Subject: Re: [linux-next: Tree for Jun 1] __khugepaged_exit rwsem_down_write_failed lockup

From: Sergey Senozhatsky <[email-masked]>
Date: Fri, 3 Jun 2016 10:29:19 +0900

On (06/03/16 10:00), Sergey Senozhatsky wrote:
> a good find by Vlastimil.
> 
> Ebru, can you also re-visit __collapse_huge_page_swapin()? it's called
> from collapse_huge_page() under the down_read(&mm->mmap_sem), is there
> any reason to do the nested down_read(&mm->mmap_sem)?
> 
> collapse_huge_page()
> ...
> 	down_read(&mm->mmap_sem);
> 	result = hugepage_vma_revalidate(mm, vma, address);
> 	if (result)
> 		goto out;
> 
> 	pmd = mm_find_pmd(mm, address);
> 	if (!pmd) {
> 		result = SCAN_PMD_NULL;
> 		goto out;
> 	}
> 
> 	if (allocstall == curr_allocstall && swap != 0) {
> 		if (!__collapse_huge_page_swapin(mm, vma, address, pmd)) {
> 			{
> 			:	if (ret & VM_FAULT_RETRY) {
> 			:		down_read(&mm->mmap_sem);
> 			:		^^^^^^^^^

oh... it's in a loop

		for (_address = address; _address < address + HPAGE_PMD_NR*PAGE_SIZE;
						pte++, _address += PAGE_SIZE) {
			ret = do_swap_page()
			if (ret & VM_FAULT_RETRY) {
				down_read(&mm->mmap_sem);
				^^^^^^^^^
				...
			}
		}

so there can be multiple sem->count++ in __collapse_huge_page_swapin(),
and you don't know how many sem->count-- you need to do later? is this
correct or am I hallucinating?

	-ss

> 			:		if (hugepage_vma_revalidate(mm, vma, address))
> 			:			return false;
> 			:	}
> 			}
> 
> 			up_read(&mm->mmap_sem);
> 			goto out;
> 		}
> 	}
> 
> 	up_read(&mm->mmap_sem);
> 
> 
> 
> so if __collapse_huge_page_swapin() retruns true we have:
> 	- down_read() twice, up_read() once?
> 
> the locking rules here are a bit confusing. (I didn't have my morning coffee yet).
> 
> 	-ss
> 

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