Re: [PATCH 3/6] EXT4: Remove ENOMEM/congestion_wait() loops.

[Dave Chinner <david@fromorbit.com>]

On Thu, Sep 16, 2021 at 08:35:40AM +1000, NeilBrown wrote:
> On Wed, 15 Sep 2021, Michal Hocko wrote:
> > On Wed 15-09-21 07:48:11, Neil Brown wrote:
> > > 
> > > Why does __GFP_NOFAIL access the reserves? Why not require that the
> > > relevant "Try harder" flag (__GFP_ATOMIC or __GFP_MEMALLOC) be included
> > > with __GFP_NOFAIL if that is justified?
> > 
> > Does 5020e285856c ("mm, oom: give __GFP_NOFAIL allocations access to
> > memory reserves") help?
> 
> Yes, that helps.  A bit.
> 
> I'm not fond of the clause "the allocation request might have come with some
> locks held".  What if it doesn't?  Does it still have to pay the price.
> 
> Should we not require that the caller indicate if any locks are held?
> That way callers which don't hold locks can use __GFP_NOFAIL without
> worrying about imposing on other code.
> 
> Or is it so rare that __GFP_NOFAIL would be used without holding a lock
> that it doesn't matter?
> 
> The other commit of interest is
> 
> Commit: 6c18ba7a1899 ("mm: help __GFP_NOFAIL allocations which do not trigger OOM killer")
> 
> I don't find the reasoning convincing.  It is a bit like "Robbing Peter
> to pay Paul".  It takes from the reserves to allow a __GFP_NOFAIL to
> proceed, with out any reason to think this particular allocation has any
> more 'right' to the reserves than anything else.
> 
> While I don't like the reasoning in either of these, they do make it
> clear (to me) that the use of reserves is entirely an internal policy
> decision.  They should *not* be seen as part of the API and callers
> should not have to be concerned about it when deciding whether to use
> __GFP_NOFAIL or not.

Agree totally with this - we just want to block until allocation
succeeds, and if the -filesystem- deadlocks because allocation never
succeeds then that's a problem that needs to be solved in the
filesystem with a different memory allocation strategy...

OTOH, setting up a single __GFP_NOFAIL call site with the ability to
take the entire system down seems somewhat misguided.

> The use of these reserves is, at most, a hypothetical problem.  If it
> ever looks like becoming a real practical problem, it needs to be fixed
> internally to the page allocator.  Maybe an extra water-mark which isn't
> quite as permissive as ALLOC_HIGH...
> 
> I'm inclined to drop all references to reserves from the documentation
> for __GFP_NOFAIL.  I think there are enough users already that adding a
> couple more isn't going to make problems substantially more likely.  And
> more will be added anyway that the mm/ team won't have the opportunity
> or bandwidth to review.

Yup, we've been replacing open coded loops like in kmem_alloc() with
explicit __GFP_NOFAIL usage for a while now:

$ ▶ git grep __GFP_NOFAIL fs/xfs |wc -l
33
$

ANd we've got another 100 or so call sites planned for conversion to
__GFP_NOFAIL. Hence the suggestion to remove the use of
reserves from __GFP_NOFAIL seems like a sensible plan because it has
never been necessary in the past for all the allocation sites we are
converting from open coded loops to __GFP_NOFAIL...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com