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 13:14:19 +0900

On (06/03/16 10:29), Sergey Senozhatsky wrote:
> > 	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?

No, I was wrong, sorry for the noise.

it's getting unlocked in

__collapse_huge_page_swapin()
	do_swap_page()
		lock_page_or_retry()
			if (flags & FAULT_FLAG_ALLOW_RETRY)
				up_read(&mm->mmap_sem);
	return VM_FAULT_RETRY

	-ss