Re: [PATCH v3] mm, memcg: fix reclaim deadlock with writeback

[Michal Hocko]

On Thu 13-12-18 17:04:00, Johannes Weiner wrote:
[...]
> Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>

Thanks!

> Just one nit:
> 
> > @@ -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() */
> > +	}
> 
> Could you be more specific in the deadlock comment? git blame will
> work fine for a while, but it becomes a pain to find corresponding
> patches after stuff gets moved around for years.
> 
> In particular the race diagram between reclaim with a page lock held
> and the fs doing SetPageWriteback batches before kicking off IO would
> be useful directly in the code, IMO.

This?

diff --git a/mm/memory.c b/mm/memory.c
index bb78e90a9b70..ece221e4da6d 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -2995,7 +2995,18 @@ static vm_fault_t __do_fault(struct vm_fault *vmf)
 
 	/*
 	 * Preallocate pte before we take page_lock because this might lead to
-	 * deadlocks for memcg reclaim which waits for pages under writeback.
+	 * deadlocks for memcg reclaim which waits for pages under writeback:
+	 * 				lock_page(A)
+	 * 				SetPageWriteback(A)
+	 * 				unlock_page(A)
+	 * lock_page(B)
+	 * 				lock_page(B)
+	 * pte_alloc_pne
+	 *   shrink_page_list
+	 *     wait_on_page_writeback(A)
+	 *     				SetPageWriteback(B)
+	 *     				unlock_page(B)
+	 *     				# flush A, B to clear the writeback
 	 */
 	if (pmd_none(*vmf->pmd) && !vmf->prealloc_pte) {
 		vmf->prealloc_pte = pte_alloc_one(vmf->vma->vm_mm, vmf->address);
-- 
Michal Hocko
SUSE Labs