Re: mm/memory.c: Add update local tlb for smp race

[David Hildenbrand]

On 14.05.20 08:50, Bibo Mao wrote:
> If there are two threads hitting page fault at the address, one
> thread updates pte entry and local tlb, the other thread can update
> local tlb also, rather than give up and let page fault happening
> again.

Let me suggest

"mm/memory: optimize concurrent page faults at same address

If two threads concurrently fault at the same address, the thread that
won the race updates the PTE and its local TLB. For now, the other
thread gives up, simply does nothing, and continues.

It could happen that this second thread triggers another fault, whereby
it only updates its local TLB while handling the fault. Instead of
triggering another fault, let's directly update the local TLB of the
second thread.
"

If I got the intention of this patch correctly.

Are there any performance numbers to support this patch?

(I can't say too much about the correctness and/or usefulness of this patch)

> 
> 	modified:   mm/memory.c

This does not belong into a patch description.


> Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
> ---
>  mm/memory.c | 30 ++++++++++++++++++++++--------
>  1 file changed, 22 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index f703fe8..3a741ce 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2436,11 +2436,10 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>  		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
>  			/*
>  			 * Other thread has already handled the fault
> -			 * and we don't need to do anything. If it's
> -			 * not the case, the fault will be triggered
> -			 * again on the same address.
> +			 * and update local tlb only
>  			 */
>  			ret = false;
> +			update_mmu_cache(vma, addr, vmf->pte);
>  			goto pte_unlock;
>  		}
>  
> @@ -2463,8 +2462,9 @@ static inline bool cow_user_page(struct page *dst, struct page *src,
>  		vmf->pte = pte_offset_map_lock(mm, vmf->pmd, addr, &vmf->ptl);
>  		locked = true;
>  		if (!likely(pte_same(*vmf->pte, vmf->orig_pte))) {
> -			/* The PTE changed under us. Retry page fault. */
> +			/* The PTE changed under us. update local tlb */
>  			ret = false;
> +			update_mmu_cache(vma, addr, vmf->pte);
>  			goto pte_unlock;
>  		}
>  
> @@ -2704,6 +2704,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		}
>  		flush_cache_page(vma, vmf->address, pte_pfn(vmf->orig_pte));
>  		entry = mk_pte(new_page, vma->vm_page_prot);
> +		entry = pte_mkyoung(entry);
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>  		/*
>  		 * Clear the pte entry and flush it first, before updating the
> @@ -2752,6 +2753,7 @@ static vm_fault_t wp_page_copy(struct vm_fault *vmf)
>  		new_page = old_page;
>  		page_copied = 1;
>  	} else {
> +		update_mmu_cache(vma, vmf->address, vmf->pte);
>  		mem_cgroup_cancel_charge(new_page, memcg, false);
>  	}
>  
> @@ -2812,6 +2814,7 @@ vm_fault_t finish_mkwrite_fault(struct vm_fault *vmf)
>  	 * pte_offset_map_lock.
>  	 */
>  	if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> +		update_mmu_cache(vmf->vma, vmf->address, vmf->pte);
>  		pte_unmap_unlock(vmf->pte, vmf->ptl);
>  		return VM_FAULT_NOPAGE;
>  	}
> @@ -2936,6 +2939,7 @@ static vm_fault_t do_wp_page(struct vm_fault *vmf)
>  			vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>  					vmf->address, &vmf->ptl);
>  			if (!pte_same(*vmf->pte, vmf->orig_pte)) {
> +				update_mmu_cache(vma, vmf->address, vmf->pte);
>  				unlock_page(vmf->page);
>  				pte_unmap_unlock(vmf->pte, vmf->ptl);
>  				put_page(vmf->page);
> @@ -3341,8 +3345,10 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  						vma->vm_page_prot));
>  		vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd,
>  				vmf->address, &vmf->ptl);
> -		if (!pte_none(*vmf->pte))
> +		if (!pte_none(*vmf->pte)) {
> +			update_mmu_cache(vma, vmf->address, vmf->pte);
>  			goto unlock;
> +		}
>  		ret = check_stable_address_space(vma->vm_mm);
>  		if (ret)
>  			goto unlock;
> @@ -3373,13 +3379,16 @@ static vm_fault_t do_anonymous_page(struct vm_fault *vmf)
>  	__SetPageUptodate(page);
>  
>  	entry = mk_pte(page, vma->vm_page_prot);
> +	entry = pte_mkyoung(entry);
>  	if (vma->vm_flags & VM_WRITE)
>  		entry = pte_mkwrite(pte_mkdirty(entry));
>  
>  	vmf->pte = pte_offset_map_lock(vma->vm_mm, vmf->pmd, vmf->address,
>  			&vmf->ptl);
> -	if (!pte_none(*vmf->pte))
> +	if (!pte_none(*vmf->pte)) {
> +		update_mmu_cache(vma, vmf->address, vmf->pte);
>  		goto release;
> +	}
>  
>  	ret = check_stable_address_space(vma->vm_mm);
>  	if (ret)
> @@ -3646,11 +3655,14 @@ vm_fault_t alloc_set_pte(struct vm_fault *vmf, struct mem_cgroup *memcg,
>  	}
>  
>  	/* Re-check under ptl */
> -	if (unlikely(!pte_none(*vmf->pte)))
> +	if (unlikely(!pte_none(*vmf->pte))) {
> +		update_mmu_cache(vma, vmf->address, vmf->pte);
>  		return VM_FAULT_NOPAGE;
> +	}
>  
>  	flush_icache_page(vma, page);
>  	entry = mk_pte(page, vma->vm_page_prot);
> +	entry = pte_mkyoung(entry);
>  	if (write)
>  		entry = maybe_mkwrite(pte_mkdirty(entry), vma);
>  	/* copy-on-write page */
> @@ -4224,8 +4236,10 @@ static vm_fault_t handle_pte_fault(struct vm_fault *vmf)
>  	vmf->ptl = pte_lockptr(vmf->vma->vm_mm, vmf->pmd);
>  	spin_lock(vmf->ptl);
>  	entry = vmf->orig_pte;
> -	if (unlikely(!pte_same(*vmf->pte, entry)))
> +	if (unlikely(!pte_same(*vmf->pte, entry))) {
> +		update_mmu_cache(vmf->vma, vmf->address, vmf->pte);
>  		goto unlock;
> +	}
>  	if (vmf->flags & FAULT_FLAG_WRITE) {
>  		if (!pte_write(entry))
>  			return do_wp_page(vmf);
> 


-- 
Thanks,

David / dhildenb