linux-kernel - Re: Need to take mmap_sem lock in move

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Date:	Wed, 4 Feb 2009 18:40:28 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	"Swamy Gowda" <swamy@...ident.com>, <linux-kernel@...r.kernel.org>,
	cl@...ux-foundation.org, Brice.Goglin@...ia.fr
Subject: Re: Need to take mmap_sem lock in move_pages.

On Wed, 4 Feb 2009 18:36:00 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> On Wed, 4 Feb 2009 01:02:35 -0800
> "Swamy Gowda" <swamy@...ident.com> wrote:
> 
> > Hi,
> > 
> >  
> > 
> > I believe that migrate_pages related race conditions were fixed as part
> > of rcu_read_lock in unmap_and_move. But it seems we are still taking the
> > mmap_sem lock in do_move_pages function, is this really required? If it
> > is so why it is not needed in hot remove path? 
> > 
> 
> Hmm , CC: to Christoph and Brice.
> 
> My understanding is following.
> 
>  1. do_move_page_to_node_array() at el. needs mmap_sem (read-side) because it
>     scans page table and vma. While we have to scan vmas to find pages for
>     migration, it needs mmap_sem.
> 
>     So, this part,
>     == do_move_page_to_node_array()
>  903         err = 0;
>  904         if (!list_empty(&pagelist))
>  905                 err = migrate_pages(&pagelist, new_page_node,
>  906                                 (unsigned long)pm);
>  907 
>  908         up_read(&mm->mmap_sem);
>    ==
>    can be
>    ==
>  903         err = 0;
>  904         up_read(&mm->mmap_sem);
>  905         if (!list_empty(&pagelist))
>  906                 err = migrate_pages(&pagelist, new_page_node,
>  907                                 (unsigned long)pm);
>  908 
>     ==
>    ?
> 
>    But, by above move of semaphore, reliability of  sys_migrate_page() goes down.
>    Assume 2 threads. Thread1 calls sys_move_pages(), thread2 does page fault,
>    touches pte under migraton.
>    ==
>        Thread 1                    Thread2                 
> 
>        up_read(&mm->mmap_sem)
>                                     page fault => map new page.
>        end of migration.
>     ==
> 

      please ignore following ;( Sigh..
      Maybe up_read() can be moved before do_migrate_pages(), I think.

Sorry.
-kame

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