs to be persisted though for writable and
>>> shared pud mappings like PFNMAP ones, otherwise a follow up write in either
>>> parent or child process will trigger a write fault.
>>>
>>> Do the same for pmd level.
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>>> ---
>>>    mm/huge_memory.c | 27 ++++++++++++++++++++++++---
>>>    1 file changed, 24 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
>>> index 6568586b21ab..015c9468eed5 100644
>>> --- a/mm/huge_memory.c
>>> +++ b/mm/huge_memory.c
>>> @@ -1375,6 +1375,22 @@ int copy_huge_pmd(struct mm_struct *dst_mm, struct mm_struct *src_mm,
>>>    	pgtable_t pgtable = NULL;
>>>    	int ret = -ENOMEM;
>>> +	pmd = pmdp_get_lockless(src_pmd);
>>> +	if (unlikely(pmd_special(pmd))) {
>>> +		dst_ptl = pmd_lock(dst_mm, dst_pmd);
>>> +		src_ptl = pmd_lockptr(src_mm, src_pmd);
>>> +		spin_lock_nested(src_ptl, SINGLE_DEPTH_NESTING);
>>> +		/*
>>> +		 * No need to recheck the pmd, it can't change with write
>>> +		 * mmap lock held here.
>>> +		 */
>>> +		if (is_cow_mapping(src_vma->vm_flags) && pmd_write(pmd)) {
>>> +			pmdp_set_wrprotect(src_mm, addr, src_pmd);
>>> +			pmd = pmd_wrprotect(pmd);
>>> +		}
>>> +		goto set_pmd;
>>> +	}
>>> +
>>
>> I strongly assume we should be using using vm_normal_page_pmd() instead of
>> pmd_page() further below. pmd_special() should be mostly limited to GUP-fast
>> and vm_normal_page_pmd().
> 
> One thing to mention that it has this:
> 
> 	if (!vma_is_anonymous(dst_vma))
> 		return 0;

Another obscure thing in this function. It's not the job of 
copy_huge_pmd() to make the decision whether to copy, it's the job of 
vma_needs_copy() in copy_page_range().

And now I have to suspect that uffd-wp is broken with this function, 
because as vma_needs_copy() clearly states, we must copy, and we don't 
do that for PMDs. Ugh.

What a mess, we should just do what we do for PTEs and we will be fine ;)

Also, we call copy_huge_pmd() only if "is_swap_pmd(*src_pmd) || 
pmd_trans_huge(*src_pmd) || pmd_devmap(*src_pmd)"

Would that even be the case with PFNMAP? I suspect that pmd_trans_huge() 
would return "true" for special pfnmap, which is rather "surprising", 
but fortunate for us.

Likely we should be calling copy_huge_pmd() if pmd_leaf() ... cleanup 
for another day.

> 
> So it's only about anonymous below that.  In that case I feel like the
> pmd_page() is benign, and actually good.

Yes, it would likely currently work.

> 
> Though what you're saying here made me notice my above check doesn't seem
> to be necessary, I mean, "(is_cow_mapping(src_vma->vm_flags) &&
> pmd_write(pmd))" can't be true when special bit is set, aka, pfnmaps.. and
> if it's writable for CoW it means it's already an anon.
> 
> I think I can probably drop that line there, perhaps with a
> VM_WARN_ON_ONCE() making sure it won't happen.
> 
>>
>> Again, we should be doing this similar to how we handle PTEs.
>>
>> I'm a bit confused about the "unlikely(!pmd_trans_huge(pmd)" check, below:
>> what else should we have here if it's not a migration entry but a present
>> entry?
> 
> I had a feeling that it was just a safety belt since the 1st day of thp
> when Andrea worked that out, so that it'll work with e.g. file truncation
> races.
> 
> But with current code it looks like it's only anonymous indeed, so looks
> not possible at least from that pov.

Yes, as stated above, likely broken with UFFD-WP ...

I really think we should make this code just behave like it would with 
PTEs, instead of throwing in more "different" handling.

-- 
Cheers,

David / dhildenb