Re: [RFC PATCH] mm: filemap: avoid unnecessary major faults in filemap_fault()

[Yin Fengwei <fengwei.yin@intel.com>]

Hi Peng,

On 11/22/23 22:00, Peng Zhang wrote:
> From: ZhangPeng <zhangpeng362@huawei.com>
> 
> The major fault occurred when using mlockall(MCL_CURRENT | MCL_FUTURE)
> in application, which leading to an unexpected performance issue[1].
> 
> This caused by temporarily cleared pte during a read/modify/write update
> of the pte, eg, do_numa_page()/change_pte_range().
> 
> For the data segment of the user-mode program, the global variable area
> is a private mapping. After the pagecache is loaded, the private anonymous
> page is generated after the COW is triggered. Mlockall can lock COW pages
> (anonymous pages), but the original file pages cannot be locked and may
> be reclaimed. If the global variable (private anon page) is accessed when
> vmf->pte is zeroed in numa fault, a file page fault will be triggered.
> 
> At this time, the original private file page may have been reclaimed.
> If the page cache is not available at this time, a major fault will be
> triggered and the file will be read, causing additional overhead.
> 
> Fix this by rechecking the pte by holding ptl in filemap_fault() before
> triggering a major fault.
> 
> [1] https://lore.kernel.org/linux-mm/9e62fd9a-bee0-52bf-50a7-498fa17434ee@huawei.com/
> 
> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
> Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
> ---
>  mm/filemap.c | 14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 71f00539ac00..bb5e6a2790dc 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -3226,6 +3226,20 @@ vm_fault_t filemap_fault(struct vm_fault *vmf)
>  			mapping_locked = true;
>  		}
>  	} else {
> +		pte_t *ptep = pte_offset_map_lock(vmf->vma->vm_mm, vmf->pmd,
> +						  vmf->address, &vmf->ptl);
> +		if (ptep) {
> +			/*
> +			 * Recheck pte with ptl locked as the pte can be cleared
> +			 * temporarily during a read/modify/write update.
> +			 */
> +			if (unlikely(!pte_none(ptep_get(ptep))))
> +				ret = VM_FAULT_NOPAGE;
> +			pte_unmap_unlock(ptep, vmf->ptl);
> +			if (unlikely(ret))
> +				return ret;
> +		}
I am curious. Did you try not to take PTL here and just check whether PTE is not NONE?


Regards
Yin, Fengwei

> +
>  		/* No page in the page cache at all */
>  		count_vm_event(PGMAJFAULT);
>  		count_memcg_event_mm(vmf->vma->vm_mm, PGMAJFAULT);