Re: [PATCH] shmem: support huge_fault to avoid pmd split

[Matthew Wilcox <willy@infradead.org>]

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@huawei.com>
> ---
>  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
> 
>