On Tue, 2 Feb 2021, Axel Rasmussen wrote: > For background, mm/userfaultfd.c provides a general mcopy_atomic > implementation. But some types of memory (e.g., hugetlb and shmem) need > a slightly different implementation, so they provide their own helpers > for this. In other words, userfaultfd is the only caller of this > function. > > This patch achieves two things: > > 1. Don't spend time compiling code which will end up never being > referenced anyway (a small build time optimization). > > 2. In future patches (e.g. [1]), we plan to extend the signature of > these helpers with UFFD-specific state (e.g., enums or structs defined > conditionally in userfaultfd_k.h). Once this happens, this patch will be > needed to avoid build errors (or, we'd need to define more UFFD-only > stuff unconditionally, which seems messier to me). > > Peter Xu suggested this be sent as a standalone patch, in the mailing > list discussion for [1]. > > [1] https://patchwork.kernel.org/project/linux-mm/list/?series=424091 > > Signed-off-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx> > --- > include/linux/hugetlb.h | 4 ++++ > mm/hugetlb.c | 2 ++ > 2 files changed, 6 insertions(+) Hi Axel, please also do the same to mm/shmem.c (perhaps you missed it because I did that long ago to our internal copy of mm/shmem.c). But please also comment the endifs #endif /* CONFIG_USERFAULTFD */ to help find one's way around them. I see you've done include/linux/hugetlb.h: okay, that's not necessary, but a matter of taste; up to you whether to do include/linux/shmem_fs.h. Thanks, Hugh > > diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h > index ebca2ef02212..749701b5c153 100644 > --- a/include/linux/hugetlb.h > +++ b/include/linux/hugetlb.h > @@ -134,11 +134,13 @@ void hugetlb_show_meminfo(void); > unsigned long hugetlb_total_pages(void); > vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > unsigned long address, unsigned int flags); > +#ifdef CONFIG_USERFAULTFD > int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, pte_t *dst_pte, > struct vm_area_struct *dst_vma, > unsigned long dst_addr, > unsigned long src_addr, > struct page **pagep); > +#endif > int hugetlb_reserve_pages(struct inode *inode, long from, long to, > struct vm_area_struct *vma, > vm_flags_t vm_flags); > @@ -308,6 +310,7 @@ static inline void hugetlb_free_pgd_range(struct mmu_gather *tlb, > BUG(); > } > > +#ifdef CONFIG_USERFAULTFD > static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, > pte_t *dst_pte, > struct vm_area_struct *dst_vma, > @@ -318,6 +321,7 @@ static inline int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, > BUG(); > return 0; > } > +#endif > > static inline pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr, > unsigned long sz) > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 18f6ee317900..821bfa9c0c80 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4615,6 +4615,7 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma, > return ret; > } > > +#ifdef CONFIG_USERFAULTFD > /* > * Used by userfaultfd UFFDIO_COPY. Based on mcopy_atomic_pte with > * modifications for huge pages. > @@ -4745,6 +4746,7 @@ int hugetlb_mcopy_atomic_pte(struct mm_struct *dst_mm, > put_page(page); > goto out; > } > +#endif > > long follow_hugetlb_page(struct mm_struct *mm, struct vm_area_struct *vma, > struct page **pages, struct vm_area_struct **vmas, > -- > 2.30.0.365.g02bc693789-goog
