linux-kernel - Re: [PATCH v10 1/5] kasan: support backing vmalloc space with real shadow memory

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Date:   Wed, 30 Oct 2019 15:29:51 +0100
From:   Uladzislau Rezki <urezki@...il.com>
To:     Daniel Axtens <dja@...ens.net>
Cc:     kasan-dev@...glegroups.com, linux-mm@...ck.org, x86@...nel.org,
        aryabinin@...tuozzo.com, glider@...gle.com, luto@...nel.org,
        linux-kernel@...r.kernel.org, mark.rutland@....com,
        dvyukov@...gle.com, christophe.leroy@....fr,
        linuxppc-dev@...ts.ozlabs.org, gor@...ux.ibm.com
Subject: Re: [PATCH v10 1/5] kasan: support backing vmalloc space with real
 shadow memory

Hello, Daniel

>  
> @@ -1294,14 +1299,19 @@ static bool __purge_vmap_area_lazy(unsigned long start, unsigned long end)
>  	spin_lock(&free_vmap_area_lock);
>  	llist_for_each_entry_safe(va, n_va, valist, purge_list) {
>  		unsigned long nr = (va->va_end - va->va_start) >> PAGE_SHIFT;
> +		unsigned long orig_start = va->va_start;
> +		unsigned long orig_end = va->va_end;
>  
>  		/*
>  		 * Finally insert or merge lazily-freed area. It is
>  		 * detached and there is no need to "unlink" it from
>  		 * anything.
>  		 */
> -		merge_or_add_vmap_area(va,
> -			&free_vmap_area_root, &free_vmap_area_list);
> +		va = merge_or_add_vmap_area(va, &free_vmap_area_root,
> +					    &free_vmap_area_list);
> +
> +		kasan_release_vmalloc(orig_start, orig_end,
> +				      va->va_start, va->va_end);
>  
I have some questions here. I have not analyzed kasan_releace_vmalloc()
logic in detail, sorry for that if i miss something. __purge_vmap_area_lazy()
deals with big address space, so not only vmalloc addresses it frees here,
basically it can be any, starting from 1 until ULONG_MAX, whereas vmalloc
space spans from VMALLOC_START - VMALLOC_END:

1) Should it be checked that vmalloc only address is freed or you handle
it somewhere else?

if (is_vmalloc_addr(va->va_start))
    kasan_release_vmalloc(...)

2) Have you run any bencmarking just to see how much overhead it adds?
I am asking, because probably it make sense to add those figures to the
backlog(commit message). For example you can run:

<snip>
sudo ./test_vmalloc.sh performance
and
sudo ./test_vmalloc.sh sequential_test_order=1
<snip>

Thanks!

--
Vlad Rezki