linux-kernel - Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch added to -mm tree

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Message-ID: <alpine.LSU.2.11.1407091000410.11705@eggly.anvils>
Date:	Wed, 9 Jul 2014 10:05:35 -0700 (PDT)
From:	Hugh Dickins <hughd@...gle.com>
To:	Vlastimil Babka <vbabka@...e.cz>
cc:	Sasha Levin <sasha.levin@...cle.com>,
	Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org,
	davej@...hat.com, koct9i@...il.com, lczerner@...hat.com,
	stable@...r.kernel.org, "linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: + shmem-fix-faulting-into-a-hole-while-its-punched-take-2.patch
 added to -mm tree

On Wed, 9 Jul 2014, Vlastimil Babka wrote:
> On 07/09/2014 06:03 PM, Sasha Levin wrote:
> > 
> > We can see that it's not blocked since it's in the middle of a spinlock
> > unlock
> > call, and we can guess it's been in that function for a while because of
> > the hung
> > task timer, and other processes waiting on that i_mmap_mutex:
> 
> Hm, zap_pte_range has potentially an endless loop due to the 'goto again'
> path. Could it be a somewhat similar situation to the fallocate problem, but
> where parallel faulters on shared memory are preventing a process from
> exiting? Although they don't fault the pages into the same address space,
> they could maybe somehow interact through the TLB flushing code? And only
> after fixing the original problem we can observe this one?

That's a good thought.  It ought to make forward progress nonetheless,
but I believe (please check, I'm rushing) that there's an off-by-one in
that path which could leave us hanging - but only when __tlb_remove_page()
repeatedly fails, which would only happen if exceptionally low on memory??

Does this patch look good, and does it make any difference to the hang?

--- mmotm/mm/memory.c	2014-07-02 15:32:22.212311544 -0700
+++ linux/mm/memory.c	2014-07-09 09:56:33.724159443 -0700
@@ -1145,6 +1145,7 @@ again:
 			if (unlikely(page_mapcount(page) < 0))
 				print_bad_pte(vma, addr, ptent, page);
 			if (unlikely(!__tlb_remove_page(tlb, page))) {
+				addr += PAGE_SIZE;
 				force_flush = 1;
 				break;
 			}
--
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