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

["zhangpeng (AS)" <zhangpeng362@huawei.com>]

On 2023/11/23 23:33, Matthew Wilcox wrote:

> On Thu, Nov 23, 2023 at 05:09:04PM +0800, zhangpeng (AS) wrote:
>>>> +		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;
>>>> +		}
>>>> +
>>> Need to deal with ptep == NULL.  Although that is high impossible.
>> If ptep == NULL, we may just need to return VM_FAULT_SIGBUS.
>> I'll add it in the next version.
> no?  wouldn't ptep being NULL mean that the ptep has been replaced with
> a PMD entry, and thus should return NOPAGE?

Yes, ptep == NULL means that the ptep has been replaced with a PMD entry.
I'll add return NOPAGE in the next version.

Thanks!

-- 
Best Regards,
Peng