Re: [PATCH v2 12/20] mm/mshare: prepare for page table sharing support

[Anthony Yznaga <anthony.yznaga@oracle.com>]

On 5/30/25 7:56 AM, Jann Horn wrote:
> On Fri, Apr 4, 2025 at 4:18 AM Anthony Yznaga <anthony.yznaga@oracle.com> wrote:
>> In preparation for enabling the handling of page faults in an mshare
>> region provide a way to link an mshare shared page table to a process
>> page table and otherwise find the actual vma in order to handle a page
>> fault. Modify the unmap path to ensure that page tables in mshare regions
>> are unlinked and kept intact when a process exits or an mshare region
>> is explicitly unmapped.
>>
>> Signed-off-by: Khalid Aziz <khalid@kernel.org>
>> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
>> Signed-off-by: Anthony Yznaga <anthony.yznaga@oracle.com>
> [...]
>> diff --git a/mm/memory.c b/mm/memory.c
>> index db558fe43088..68422b606819 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
> [...]
>> @@ -259,7 +260,10 @@ static inline void free_p4d_range(struct mmu_gather *tlb, pgd_t *pgd,
>>                  next = p4d_addr_end(addr, end);
>>                  if (p4d_none_or_clear_bad(p4d))
>>                          continue;
>> -               free_pud_range(tlb, p4d, addr, next, floor, ceiling);
>> +               if (unlikely(shared_pud))
>> +                       p4d_clear(p4d);
>> +               else
>> +                       free_pud_range(tlb, p4d, addr, next, floor, ceiling);
>>          } while (p4d++, addr = next, addr != end);
>>
>>          start &= PGDIR_MASK;
> [...]
>> +static void mshare_vm_op_unmap_page_range(struct mmu_gather *tlb,
>> +                               struct vm_area_struct *vma,
>> +                               unsigned long addr, unsigned long end,
>> +                               struct zap_details *details)
>> +{
>> +       /*
>> +        * The msharefs vma is being unmapped. Do not unmap pages in the
>> +        * mshare region itself.
>> +        */
>> +}
> 
> Unmapping a VMA has three major phases:
> 
> 1. unlinking the VMA from the VMA tree
> 2. removing the VMA contents
> 3. removing unneeded page tables
> 
> The MM subsystem broadly assumes that after phase 2, no stuff is
> mapped in the region anymore and therefore changes to the backing file
> don't need to TLB-flush this VMA anymore, and unlinks the mapping from
> rmaps and such. If munmap() of an mshare region only removes the
> mapping of shared page tables in step 3, as implemented here, that
> means things like TLB flushes won't be able to discover all
> currently-existing mshare mappings of a host MM through rmap walks.
> 
> I think it would make more sense to remove the links to shared page
> tables in step 2 (meaning in mshare_vm_op_unmap_page_range), just like
> hugetlb does, and not modify free_pgtables().

That makes sense. I'll make this change.

Thanks!

> 
>>   static const struct vm_operations_struct msharefs_vm_ops = {
>>          .may_split = mshare_vm_op_split,
>>          .mprotect = mshare_vm_op_mprotect,
>> +       .unmap_page_range = mshare_vm_op_unmap_page_range,
>>   };
>>
>>   /*
>> --
>> 2.43.5
>>