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

Michal Hocko
2018-12-14T08:49:54Z

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

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