On Thu, Apr 22, 2010 at 6:46 PM, KAMEZAWA Hiroyuki <kamezawa.hiroyu@xxxxxxxxxxxxxx> wrote: > 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. I looked do_swap_page, again. lock_page is called long after migration_entry_wait. It means lock_page can't close the race. So I think this BUG is possible. What do you think? > -Kame > > > > -- Kind regards, Minchan Kim -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href