Re: linux-next: kernel BUG at mm/slub.c:1447!

[Andrew Morton]

On Mon, 5 Oct 2015 15:47:13 +0200 Michal Hocko <mhocko@xxxxxxxxxx> wrote:

> > The fourth best way of fixing this is a nasty short-term bodge, such a
> > the one you just sent ;) But if we're going to do this, it should be
> > the minimal bodge which fixes this deadlock.  Is it possible to come up
> > with a one-liner (plus suitable comment) to get us out of this mess?
> 
> Yes I do agree that the fix I am proposing is short-term but this seems
> like the easiest way to go for stable and older kernels that might be
> affected. I thought your proposal for mapping_gfp_constraint was exactly
> to have all such places annotated for an easier future transition to
> something more reasonable.

hm, OK, let's go that way.  But I expect this mess will continue to
float around for a long time - fixing it nicely will be somewhat
intrusive.

> > Longer-term I suggest we look at generalising the memalloc_noio_foo()
> > stuff so as to permit callers to mask off (ie: zero) __GFP_ flags in
> > callees.  I have a suspicion we should have done this 15 years ago
> > (which is about when I started wanting to do it).
> 
> I am not sure memalloc_noio_foo is a huge win. It is an easy hack where
> the whole allocation transaction is clear - like in the PM code. I am
> not sure this is true also for the FS.

mm..  I think it'll work out OK - a set/restore around particular
callsites.

It might get messy in core MM though.  Do we apply current->mask at the
very low levels of the page allocator?  If so, that might muck up
intermediate callers who are peeking into specific gfp_t flags.

Perhaps it would be better to apply the mask at the highest possible
level: wherever a function which was not passed a gfp_t decides to
create one.  Basically a grep for "GFP_".  But then we need to decide
*which* gfp_t-creators need the treatment.  All of them (yikes) or is
this mechanism only for called-via-address_space_operations code?  That
might work.

Maybe it would be better to add the gfp_t argument to the
address_space_operations.  At a minimum, writepage(), readpage(),
writepages(), readpages().  What a pickle.
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