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

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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





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