Re: [PATCH 04/14] mm,migration: Allow the migration ofPageSwapCache pages

[KAMEZAWA Hiroyuki]

On Thu, 22 Apr 2010 10:28:20 +0100
Mel Gorman <mel@xxxxxxxxx> wrote:

> On Wed, Apr 21, 2010 at 10:46:45AM -0500, Christoph Lameter wrote:
> > On Wed, 21 Apr 2010, Mel Gorman wrote:
> > 
> > > > > 2. Is the BUG_ON check in
> > > > >    include/linux/swapops.h#migration_entry_to_page() now wrong? (I
> > > > >    think yes, but I'm not sure and I'm having trouble verifying it)
> > > >
> > > > The bug check ensures that migration entries only occur when the page
> > > > is locked. This patch changes that behavior. This is going too oops
> > > > therefore in unmap_and_move() when you try to remove the migration_ptes
> > > > from an unlocked page.
> > > >
> > >
> > > It's not unmap_and_move() that the problem is occurring on but during a
> > > page fault - presumably in do_swap_page but I'm not 100% certain.
> > 
> > remove_migration_pte() calls migration_entry_to_page(). So it must do that
> > only if the page is still locked.
> > 
> 
> Correct, but the other call path is
> 
> do_swap_page
>   -> migration_entry_wait
>     -> migration_entry_to_page
> 
> with migration_entry_wait expecting the page to be locked. There is a dangling
> migration PTEs coming from somewhere. I thought it was from unmapped swapcache
> first, but that cannot be the case. There is a race somewhere.
> 
> > You need to ensure that the page is not unlocked in move_to_new_page() if
> > the migration ptes are kept.
> > 
> > move_to_new_page() only unlocks the new page not the original page. So that is safe.
> > 
> > And it seems that the old page is also unlocked in unmap_and_move() only
> > after the migration_ptes have been removed? So we are fine after all...?
> > 
> 
> You'd think but migration PTEs are being left behind in some circumstance. I
> thought it was due to this series, but it's unlikely. It's more a case that
> compaction heavily exercises migration.
> 
> We can clean up the old migration PTEs though when they are encountered
> like in the following patch for example? I'll continue investigating why
> this dangling migration pte exists as closing that race would be a
> better fix.
> 
> ==== CUT HERE ====
> mm,migration: Remove dangling migration ptes pointing to unlocked pages
> 
> Due to some yet-to-be-identified race, it is possible for migration PTEs
> to be left behind, When later paged-in, a BUG is triggered that assumes
> that all migration PTEs are point to a page currently being migrated and
> so must be locked.
> 
> Rather than calling BUG, this patch notes the existance of dangling migration
> PTEs in migration_entry_wait() and cleans them up.
> 

I use similar patch for debugging. In my patch, this when this function founds
dangling migration entry, return error code and do_swap_page() returns
VM_FAULT_SIGBUS.


Hmm..in my test, the case was.

Before try_to_unmap:
	mapcount=1, SwapCache, remap_swapcache=1
After remap
	mapcount=0, SwapCache, rc=0.

So, I think there may be some race in rmap_walk() and vma handling or
anon_vma handling. migration_entry isn't found by rmap_walk.

Hmm..it seems this kind patch will be required for debug.

-Kame



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