Re: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and free_percpu()

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On May 4, 2020, at 11:12 AM, Joerg Roedel jroedel@suse.de wrote:

> On Thu, Apr 30, 2020 at 10:39:19PM -0400, Steven Rostedt wrote:
>> What's so damn special about alloc_percpu()? It's definitely not a fast
>> path. And it's not used often.
> 
> Okay, I fixed it in the percpu code. It is definitly not a nice
> solution, but having to call vmalloc_sync_mappings/unmappings() is not a
> nice solution at any place in the code. Here is the patch which fixes
> this issue for me. I am also not sure what to put in the Fixes tag, as
> it is related to tracing code accessing per-cpu data from the page-fault
> handler, not sure when this got introduced. Maybe someone else can
> provide a meaningful Fixes- or stable tag.
> 
> I also have an idea in mind how to make this all more robust and get rid
> of the vmalloc_sync_mappings/unmappings() interface, will show more when
> I know it works the way I think it does.
> 
> Regards,
> 
>	Joerg
> 
> From c616a9a09499f9c9d682775767d3de7db81fb2ed Mon Sep 17 00:00:00 2001
> From: Joerg Roedel <jroedel@suse.de>
> Date: Mon, 4 May 2020 17:11:41 +0200
> Subject: [PATCH] percpu: Sync vmalloc mappings in pcpu_alloc() and
> free_percpu()
> 
> Sync the vmalloc mappings for all page-tables in the system when
> allocating and freeing per-cpu memory. This is necessary for
> architectures which use page-faults on vmalloc areas.
> 
> The page-fault handlers accesses per-cpu data when tracing is enabled,
> and fauling again in the page-fault handler on a vmalloc'ed per-cpu area
> will result in a recursive fault.
> 
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> mm/percpu.c | 20 ++++++++++++++++++++
> 1 file changed, 20 insertions(+)
> 
> diff --git a/mm/percpu.c b/mm/percpu.c
> index d7e3bc649f4e..6ab035bc6977 100644
> --- a/mm/percpu.c
> +++ b/mm/percpu.c
> @@ -1710,6 +1710,20 @@ static void __percpu *pcpu_alloc(size_t size, size_t
> align, bool reserved,
> 	trace_percpu_alloc_percpu(reserved, is_atomic, size, align,
> 			chunk->base_addr, off, ptr);
> 
> +	/*
> +	 * The per-cpu buffers might be allocated in the vmalloc area of the
> +	 * address space. When the architecture allows faulting on the vmalloc
> +	 * area and the memory allocated here is accessed in the page-fault
> +	 * handler, the vmalloc area fault may be recursive and could never be
> +	 * resolved.
> +	 * This happens for example in the tracing code which allocates per-cpu
> +	 * and accesses them when tracing page-faults.
> +	 * To prevent this, make sure the per-cpu buffers allocated here are
> +	 * mapped in all PGDs so that the page-fault handler will never fault
> +	 * again on them.
> +	 */
> +	vmalloc_sync_mappings();

Placing this here is inefficient. It syncs mappings for each percpu allocation.
I would recommend moving it right after __vmalloc() is called to allocate the
underlying memory chunk instead:

static void *pcpu_mem_zalloc(size_t size, gfp_t gfp)
{
        if (WARN_ON_ONCE(!slab_is_available()))
                return NULL;

        if (size <= PAGE_SIZE)
                return kzalloc(size, gfp);
        else {
                void *p = __vmalloc(size, gfp | __GFP_ZERO, PAGE_KERNEL);
                /* Add comments here ... */
                vmalloc_sync_mappings();
                return p;
        }
}

> +
> 	return ptr;
> 
> fail_unlock:
> @@ -1958,6 +1972,12 @@ void free_percpu(void __percpu *ptr)
> 
> 	trace_percpu_free_percpu(chunk->base_addr, off, ptr);
> 
> +	/*
> +	 * See comment at the vmalloc_sync_mappings() call in pcpu_alloc() for
> +	 * why this is necessary.
> +	 */
> +	vmalloc_sync_unmappings();

I wonder why we'd ever need to explicitly invoke vmalloc_sync_unmappings().
Leaving a stale PTE mapping in place to be lazily unmapped does not seem to
hurt even the tracing use-cases. Why add this call to vmalloc_sync_unmappings()
at all ?

*If* this ends up being needed, it should be moved to:

static void pcpu_mem_free(void *ptr)
{
        /* Add comments here... */
        if (is_vmalloc_addr(ptr))
                vmalloc_sync_unmappings();
        kvfree(ptr);
}

So it is only called before the underlying vmalloc'd chunk is freed, rather than
at each and every percpu free.

Thanks,

Mathieu


> +
> 	spin_unlock_irqrestore(&pcpu_lock, flags);
> 
> 	if (need_balance)
> --
> 2.12.3

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com