Messages in this thread | | | Date | Tue, 27 Apr 2010 17:35:49 +0100 | From | Mel Gorman <> | Subject | Re: [PATCH 2/2] mm,migration: Prevent rmap_walk_[anon|ksm] seeing the wrong VMA information |
| |
On Tue, Apr 27, 2010 at 05:37:59PM +0200, Andrea Arcangeli wrote: > On Tue, Apr 27, 2010 at 11:29:05AM +0100, Mel Gorman wrote: > > It could have been in both but the vma lock should have been held across > > the rmap_one. It still reproduces but it's still the right thing to do. > > This is the current version of patch 2/2. > > Well, keep in mind I reproduced the swapops bug with 2.6.33 anon-vma > code, it's unlikely that focusing on patch 2 you'll fix bug in > swapops.h. If this is a bug in the new anon-vma code, it needs fixing > of course! But I doubt this bug is related to swapops in execve on the > bprm->p args. >
Why do you doubt it's unrelated to execve? From what I've seen during the day, there is a race in execve where the VMA gets moved (under the anon_vma lock) before the page-tables are copied with move_ptes (without a lock). In that case, execve can encounter and copy migration ptes before migration removes them. This also applies to mainline because it is only taking the RCU lock and not the anon_vma->lock.
I have a prototype that "handles" the situation with the new anon_vma code by removing the migration ptes it finds while moving page tables but it needs more work before releasing.
An alternative would be to split vma_adjust() into locked and unlocked versions. shift_arg_pages() could then take the anon_vma lock to lock both the VMA move and the pagetable copy here.
/* * cover the whole range: [new_start, old_end) */ if (vma_adjust(vma, new_start, old_end, vma->vm_pgoff, NULL)) return -ENOMEM;
/* * move the page tables downwards, on failure we rely on * process cleanup to remove whatever mess we made. */ if (length != move_page_tables(vma, old_start, vma, new_start, length)) return -ENOMEM;
It'd be messy to split up the locking of vma_adjust like this though and exec() will hold the anon_vma locks for longer just to guard against migration. It's not clear this is better than having move_ptes handle the
> I've yet to check in detail patch 1 sorry, I'll let you know my > opinion about it as soon as I checked it in detail. >
No problem. I still need to revisit all of these patches once I am confident the swapops bug cannot be triggered any more.
| |