linux-kernel - Re: [PATCH 4/4] mm: migrate: Batch TLB flushing when unmapping pages for migration

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Date:	Thu, 16 Apr 2015 20:34:54 +0100
From:	Mel Gorman <mgorman@...e.de>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...hat.com>,
	Minchan Kim <minchan@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/4] mm: migrate: Batch TLB flushing when unmapping pages
 for migration

On Thu, Apr 16, 2015 at 11:57:15AM -0700, Hugh Dickins wrote:
> > @@ -1098,6 +1098,8 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
> >  	if (!swapwrite)
> >  		current->flags |= PF_SWAPWRITE;
> >  
> > +	alloc_tlb_ubc();
> > +
> >  	for(pass = 0; pass < 10 && retry; pass++) {
> >  		retry = 0;
> >  
> > @@ -1144,6 +1146,8 @@ out:
> >  	if (!swapwrite)
> >  		current->flags &= ~PF_SWAPWRITE;
> >  
> > +	try_to_unmap_flush();
> 
> This is the right place to aim to flush, but I think you have to make
> more changes before it is safe to do so here.
> 
> The putback_lru_page(page) in unmap_and_move() is commented "A page
> that has been migrated has all references removed and will be freed".
> 
> If you leave TLB flushing until after the page has been freed, then
> there's a risk that userspace will see, not the data it expects at
> whatever virtual address, but data placed in there by the next user
> of this freed page.
> 
> So you'll need to do a little restructuring first.
> 

Well spotted. I believe you are correct and it almost certainly applies to
patch 2 as well for similar reasons. It also impacts the maximum reasonable
batch size that can be managed while maintaing safety. I'll do the necessary
shuffling tomorrow or Monday.

-- 
Mel Gorman
SUSE Labs
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