On 2/17/21 11:32 AM, Peter Xu wrote:
> On Wed, Feb 10, 2021 at 04:03:19PM -0800, Mike Kravetz wrote:
>> hugetlb fault processing code would COW all write faults where the
>> pte was not writable. Soft dirty will write protect ptes as part
>> of it's tracking mechanism. The existing hugetlb_cow code will do
>> the right thing for PRIVATE mappings as it checks map_count. However,
>> for SHARED mappings it would actually allocate and install a COW page.
>> Modify the code to not call hugetlb_cow for SHARED mappings and just
>> update the pte.
>>
>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>> ---
>> mm/hugetlb.c | 23 ++++++++++++++++-------
>> 1 file changed, 16 insertions(+), 7 deletions(-)
>>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 47f3123afd1a..b561b6867ec1 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -4584,8 +4584,10 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>> * spinlock. For private mappings, we also lookup the pagecache
>> * page now as it is used to determine if a reservation has been
>> * consumed.
>> + * Only non-shared mappings are sent to hugetlb_cow.
>> */
>> - if ((flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
>> + if ((flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry) &&
>> + !(vma->vm_flags & VM_SHARED)) {
>> if (vma_needs_reservation(h, vma, haddr) < 0) {
>> ret = VM_FAULT_OOM;
>> goto out_mutex;
>> @@ -4593,9 +4595,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>> /* Just decrements count, does not deallocate */
>> vma_end_reservation(h, vma, haddr);
>>
>> - if (!(vma->vm_flags & VM_MAYSHARE))
>> - pagecache_page = hugetlbfs_pagecache_page(h,
>> - vma, haddr);
>> + pagecache_page = hugetlbfs_pagecache_page(h, vma, haddr);
>
> Pure question: I see that the check actually changed from VM_MAYSHARE into
> VM_SHARE, then I noticed I'm actually unclear on the difference.. Say, when
> VM_MAYSHARE is set, could VM_SHARED be cleared in any case? Or say, is this
> change intended?
The change was not intended. I will use VM_MAYSHARE.
>
> I see that vma_set_page_prot() tried to remove VM_SHARED if soft dirty enabled
> (which should cause vma_wants_writenotify() to return true, iiuc), however
> that's temporary just to calculate vm_page_prot, and it's not applied to the
> vma->vm_flags. I failed to find a place where VM_SHARED of the vma is cleared
> while VM_MAYSHARE is set..
I am not 100% sure about differences. Here is a snippet from do_mmap() where
you can have VM_MAYSHARE and not VM_SHARED
vm_flags |= VM_SHARED | VM_MAYSHARE;
if (!(file->f_mode & FMODE_WRITE))
vm_flags &= ~(VM_MAYWRITE | VM_SHARED);
fallthrough;
>
>> }
>>
>> ptl = huge_pte_lock(h, mm, ptep);
>> @@ -4620,9 +4620,18 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
>>
>> if (flags & FAULT_FLAG_WRITE) {
>> if (!huge_pte_write(entry)) {
>> - ret = hugetlb_cow(mm, vma, address, ptep,
>> - pagecache_page, ptl);
>> - goto out_put_page;
>> + if (!(vma->vm_flags & VM_SHARED)) {
>> + ret = hugetlb_cow(mm, vma, address, ptep,
>> + pagecache_page, ptl);
>> + goto out_put_page;
>> + }
>> +
>> + /* write protected for soft dirty processing */
>> + if ((vma->vm_flags & VM_WRITE) &&
>
> This VM_WRITE check seems to be redundant. As example, do_user_addr_fault() of
> x86 code will check this right after vma lookup by access_error(). So when
> reach here if "flags & FAULT_FLAG_WRITE", then VM_WRITE must be set, imho.
Thanks, that sounds reasonable. I will check to make sure and drop the
redundant check.
>
>> + (vma->vm_flags & VM_SHARED))
>> + entry = huge_pte_mkwrite(entry);
>
> Same question to VM_SHARED, since "(vma->vm_flags & VM_SHARED)" is just checked
> above and we'll go hugetlb_cow() otherwise.
Yes, certainly redundant here.
>
>> +
>> + entry = huge_pte_mkdirty(entry);
>
> There's another huge_pte_mkdirty() right below; likely we could merge them somehow?
>
Yes,
Thanks for taking a look!
--
Mike Kravetz
> Thanks,
>
>> }
>> entry = huge_pte_mkdirty(entry);
>> }
>> --
>> 2.29.2
>>
>
