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SubjectRe: [PATCH v17 3/6] mm/balloon_compaction.c: split balloon page allocation and enqueue
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Wei Wang wrote:
> Here's a detailed analysis of the deadlock by Tetsuo Handa:
>
> In leak_balloon(), mutex_lock(&vb->balloon_lock) is called in order to
> serialize against fill_balloon(). But in fill_balloon(),
> alloc_page(GFP_HIGHUSER[_MOVABLE] | __GFP_NOMEMALLOC | __GFP_NORETRY) is
> called with vb->balloon_lock mutex held. Since GFP_HIGHUSER[_MOVABLE]
> implies __GFP_DIRECT_RECLAIM | __GFP_IO | __GFP_FS, despite __GFP_NORETRY
> is specified, this allocation attempt might indirectly depend on somebody
> else's __GFP_DIRECT_RECLAIM memory allocation. And such indirect
> __GFP_DIRECT_RECLAIM memory allocation might call leak_balloon() via
> virtballoon_oom_notify() via blocking_notifier_call_chain() callback via
> out_of_memory() when it reached __alloc_pages_may_oom() and held oom_lock
> mutex. Since vb->balloon_lock mutex is already held by fill_balloon(), it
> will cause OOM lockup. Thus, do not wait for vb->balloon_lock mutex if
> leak_balloon() is called from out_of_memory().

Please drop "Thus, do not wait for vb->balloon_lock mutex if leak_balloon()
is called from out_of_memory()." part. This is not what this patch will do.

>
> Thread1 Thread2
> fill_balloon()
> takes a balloon_lock
> balloon_page_enqueue()
> alloc_page(GFP_HIGHUSER_MOVABLE)
> direct reclaim (__GFP_FS context) takes a fs lock
> waits for that fs lock alloc_page(GFP_NOFS)
> __alloc_pages_may_oom()
> takes the oom_lock
> out_of_memory()
> blocking_notifier_call_chain()
> leak_balloon()
> tries to take that
> balloon_lock and deadlocks

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