Re: [PATCH RFC] Ext4: fix deadlock on dirty pages between fault and writeback

[Michal Hocko <mhocko@kernel.org>]

On Fri 07-12-18 08:16:15, Michal Hocko wrote:
[...]
> Memcg v1 indeed doesn't have any dirty IO throttling and this is a
> poor's man workaround. We still do not have that AFAIK and I do not know
> of an elegant way around that. Fortunatelly we shouldn't have that many
> GFP_KERNEL | __GFP_ACCOUNT allocations under page lock and we can work
> around this specific one quite easily. I haven't tested this yet but the
> following should work
> 
> diff --git a/mm/memory.c b/mm/memory.c
> index 4ad2d293ddc2..59c98eeb0260 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -2993,6 +2993,16 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
>  	struct vm_area_struct *vma = vmf->vma;
>  	vm_fault_t ret;
>  
> +	/*
> +	 * Preallocate pte before we take page_lock because this might lead to
> +	 * deadlocks for memcg reclaim which waits for pages under writeback.
> +	 */
> +	if (!vmf->prealloc_pte) {
> +		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm>mm, vmf->address);
> +		if (!vmf->prealloc_pte)
> +			return VM_FAULT_OOM;
> +	}
> +
>  	ret = vma->vm_ops->fault(vmf);
>  	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
>  			    VM_FAULT_DONE_COW)))

This is too eager to allocate pte even when it is not really needed.
Jack has also pointed out that I am missing a write barrier. So here we
go with an updated patch. This is essentially what fault around code
does.

diff --git a/mm/memory.c b/mm/memory.c
index 4ad2d293ddc2..1a73d2d4659e 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2993,6 +2993,17 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 	struct vm_area_struct *vma = vmf->vma;
 	vm_fault_t ret;
 
+	/*
+	 * Preallocate pte before we take page_lock because this might lead to
+	 * deadlocks for memcg reclaim which waits for pages under writeback.
+	 */
+	if (pmd_none(*vmf->pmd) && !vmf->prealloc_pte) {
+		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm>mm, vmf->address);
+		if (!vmf->prealloc_pte)
+			return VM_FAULT_OOM;
+		smp_wmb(); /* See comment in __pte_alloc() */
+	}
+
 	ret = vma->vm_ops->fault(vmf);
 	if (unlikely(ret & (VM_FAULT_ERROR | VM_FAULT_NOPAGE | VM_FAULT_RETRY |
 			    VM_FAULT_DONE_COW)))
-- 
Michal Hocko
SUSE Labs