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

[Baoquan He <bhe@redhat.com>]

On 12/13/21 at 01:43pm, Hyeonggon Yoo wrote:
> 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.

The problem is this is a generic issue on x86_64, and will be warned out
always on all x86_64 systems, but not on a certain machine or a certain
type of machine. If not fixed, we can always see it in kdump kernel. The
way things are, it doesn't casue system or device collapse even if
dma-kmalloc can't provide buffer or provide buffer from zone NORMAL.


I have got bug reports several times from different people, and we have
several bugs tracking this inside Redhat. I think nobody want to see
this appearing in customers' monitor w or w/o a note. If we have to
leave it with that, it's a little embrassing.


> 
> > ---
> >  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?

This is a great question. Honestly, no,

On the surface, it's obviously not what we want, We should never give
user a zone NORMAL memory when they ask for zone DMA memory. If going to
this specific x86_64 ARCH where this problem is observed, I prefer to give
it zone DMA32 memory if zone DMA allocation failed. Because we rarely
have ISA device deployed which requires low 16M DMA buffer. The zone DMA
is just in case. Thus, for kdump kernel, we have been trying to make sure
zone DMA32 has enough memory to satisfy PCIe device DMA buffer allocation,
I don't remember we made any effort to do that for zone DMA.

Now the thing is that the nothing serious happened even if sr_probe()
doesn't get DMA buffer from zone DMA. And it works well when I feed it
with zone NORMAL memory instead with this patch applied.
> 
> Maybe the function get_capabilities() want to allocate memory
> even if it's not from DMA zone, but other callers will not expect that.

Yeah, I have the same guess too for get_capabilities(), not sure about other
callers. Or, as ChristophL and ChristophH said(Sorry, not sure if this is
the right way to call people when the first name is the same. Correct me if
it's wrong), any buffer requested from kmalloc can be used by device driver.
Means device enforces getting memory inside addressing limit for those
DMA transferring buffer which is usually large, Megabytes level with
vmalloc() or alloc_pages(), but doesn't care about this kind of small
piece buffer memory allocated with kmalloc()? Just a guess, please tell
a counter example if anyone happens to know, it could be easy.


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