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

From: Florian Westphal <[email-masked]>
Date: Fri, 23 Sep 2022 16:54:09 +0200

Uladzislau Rezki <[email-masked]> wrote:
> On Fri, Sep 23, 2022 at 03:35:12PM +0200, Florian Westphal wrote:
> > Michal Hocko <[email-masked]> 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.