Re: [PATCH mm] mm: fix BUG with kvzalloc+GFP_ATOMIC

[Vlastimil Babka]

On 9/23/22 17:10, Uladzislau Rezki wrote:
> On Fri, Sep 23, 2022 at 04:54:09PM +0200, Florian Westphal wrote:
> > Uladzislau Rezki <urezki@xxxxxxxxx> wrote:
> > > On Fri, Sep 23, 2022 at 03:35:12PM +0200, Florian Westphal wrote:
> > > > Michal Hocko <mhocko@xxxxxxxx> wrote:
> > > > > On Fri 23-09-22 12:38:58, Florian Westphal wrote:
> > > > > > Martin Zaharinov reports BUG() in mm land for 5.19.10 kernel:
> > > > > >   kernel BUG at mm/vmalloc.c:2437!
> > > > > >   invalid opcode: 0000 [#1] SMP
> > > > > >   CPU: 28 PID: 0 Comm: swapper/28 Tainted: G        W  O      5.19.9 #1
> > > > > >   [..]
> > > > > >   RIP: 0010:__get_vm_area_node+0x120/0x130
> > > > > >    __vmalloc_node_range+0x96/0x1e0
> > > > > >    kvmalloc_node+0x92/0xb0
> > > > > >    bucket_table_alloc.isra.0+0x47/0x140
> > > > > >    rhashtable_try_insert+0x3a4/0x440
> > > > > >    rhashtable_insert_slow+0x1b/0x30
> > > > > >   [..]
> > > > > >
> > > > > > bucket_table_alloc uses kvzallocGPF_ATOMIC).  If kmalloc fails, this now
> > > > > > falls through to vmalloc and hits code paths that assume GFP_KERNEL.
> > > > > >
> > > > > > Revert the problematic change and stay with slab allocator.
> > > > >
> > > > > Why don't you simply fix the caller?
> > > >
> > > > Uh, not following?
> > > >
> > > > kvzalloc(GFP_ATOMIC) was perfectly fine, is this illegal again?
> > > <snip>
> > > static struct vm_struct *__get_vm_area_node(unsigned long size,
> > > 		unsigned long align, unsigned long shift, unsigned long flags,
> > > 		unsigned long start, unsigned long end, int node,
> > > 		gfp_t gfp_mask, const void *caller)
> > > {
> > > 	struct vmap_area *va;
> > > 	struct vm_struct *area;
> > > 	unsigned long requested_size = size;
> > >
> > > 	BUG_ON(in_interrupt());
> > > ...
> > > <snip>
> > >
> > > vmalloc is not supposed to be called from the IRQ context.
> >
> > It uses kvzalloc, not vmalloc api.
> >
> > Before 2018, rhashtable did use kzalloc OR kvzalloc, depending on gfp_t.
> >
> > Quote from 93f976b5190df327939 changelog:
> >    As of ce91f6ee5b3b ("mm: kvmalloc does not fallback to vmalloc for
> >    incompatible gfp flags") we can simplify the caller
> >    and trust kvzalloc() to just do the right thing.
> >
> > I fear that if this isn't allowed it will result in hard-to-spot bugs
> > because things will work fine until a fallback to vmalloc happens.
> >
> > rhashtable may not be the only user of kvmalloc api that rely on
> > ability to call it from (soft)irq.
> Doing the "p = kmalloc(sizeof(*p), GFP_ATOMIC);" from an atomic context
> is also a problem nowadays. Such code should be fixed across the kernel
> because of PREEMPT_RT support.

But the "atomic context" here is different, no? Calling kmalloc() from 
IRQ handlers AFAIK is ok as IRQ handlers are threaded on PREEMPT_RT. 
Calling it inside an local_irq_disable() would be a problem on the other 
hand. But then under e.g. spin_lock_irqsave() could be ok as those don't 
really disable irqs on RT.

> --
> Uladzislau Rezki