Re: dm bufio: Reduce dm_bufio_lock contention

[Mikulas Patocka <mpatocka@redhat.com>]

On Tue, 19 Jun 2018, Michal Hocko wrote:

> On Mon 18-06-18 18:11:26, Mikulas Patocka wrote:
> [...]
> > I grepped the kernel for __GFP_NORETRY and triaged them. I found 16 cases 
> > without a fallback - those are bugs that make various functions randomly 
> > return -ENOMEM.
> 
> Well, maybe those are just optimistic attempts to allocate memory and
> have a fallback somewhere else. So I would be careful calling them
> outright bugs. But maybe you are right.

I was trying to find the fallback code when I triaged them and maked as 
"BUG" those cases where I didn't find it. You can search harder and 
perhaps you'll find something that I didn't.

> > Most of the callers provide callback.
> > 
> > There is another strange flag - __GFP_RETRY_MAYFAIL - it provides two 
> > different functions - if the allocation is larger than 
> > PAGE_ALLOC_COSTLY_ORDER, it retries the allocation as if it were smaller. 
> > If the allocations is smaller than PAGE_ALLOC_COSTLY_ORDER, 
> > __GFP_RETRY_MAYFAIL will avoid the oom killer (larger order allocations 
> > don't trigger the oom killer at all).
> 
> Well, the primary purpose of this flag is to provide a consistent
> failure behavior for all requests regardless of the size.
> 
> > So, perhaps __GFP_RETRY_MAYFAIL could be used instead of __GFP_NORETRY in 
> > the cases where the caller wants to avoid trigerring the oom killer (the 
> > problem is that __GFP_NORETRY causes random failure even in no-oom 
> > situations but __GFP_RETRY_MAYFAIL doesn't).
> 
> myabe yes.
> 
> > So my suggestion is - fix these obvious bugs when someone allocates memory 
> > with __GFP_NORETRY without any fallback - and then, __GFP_NORETRY could be 
> > just changed to return NULL instead of sleeping.
> 
> No real objection to fixing wrong __GFP_NORETRY usage. But __GFP_NORETRY
> can sleep. Nothing will really change in that regards.  It does a
> reclaim and that _might_ sleep.
> 
> But seriously, isn't the best way around the throttling issue to use
> PF_LESS_THROTTLE?

Yes - it could be done by setting PF_LESS_THROTTLE. But I think it would 
be better to change it just in one place than to add PF_LESS_THROTTLE to 
every block device driver (because adding it to every block driver results 
in more code).

What about this patch? If __GFP_NORETRY and __GFP_FS is not set (i.e. the 
request comes from a block device driver or a filesystem), we should not 
sleep.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Cc: stable@vger.kernel.org

Index: linux-2.6/mm/vmscan.c
===================================================================
--- linux-2.6.orig/mm/vmscan.c
+++ linux-2.6/mm/vmscan.c
@@ -2674,6 +2674,7 @@ static bool shrink_node(pg_data_t *pgdat
 		 * the LRU too quickly.
 		 */
 		if (!sc->hibernation_mode && !current_is_kswapd() &&
+		   (sc->gfp_mask & (__GFP_NORETRY | __GFP_FS)) != __GFP_NORETRY &&
 		   current_may_throttle() && pgdat_memcg_congested(pgdat, root))
 			wait_iff_congested(BLK_RW_ASYNC, HZ/10);