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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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