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

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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