linux-kernel - Re: [PATCH 2/5] vrange: Add purged page detection on setting memory non-volatile

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Date:	Sun, 23 Mar 2014 14:50:28 -0700
From:	KOSAKI Motohiro <kosaki.motohiro@...il.com>
To:	John Stultz <john.stultz@...aro.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>, Michel Lespinasse <walken@...gle.com>,
	Minchan Kim <minchan@...nel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: [PATCH 2/5] vrange: Add purged page detection on setting memory non-volatile

On Sun, Mar 23, 2014 at 1:26 PM, John Stultz <john.stultz@...aro.org> wrote:
> On Sun, Mar 23, 2014 at 10:50 AM, KOSAKI Motohiro
> <kosaki.motohiro@...il.com> wrote:
>>> +/**
>>> + * vrange_check_purged_pte - Checks ptes for purged pages
>>> + *
>>> + * Iterates over the ptes in the pmd checking if they have
>>> + * purged swap entries.
>>> + *
>>> + * Sets the vrange_walker.pages_purged to 1 if any were purged.
>>> + */
>>> +static int vrange_check_purged_pte(pmd_t *pmd, unsigned long addr,
>>> +                                       unsigned long end, struct mm_walk *walk)
>>> +{
>>> +       struct vrange_walker *vw = walk->private;
>>> +       pte_t *pte;
>>> +       spinlock_t *ptl;
>>> +
>>> +       if (pmd_trans_huge(*pmd))
>>> +               return 0;
>>> +       if (pmd_trans_unstable(pmd))
>>> +               return 0;
>>> +
>>> +       pte = pte_offset_map_lock(walk->mm, pmd, addr, &ptl);
>>> +       for (; addr != end; pte++, addr += PAGE_SIZE) {
>>> +               if (!pte_present(*pte)) {
>>> +                       swp_entry_t vrange_entry = pte_to_swp_entry(*pte);
>>> +
>>> +                       if (unlikely(is_vpurged_entry(vrange_entry))) {
>>> +                               vw->page_was_purged = 1;
>>> +                               break;
>>
>> This function only detect there is vpurge entry or not. But
>> VRANGE_NONVOLATILE should remove all vpurge entries.
>> Otherwise, non-volatiled range still makes SIGBUS.
>
> So in the following patch (3/5), we only SIGBUS if the swap entry
> is_vpurged_entry()  && the vma is still marked volatile, so this
> shouldn't be an issue.

When VOLATILE -> NON-VOLATILE -> VOLATILE transition happen,
the page immediately marked "was purged"?

I don't understand why vma check help.
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