netdev - Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

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Date:   Tue, 4 Dec 2018 00:04:21 +0000
From:   "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To:     "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
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        "Keshavamurthy, Anil S" <anil.s.keshavamurthy@...el.com>
Subject: Re: [PATCH 1/2] vmalloc: New flag for flush before releasing pages

It looks like this new flag is in linux-next now. As I am reading it, these
architectures have a module_alloc that uses some sort of executable flag and
are not using the default module_alloc which is already covered, and so may need
it plugged in:
arm
arm64
parisc
s390
unicore32

Thanks,

Rick

On Tue, 2018-11-27 at 16:07 -0800, Rick Edgecombe wrote:
> Since vfree will lazily flush the TLB, but not lazily free the underlying
> pages,
> it often leaves stale TLB entries to freed pages that could get re-used. This
> is
> undesirable for cases where the memory being freed has special permissions
> such
> as executable.
> 
> Having callers flush the TLB after calling vfree still leaves a window where
> the pages are freed, but the TLB entry remains. Also the entire operation can
> be
> deferred if the vfree is called from an interrupt and so a TLB flush after
> calling vfree would miss the entire operation. So in order to support this use
> case, a new flag VM_IMMEDIATE_UNMAP is added, that will cause the free
> operation
> to take place like this:
>         1. Unmap
>         2. Flush TLB/Unmap aliases
>         3. Free pages
> In the deferred case these steps are all done by the work queue.
> 
> This implementation derives from two sketches from Dave Hansen and
> Andy Lutomirski.
> 
> Suggested-by: Dave Hansen <dave.hansen@...el.com>
> Suggested-by: Andy Lutomirski <luto@...nel.org>
> Suggested-by: Will Deacon <will.deacon@....com>
> Signed-off-by: Rick Edgecombe <rick.p.edgecombe@...el.com>
> ---
>  include/linux/vmalloc.h |  1 +
>  mm/vmalloc.c            | 13 +++++++++++--
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/vmalloc.h b/include/linux/vmalloc.h
> index 398e9c95cd61..cca6b6b83cf0 100644
> --- a/include/linux/vmalloc.h
> +++ b/include/linux/vmalloc.h
> @@ -21,6 +21,7 @@ struct notifier_block;		/* in notifier.h */
>  #define VM_UNINITIALIZED	0x00000020	/* vm_struct is not fully
> initialized */
>  #define VM_NO_GUARD		0x00000040      /* don't add guard page */
>  #define VM_KASAN		0x00000080      /* has allocated kasan shadow
> memory */
> +#define VM_IMMEDIATE_UNMAP	0x00000200	/* flush before releasing
> pages */
>  /* bits [20..32] reserved for arch specific ioremap internals */
>  
>  /*
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index 97d4b25d0373..68766651b5a7 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -1516,6 +1516,14 @@ static void __vunmap(const void *addr, int
> deallocate_pages)
>  	debug_check_no_obj_freed(area->addr, get_vm_area_size(area));
>  
>  	remove_vm_area(addr);
> +
> +	/*
> +	 * Need to flush the TLB before freeing pages in the case of this flag.
> +	 * As long as that's happening, unmap aliases.
> +	 */
> +	if (area->flags & VM_IMMEDIATE_UNMAP)
> +		vm_unmap_aliases();
> +
>  	if (deallocate_pages) {
>  		int i;
>  
> @@ -1925,8 +1933,9 @@ EXPORT_SYMBOL(vzalloc_node);
>  
>  void *vmalloc_exec(unsigned long size)
>  {
> -	return __vmalloc_node(size, 1, GFP_KERNEL, PAGE_KERNEL_EXEC,
> -			      NUMA_NO_NODE, __builtin_return_address(0));
> +	return __vmalloc_node_range(size, 1, VMALLOC_START, VMALLOC_END,
> +			GFP_KERNEL, PAGE_KERNEL_EXEC, VM_IMMEDIATE_UNMAP,
> +			NUMA_NO_NODE, __builtin_return_address(0));
>  }
>  
>  #if defined(CONFIG_64BIT) && defined(CONFIG_ZONE_DMA32)