cgroups: warning for metadata allocation with GFP_NOFAIL (was Re: folio_alloc_buffers() doing allocations > order 1 with GFP_NOFAIL)

[Christoph Lameter <cl@linux.com>]

Right.. Well lets add the cgoup folks to this.

The code that simply uses the GFP_NOFAIL to allocate cgroup metadata 
using an order > 1:

int memcg_alloc_slab_cgroups(struct slab *slab, struct kmem_cache *s,
 				 gfp_t gfp, bool new_slab)
{
 	unsigned int objects = objs_per_slab(s, slab);
 	unsigned long memcg_data;
 	void *vec;

 	gfp &= ~OBJCGS_CLEAR_MASK;
 	vec = kcalloc_node(objects, sizeof(struct obj_cgroup *), gfp,
 			   slab_nid(slab));




On Wed, 1 Nov 2023, Matthew Wilcox wrote:

> On Tue, Oct 31, 2023 at 05:13:57PM -0700, Christoph Lameter (Ampere) wrote:
>> Hi Matthew,
>>
>> There is a strange warning on bootup related to folios. Seen it a couple of
>> times before. Why does this occur?
>
> Filesystems generally can't cope with failing to allocate a bufferhead.
> So the buffer head code sets __GFP_NOFAIL.  That's better than trying
> to implement __GFP_NOFAIL semantics in the fs code, right?
>
>> [   20.878110] Call trace:
>> [   20.878111]  get_page_from_freelist+0x214/0x17f8
>> [   20.878116]  __alloc_pages+0x17c/0xe08
>> [   20.878120]  __kmalloc_large_node+0xa0/0x170
>> [   20.878123]  __kmalloc_node+0x120/0x1d0
>> [   20.878125]  memcg_alloc_slab_cgroups+0x48/0xc0
>
> Oho.  It's not buffer's fault, specifically.  memcg is allocating
> its own metadata for the slab.  I decree this Not My Fault.
>
>> [   20.878128]  memcg_slab_post_alloc_hook+0xa8/0x1c8
>> [   20.878132]  kmem_cache_alloc+0x18c/0x338
>> [   20.878135]  alloc_buffer_head+0x28/0xa0
>> [   20.878138]  folio_alloc_buffers+0xe8/0x1c0
>> [   20.878141]  folio_create_empty_buffers+0x2c/0x1e8
>> [   20.878143]  folio_create_buffers+0x58/0x80
>> [   20.878145]  block_read_full_folio+0x80/0x450
>> [   20.878148]  blkdev_read_folio+0x24/0x38
>> [   20.956921]  filemap_read_folio+0x60/0x138
>> [   20.956925]  do_read_cache_folio+0x180/0x298
>> [   20.965270]  read_cache_page+0x24/0x90
>> [   20.965273]  __arm64_sys_swapon+0x2e0/0x1208
>> [   20.965277]  invoke_syscall+0x78/0x108
>> [   20.965282]  el0_svc_common.constprop.0+0x48/0xf0
>> [   20.981702]  do_el0_svc+0x24/0x38
>> [   20.993773]  el0t_64_sync_handler+0x100/0x130
>> [   20.993776]  el0t_64_sync+0x190/0x198
>> [   20.993779] ---[ end trace 0000000000000000 ]---
>> [   20.999972] Adding 999420k swap on /dev/mapper/eng07sys--r113--vg-swap_1.
>> Priority:-2 extents:1 across:999420k SS
>>
>> This is due to
>>
>>
>>
>> folio_alloc_buffers() setting GFP_NOFAIL:
>>
>>
>> struct buffer_head *folio_alloc_buffers(struct folio *folio, unsigned long
>> size,
>>                                         bool retry)
>> {
>>         struct buffer_head *bh, *head;
>>         gfp_t gfp = GFP_NOFS | __GFP_ACCOUNT;
>>         long offset;
>>         struct mem_cgroup *memcg, *old_memcg;
>>
>>         if (retry)
>>                 gfp |= __GFP_NOFAIL;
>
> This isn't new.  It was introduced by 640ab98fb362 in 2017.
> It seems reasonable to be able to kmalloc(512, GFP_NOFAIL).  It's the
> memcg code which is having problems here.
>