Message-ID: <gjuqvidcpvzwqrwogeoygwnsbvlpa4fvsvaoq6rlfzcq4wxmh5@tdhz3f2fm4ga>
Date: Mon, 6 Jan 2025 13:07:58 +1100
From: Alistair Popple <apopple@...dia.com>
To: David Hildenbrand <david@...hat.com>
Cc: akpm@...ux-foundation.org, dan.j.williams@...el.com, 
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Subject: Re: [PATCH v4 12/25] mm/memory: Enhance
 insert_page_into_pte_locked() to create writable mappings

On Fri, Dec 20, 2024 at 08:06:48PM +0100, David Hildenbrand wrote:
> On 20.12.24 20:01, David Hildenbrand wrote:
> > On 17.12.24 06:12, Alistair Popple wrote:
> > > In preparation for using insert_page() for DAX, enhance
> > > insert_page_into_pte_locked() to handle establishing writable
> > > mappings.  Recall that DAX returns VM_FAULT_NOPAGE after installing a
> > > PTE which bypasses the typical set_pte_range() in finish_fault.
> > > 
> > > Signed-off-by: Alistair Popple <apopple@...dia.com>
> > > Suggested-by: Dan Williams <dan.j.williams@...el.com>
> > > 
> > > ---
> > > 
> > > Changes since v2:
> > > 
> > >    - New patch split out from "mm/memory: Add dax_insert_pfn"
> > > ---
> > >    mm/memory.c | 45 +++++++++++++++++++++++++++++++++++++--------
> > >    1 file changed, 37 insertions(+), 8 deletions(-)
> > > 
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 06bb29e..cd82952 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -2126,19 +2126,47 @@ static int validate_page_before_insert(struct vm_area_struct *vma,
> > >    }
> > >    static int insert_page_into_pte_locked(struct vm_area_struct *vma, pte_t *pte,
> > > -			unsigned long addr, struct page *page, pgprot_t prot)
> > > +				unsigned long addr, struct page *page,
> > > +				pgprot_t prot, bool mkwrite)
> > >    {
> > >    	struct folio *folio = page_folio(page);
> > > +	pte_t entry = ptep_get(pte);
> > >    	pte_t pteval;
> > > -	if (!pte_none(ptep_get(pte)))
> > > -		return -EBUSY;
> > > +	if (!pte_none(entry)) {
> > > +		if (!mkwrite)
> > > +			return -EBUSY;
> > > +
> > > +		/*
> > > +		 * For read faults on private mappings the PFN passed in may not
> > > +		 * match the PFN we have mapped if the mapped PFN is a writeable
> > > +		 * COW page.  In the mkwrite case we are creating a writable PTE
> > > +		 * for a shared mapping and we expect the PFNs to match. If they
> > > +		 * don't match, we are likely racing with block allocation and
> > > +		 * mapping invalidation so just skip the update.
> > > +		 */
> > 
> > Would it make sense to instead have here
> > 
> > /* See insert_pfn(). */
> > 
> > But ...
> > 
> > > +		if (pte_pfn(entry) != page_to_pfn(page)) {
> > > +			WARN_ON_ONCE(!is_zero_pfn(pte_pfn(entry)));
> > > +			return -EFAULT;
> > > +		}
> > > +		entry = maybe_mkwrite(entry, vma);
> > > +		entry = pte_mkyoung(entry);
> > > +		if (ptep_set_access_flags(vma, addr, pte, entry, 1))
> > > +			update_mmu_cache(vma, addr, pte);
> > 
> > ... I am not sure if we want the above at all. Someone inserted a page,
> > which is refcounted + mapcounted already.
> > 
> > Now you ignore that and do like the second insertion "worked" ?
> > 
> > No, that feels wrong, I suspect you will run into refcount+mapcount issues.
> > 
> > If there is already something, inserting must fail IMHO. If you want to
> > change something to upgrade write permissions, then a different
> > interface should be used.
> 
> Ah, now I realize that the early exit saves you because we won't adjust the
> refcount +mapcount.

Right.
 
> I still wonder if that really belongs in here, I would prefer to not play
> such tricks to upgrade write permissions if possible.

As you have pointed out this was all inspired (ie. mostly copied)
from the existing insert_pfn() implementation which is used from
vmf_insert_mixed{_mkwrite}().

I agree a different interface to upgrade permissions would be nice. However
it's tricky because in general callers of these functions (eg. FS DAX) aren't
aware if the page is already mapped by a PTE/PMD. They only know a fault has
occured and the faulting permissions.

This wouldn't be impossible to fix - the mm does provide vm_ops->page_mkwrite()
for permission upgrades. The difficulty is that most filesystems that support
FS DAX (ie. ext4, XFS) don't treat a vm_ops->page_mkwrite() call any differently
from a vm_ops->fault() call due to write fault. Therefore the FS DAX code is
unaware of whether or not this is a permission upgrade or initial writeable
mapping of the page in the VMA.

A further issue in there is currently no vm_ops->huge_mkwrite() callback.

Obviously this could all be plumbed through the MM/FS layers, but that would
require a separate patch series. Given the current implementation has no issues
beyond the cosmetic I'd rather not delay this series any longer, especially as
the cosmetic defect is largely pre-existing (vmf_insert_mixed{_mkwrite}() could
have equally had a separate upgrade interface).

> --
>
> Cheers,
> 
> David / dhildenb
> 

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