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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

On 2022/7/26 21:09, Matthew Wilcox wrote:
> 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;

There are many vmf->vma, so better to add 'struct vm_area_struct *vma = 
vmf->vma;' and

use vma directly

>> +
>> +	/* 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;
return ret；
>> +
>> +	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);

MM_SHMEMPAGES -> mm_counter_file(page)

>> +	}
>> +
directly use GFP_TRANSHUGE_LIGHT？
>> +	new_page = &vma_alloc_folio(gfp_flags, HPAGE_PMD_ORDER,
>> +			vmf->vma, haddr, true)->page;
>> +	if (!new_page)
add   count_vm_event(THP_FAULT_FALLBACK);
>> +		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
>>
>>
> .