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?
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..
> }
>
> 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.
> + (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.
> +
> + entry = huge_pte_mkdirty(entry);
There's another huge_pte_mkdirty() right below; likely we could merge them somehow?
Thanks,
> }
> entry = huge_pte_mkdirty(entry);
> }
> --
> 2.29.2
>
--
Peter Xu
