Re: [PATCH v3 5/5] mm/slub: do not create dma-kmalloc if no managed pages in DMA zone

Hyeonggon Yoo
2021-12-13T13:43:31Z

Hello Baoquan. I have a question on your code.

On Mon, Dec 13, 2021 at 08:27:12PM +0800, Baoquan He wrote:
> Dma-kmalloc will be created as long as CONFIG_ZONE_DMA is enabled.
> However, it will fail if DMA zone has no managed pages. The failure
> can be seen in kdump kernel of x86_64 as below:
>
> CPU: 0 PID: 65 Comm: kworker/u2:1 Not tainted 5.14.0-rc2+ #9
> Hardware name: Intel Corporation SandyBridge Platform/To be filled by O.E.M., BIOS RMLSDP.86I.R2.28.D690.1306271008 06/27/2013
> Workqueue: events_unbound async_run_entry_fn
> Call Trace:
> dump_stack_lvl+0x57/0x72
> warn_alloc.cold+0x72/0xd6
> __alloc_pages_slowpath.constprop.0+0xf56/0xf70
> __alloc_pages+0x23b/0x2b0
> allocate_slab+0x406/0x630
> ___slab_alloc+0x4b1/0x7e0
> ? sr_probe+0x200/0x600
> ? lock_acquire+0xc4/0x2e0
> ? fs_reclaim_acquire+0x4d/0xe0
> ? lock_is_held_type+0xa7/0x120
> ? sr_probe+0x200/0x600
> ? __slab_alloc+0x67/0x90
> __slab_alloc+0x67/0x90
> ? sr_probe+0x200/0x600
> ? sr_probe+0x200/0x600
> kmem_cache_alloc_trace+0x259/0x270
> sr_probe+0x200/0x600
> ......
> bus_probe_device+0x9f/0xb0
> device_add+0x3d2/0x970
> ......
> __scsi_add_device+0xea/0x100
> ata_scsi_scan_host+0x97/0x1d0
> async_run_entry_fn+0x30/0x130
> process_one_work+0x2b0/0x5c0
> worker_thread+0x55/0x3c0
> ? process_one_work+0x5c0/0x5c0
> kthread+0x149/0x170
> ? set_kthread_struct+0x40/0x40
> ret_from_fork+0x22/0x30
> Mem-Info:
> ......
>
> The above failure happened when calling kmalloc() to allocate buffer with
> GFP_DMA. It requests to allocate slab page from DMA zone while no managed
> pages in there.
> sr_probe()
> --> get_capabilities()
> --> buffer = kmalloc(512, GFP_KERNEL | GFP_DMA);
>
> The DMA zone should be checked if it has managed pages, then try to create
> dma-kmalloc.
>

What is problem here?

The slab allocator requested buddy allocator with GFP_DMA,
and then buddy allocator failed to allocate page in DMA zone because
there was no page in DMA zone. and then the buddy allocator called warn_alloc
because it failed at allocating page.

Looking at warn, I don't understand what the problem is.

> ---
> mm/slab_common.c | 9 +++++++++
> 1 file changed, 9 insertions(+)
>
> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index e5d080a93009..ae4ef0f8903a 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -878,6 +878,9 @@ void __init create_kmalloc_caches(slab_flags_t flags)
> {
> int i;
> enum kmalloc_cache_type type;
> +#ifdef CONFIG_ZONE_DMA
> + bool managed_dma;
> +#endif
>
> /*
> * Including KMALLOC_CGROUP if CONFIG_MEMCG_KMEM defined
> @@ -905,10 +908,16 @@ void __init create_kmalloc_caches(slab_flags_t flags)
> slab_state = UP;
>
> #ifdef CONFIG_ZONE_DMA
> + managed_dma = has_managed_dma();
> +
> for (i = 0; i <= KMALLOC_SHIFT_HIGH; i++) {
> struct kmem_cache *s = kmalloc_caches[KMALLOC_NORMAL][i];
>
> if (s) {
> + if (!managed_dma) {
> + kmalloc_caches[KMALLOC_DMA][i] = kmalloc_caches[KMALLOC_NORMAL][i];
> + continue;
> + }

This code is copying normal kmalloc caches to DMA kmalloc caches.
With this code, the kmalloc() with GFP_DMA will succeed even if allocated
memory is not actually from DMA zone. Is that really what you want?

Maybe the function get_capabilities() want to allocate memory
even if it's not from DMA zone, but other callers will not expect that.

Thanks,
Hyeonggon.

> kmalloc_caches[KMALLOC_DMA][i] = create_kmalloc_cache(
> kmalloc_info[i].name[KMALLOC_DMA],
> kmalloc_info[i].size,
> --
> 2.17.2