On 2023/01/19 13:52, Dave Chinner wrote:
> It's a false positive, and the allocation context it comes from
> in XFS is documented as needing to avoid lockdep tracking because
> this path is know to trigger false positive memory reclaim recursion
> reports:
>
> if (!args->value) {
> args->value = kvmalloc(valuelen, GFP_KERNEL | __GFP_NOLOCKDEP);
> if (!args->value)
> return -ENOMEM;
> }
> args->valuelen = valuelen;
>
>
> XFS is telling the allocator not to track this allocation with
> lockdep, and that is getting passed down through the allocator which
> has not passed it to lockdep (correct behaviour!), but then KASAN is
> trying to track the allocation and that needs to do a memory
> allocation. __stack_depot_save() is passed the gfp mask from the
> allocation context so it has __GFP_NOLOCKDEP right there, but it
> does:
>
> if (unlikely(can_alloc && !smp_load_acquire(&next_slab_inited))) {
> /*
> * Zero out zone modifiers, as we don't have specific zone
> * requirements. Keep the flags related to allocation in atomic
> * contexts and I/O.
> */
> alloc_flags &= ~GFP_ZONEMASK;
>>>>>>>> alloc_flags &= (GFP_ATOMIC | GFP_KERNEL);
> alloc_flags |= __GFP_NOWARN;
> page = alloc_pages(alloc_flags, STACK_ALLOC_ORDER);
>
> It masks masks out anything other than GFP_ATOMIC and GFP_KERNEL
> related flags. This drops __GFP_NOLOCKDEP on the floor, hence
> lockdep tracks an allocation in a context we've explicitly said not
> to track. Hence lockdep (correctly!) explodes later when the
> false positive "lock inode in reclaim context" situation triggers.
>
> This is a KASAN bug. It should not be dropping __GFP_NOLOCKDEP from
> the allocation context flags.
OK. Thanks for the explanation !
--
Damien Le Moal
Western Digital Research
