On Tue, Jul 26, 2022 at 08:43:15PM +0800, Liu Zixian wrote:
> Transparent hugepage of tmpfs is useful to improve TLB miss, but
> it will be split during cow memory fault.
That's intentional. Possibly misguided, but there's a tradeoff to
be made between memory consumption and using large pages.
> This will happen if we mprotect and rewrite code segment (which is
> private file map) to hotpatch a running process.
>
> We can avoid the splitting by adding a huge_fault function.
>
> Signed-off-by: Liu Zixian <liuzixian4@xxxxxxxxxx>
> ---
> mm/shmem.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
> 1 file changed, 46 insertions(+)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index a6f565308..12b2b5140 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2120,6 +2120,51 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
> return ret;
> }
>
> +static vm_fault_t shmem_huge_fault(struct vm_fault *vmf, enum page_entry_size pe_size)
> +{
> + vm_fault_t ret = VM_FAULT_FALLBACK;
> + unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> + struct page *old_page, *new_page;
> + int gfp_flags = GFP_HIGHUSER_MOVABLE | __GFP_COMP;
> +
> + /* read or shared fault will not split huge pmd */
> + if (!(vmf->flags & FAULT_FLAG_WRITE)
> + || (vmf->vma->vm_flags & VM_SHARED))
> + return VM_FAULT_FALLBACK;
> + if (pe_size != PE_SIZE_PMD)
> + return VM_FAULT_FALLBACK;
> +
> + if (pmd_none(*vmf->pmd)) {
> + if (shmem_fault(vmf) & VM_FAULT_ERROR)
> + goto out;
> + if (!PageTransHuge(vmf->page))
> + goto out;
> + old_page = vmf->page;
> + } else {
> + old_page = pmd_page(*vmf->pmd);
> + page_remove_rmap(old_page, vmf->vma, true);
> + pmdp_huge_clear_flush(vmf->vma, haddr, vmf->pmd);
> + add_mm_counter(vmf->vma->vm_mm, MM_SHMEMPAGES, -HPAGE_PMD_NR);
> + }
> +
> + new_page = &vma_alloc_folio(gfp_flags, HPAGE_PMD_ORDER,
> + vmf->vma, haddr, true)->page;
> + if (!new_page)
> + goto out;
> + prep_transhuge_page(new_page);
vma_alloc_folio() does the prep_transhuge_page() for you.
> + copy_user_huge_page(new_page, old_page, haddr, vmf->vma, HPAGE_PMD_NR);
> + __SetPageUptodate(new_page);
> +
> + ret = do_set_pmd(vmf, new_page);
> +
> +out:
> + if (vmf->page) {
> + unlock_page(vmf->page);
> + put_page(vmf->page);
> + }
> + return ret;
> +}
> +
> unsigned long shmem_get_unmapped_area(struct file *file,
> unsigned long uaddr, unsigned long len,
> unsigned long pgoff, unsigned long flags)
> @@ -3884,6 +3929,7 @@ static const struct super_operations shmem_ops = {
>
> static const struct vm_operations_struct shmem_vm_ops = {
> .fault = shmem_fault,
> + .huge_fault = shmem_huge_fault,
> .map_pages = filemap_map_pages,
> #ifdef CONFIG_NUMA
> .set_policy = shmem_set_policy,
> --
> 2.33.0
>
>
