linux-kernel - Re: [PATCH 4/4] mvolatile: Add page purging logic & SIGBUS trap

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Date:	Wed, 16 Apr 2014 11:43:50 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Minchan Kim <minchan@...nel.org>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Android Kernel Team <kernel-team@...roid.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Robert Love <rlove@...gle.com>, Mel Gorman <mel@....ul.ie>,
	Hugh Dickins <hughd@...gle.com>, Dave Hansen <dave@...1.net>,
	Rik van Riel <riel@...hat.com>,
	Dmitry Adamushko <dmitry.adamushko@...il.com>,
	Neil Brown <neilb@...e.de>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mike Hommey <mh@...ndium.org>, Taras Glek <tglek@...illa.com>,
	Jan Kara <jack@...e.cz>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Michel Lespinasse <walken@...gle.com>,
	Keith Packard <keithp@...thp.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 4/4] mvolatile: Add page purging logic & SIGBUS trap

On Sun, Apr 13, 2014 at 7:51 PM, Minchan Kim <minchan@...nel.org> wrote:
> On Fri, Apr 11, 2014 at 01:15:40PM -0700, John Stultz wrote:
>> @@ -3643,6 +3644,8 @@ static int handle_pte_fault(struct mm_struct *mm,
>>
>>       entry = *pte;
>>       if (!pte_present(entry)) {
>> +             swp_entry_t mvolatile_entry;
>> +
>>               if (pte_none(entry)) {
>>                       if (vma->vm_ops) {
>>                               if (likely(vma->vm_ops->fault))
>> @@ -3652,6 +3655,11 @@ static int handle_pte_fault(struct mm_struct *mm,
>>                       return do_anonymous_page(mm, vma, address,
>>                                                pte, pmd, flags);
>>               }
>> +
>> +             mvolatile_entry = pte_to_swp_entry(entry);
>> +             if (unlikely(is_purged_entry(mvolatile_entry)))
>> +                     return VM_FAULT_SIGBUS;
>> +
>
> There is no pte lock so that is_purged_entry isn't safe so if race happens,
> do_swap_page could have a problem so it would be better to handle it
> do_swap_page with pte lock because we used swp_pte to indicate purged pte.
>
> I tried to solve it while we were in Napa(you could remember I sent
> crap patchset to you privately but failed to fix and I still didn't get
> a time to fix it :( ) but I'd like to inform this problem.

Thanks for the review and the reminder! I'll move the check appropriately.

thanks
-john
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