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

[Michal Hocko <mhocko@kernel.org>]

On Fri 07-12-18 10:51:04, Liu Bo wrote:
> On Fri, Dec 07, 2018 at 12:20:36PM +0100, Michal Hocko wrote:
> > 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.
> > 
> 
> Makes sense to me, unfortunately we don't have a local reproducer to verify it
> and we've disabled CONFIG_MEMCG_KMEM to workaround the problem.  Given the stack
> I put, the patch should address the deadlock at least.

OK, I will send a full patch with the changelog tomorrow.
-- 
Michal Hocko
SUSE Labs