Re: [PATCH v3] lockdep: Teach lockdep about memalloc_noio_save

Michal Hocko
2017-03-01T13:24:44Z

On Wed 01-03-17 13:29:57, Nikolay Borisov wrote:
> Commit 21caf2fc1931 ("mm: teach mm by current context info to not do I/O
> during memory allocation") added the memalloc_noio_(save|restore) functions
> to enable people to modify the MM behavior by disbaling I/O during memory
> allocation. This was further extended in Fixes: 934f3072c17c ("mm: clear
> __GFP_FS when PF_MEMALLOC_NOIO is set"). memalloc_noio_* functions prevent
> allocation paths recursing back into the filesystem without explicitly
> changing the flags for every allocation site. However, lockdep hasn't been
> keeping up with the changes and it entirely misses handling the memalloc_noio
> adjustments. Instead, it is left to the callers of __lockdep_trace_alloc to
> call the functino after they have shaven the respective GFP flags.

I would just add that this might lead to false positives and past the
example you have encountered.

> Let's fix this by making lockdep explicitly do the shaving of respective
> GFP flags.
>
> Fixes: 934f3072c17c ("mm: clear __GFP_FS when PF_MEMALLOC_NOIO is set")
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>

Acked-by: Michal Hocko <mhocko@suse.cz>

> ---
> kernel/locking/lockdep.c | 4 +++-
> 1 file changed, 3 insertions(+), 1 deletion(-)
>
> Changes since v2:
> * Incorporate Michal's suggestion of using memalloc_noio_flags explicitly.
> * Tune the commit message to make the problem statement a bit more
> descriptive.
>
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 9812e5dd409e..565506c9e99c 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -2861,6 +2861,8 @@ static void __lockdep_trace_alloc(gfp_t gfp_mask, unsigned long flags)
> if (unlikely(!debug_locks))
> return;
>
> + gfp_mask = memalloc_noio_flags(gfp_mask);
> +
> /* no reclaim without waiting on it */
> if (!(gfp_mask & __GFP_DIRECT_RECLAIM))
> return;
> @@ -3852,7 +3854,7 @@ EXPORT_SYMBOL_GPL(lock_unpin_lock);
>
> void lockdep_set_current_reclaim_state(gfp_t gfp_mask)
> {
> - current->lockdep_reclaim_gfp = gfp_mask;
> + current->lockdep_reclaim_gfp = memalloc_noio_flags(gfp_mask);
> }
>
> void lockdep_clear_current_reclaim_state(void)
> --
> 2.7.4

--
Michal Hocko
SUSE Labs