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SubjectRe: [PATCH 05/10] vmscan: Synchrounous lumpy reclaim use lock_page() instead trylock_page()
Date
> > example, 
> >
> > __do_fault()
> > {
> > (snip)
> > if (unlikely(!(ret & VM_FAULT_LOCKED)))
> > lock_page(vmf.page);
> > else
> > VM_BUG_ON(!PageLocked(vmf.page));
> >
> > /*
> > * Should we do an early C-O-W break?
> > */
> > page = vmf.page;
> > if (flags & FAULT_FLAG_WRITE) {
> > if (!(vma->vm_flags & VM_SHARED)) {
> > anon = 1;
> > if (unlikely(anon_vma_prepare(vma))) {
> > ret = VM_FAULT_OOM;
> > goto out;
> > }
> > page = alloc_page_vma(GFP_HIGHUSER_MOVABLE,
> > vma, address);
> >
>
> Correct, this is a problem. I already had dropped the patch but thanks for
> pointing out a deadlock because I was missing this case. Nothing stops the
> page being faulted being sent to shrink_page_list() when alloc_page_vma()
> is called. The deadlock might be hard to hit, but it's there.

Yup, unfortunatelly.



> > Afaik, detailed rule is,
> >
> > o kswapd can call lock_page() because they never take page lock outside vmscan
>
> lock_page_nosync as you point out in your next mail. While it can call
> it, kswapd shouldn't because normally it avoids stalls but it would not
> deadlock as a result of calling it.

Agreed.


> > o if try_lock() is successed, we can call lock_page_nosync() against its page after unlock.
> > because the task have gurantee of no lock taken.
> > o otherwise, direct reclaimer can't call lock_page(). the task may have a lock already.
> >
>
> I think the safer bet is simply to say "direct reclaimers should not
> call lock_page() because the fault path could be holding a lock on that
> page already".

Yup, agreed.





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