Re: [PATCH 1/3] mm,migration: During fork(), wait for migration to end if migration PTE is encountered

[Mel Gorman <mel@csn.ul.ie>]

On Wed, Apr 28, 2010 at 08:52:03AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 28 Apr 2010 00:22:45 +0200
> Andrea Arcangeli <aarcange@redhat.com> wrote:
> 
> > Ok I had a first look:
> > 
> > On Tue, Apr 27, 2010 at 10:30:50PM +0100, Mel Gorman wrote:
> > > 	CPUA			CPU B
> > > 				do_fork()
> > > 				copy_mm() (from process 1 to process2)
> > > 				insert new vma to mmap_list (if inode/anon_vma)
> > 
> > Insert to the tail of the anon_vma list...
> > 
> > > 	pte_lock(process1)
> > > 	unmap a page
> > > 	insert migration_entry
> > > 	pte_unlock(process1)
> > > 
> > > 	migrate page copy
> > > 				copy_page_range
> > > 	remap new page by rmap_walk()
> > 
> > rmap_walk will walk process1 first! It's at the head, the vmas with
> > unmapped ptes are at the tail so process1 is walked before process2.
> > 
> > > 	pte_lock(process2)
> > > 	found no pte.
> > > 	pte_unlock(process2)
> > > 				pte lock(process2)
> > > 				pte lock(process1)
> > > 				copy migration entry to process2
> > > 				pte unlock(process1)
> > > 				pte unlokc(process2)
> > > 	pte_lock(process1)
> > > 	replace migration entry
> > > 	to new page's pte.
> > > 	pte_unlock(process1)
> > 
> > rmap_walk has to lock down process1 before process2, this is the
> > ordering issue I already mentioned in earlier email. So it cannot
> > happen and this patch is unnecessary.
> > 
> > The ordering is fundamental and as said anon_vma_link already adds new
> > vmas to the _tail_ of the anon-vma. And this is why it has to add to
> > the tail. If anon_vma_link would add new vmas to the head of the list,
> > the above bug could materialize, but it doesn't so it cannot happen.
> > 
> > In mainline anon_vma_link is called anon_vma_chain_link, see the
> > list_add_tail there to provide this guarantee.
> > 
> > Because process1 is walked first by CPU A, the migration entry is
> > replaced by the final pte before copy-migration-entry
> > runs. Alternatively if copy-migration-entry runs before before
> > process1 is walked, the migration entry will be copied and found in
> > process 2.
> > 
> 
> I already explained this doesn't happend and said "I'm sorry".
> 

And after going through it again, I'm happy that this was a red herring.
The patch is now dropped.

> But considering maintainance, it's not necessary to copy migration ptes
> and we don't have to keep a fundamental risks of migration circus.
> 

Even if it's not strictly necessary, migration should (and does in this case)
cope with being able to find all its migration ptes. An extra one being copied
doesn't matter as long as it can be found on the chain. It's not like the
execve-problem where a migration PTE gets moved to a place it can't be found.

> So, I don't say "we don't need this patch."
> 

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab

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