Re: [RFC PATCH v3 3/4] mm: add do_set_pte_range()

[Chih-En Lin <shiyn.lin@gmail.com>]

On Fri, Feb 03, 2023 at 09:38:15PM +0800, Yin, Fengwei wrote:
> 
> 
> On 2/3/2023 9:32 PM, Chih-En Lin wrote:
> > On Fri, Feb 03, 2023 at 09:16:35PM +0800, Yin Fengwei wrote:
> >> do_set_pte_range() allows to setup page table entries for a
> >> specific range. It calls folio_add_file_rmap_range() to take
> >> advantage of batched rmap update for large folio.
> >>
> >> Signed-off-by: Yin Fengwei <fengwei.yin@intel.com>
> >> ---
> >>  include/linux/mm.h |  3 +++
> >>  mm/filemap.c       |  1 -
> >>  mm/memory.c        | 59 ++++++++++++++++++++++++++++++----------------
> >>  3 files changed, 42 insertions(+), 21 deletions(-)
> >>
> >> diff --git a/include/linux/mm.h b/include/linux/mm.h
> >> index d6f8f41514cc..93192f04b276 100644
> >> --- a/include/linux/mm.h
> >> +++ b/include/linux/mm.h
> >> @@ -1162,6 +1162,9 @@ static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
> >>  
> >>  vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page);
> >>  void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr);
> >> +void do_set_pte_range(struct vm_fault *vmf, struct folio *folio,
> >> +		unsigned long addr, pte_t *pte,
> >> +		unsigned long start, unsigned int nr);
> >>  
> >>  vm_fault_t finish_fault(struct vm_fault *vmf);
> >>  vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf);
> >> diff --git a/mm/filemap.c b/mm/filemap.c
> >> index f444684db9f2..74046a3a0ff5 100644
> >> --- a/mm/filemap.c
> >> +++ b/mm/filemap.c
> >> @@ -3386,7 +3386,6 @@ static vm_fault_t filemap_map_folio_range(struct vm_fault *vmf,
> >>  
> >>  		ref_count++;
> >>  		do_set_pte(vmf, page, addr);
> >> -		update_mmu_cache(vma, addr, vmf->pte);
> >>  	} while (vmf->pte++, page++, addr += PAGE_SIZE, ++count < nr_pages);
> >>  
> >>  	/* Restore the vmf->pte */
> >> diff --git a/mm/memory.c b/mm/memory.c
> >> index 7a04a1130ec1..3754b2ef166a 100644
> >> --- a/mm/memory.c
> >> +++ b/mm/memory.c
> >> @@ -4257,36 +4257,58 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> >>  }
> >>  #endif
> >>  
> >> -void do_set_pte(struct vm_fault *vmf, struct page *page, unsigned long addr)
> >> +void do_set_pte_range(struct vm_fault *vmf, struct folio *folio,
> >> +		unsigned long addr, pte_t *pte,
> >> +		unsigned long start, unsigned int nr)
> >>  {
> >>  	struct vm_area_struct *vma = vmf->vma;
> >>  	bool uffd_wp = pte_marker_uffd_wp(vmf->orig_pte);
> >>  	bool write = vmf->flags & FAULT_FLAG_WRITE;
> >> +	bool cow = write && !(vma->vm_flags & VM_SHARED);
> > 
> > Why don't use is_cow_mapping()?
> > Is there anything messed up with VM_MAYWRITE?
> My understanding is it's not related with the mapping. It's related with
> what operation triggers fault here. Say, if it's a writable mapping, and
> if the read operation triggers fault here, no cow or maybe_mkwrite() needed
> here. Thanks.

Sorry, I didn't describe the thing properly.
It makes sense for the relationship with the write/read fault.
I'm just wondering if "!(vma->vm_flags & VM_SHARED)" is enough to determine
the COW page? And, I also found it in do_fault().

Like, copy_present_pte() use is_cow_mapping() for COW page and
"vm_flags & VM_SHARED" for shared mapping.

So, I looked up the commit that introduced the is_cow_mapping(),
67121172f9753 ("Allow arbitrary read-only shared pfn-remapping too").

Here is the commit message:
"
    The VM layer (for historical reasons) turns a read-only shared mmap into
    a private-like mapping with the VM_MAYWRITE bit clear.  Thus checking
    just VM_SHARED isn't actually sufficient.
    
    So use a trivial helper function for the cases where we wanted to inquire
    if a mapping was COW-like or not.
"

hmm, but it is v2.6.15.
So is "vm_flags & VM_SHARED" enough to check the COW page now?

Thanks,
Chih-En Lin