Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?

[Minchan Kim <minchan@kernel.org>]

On Thu, Mar 12, 2020 at 02:41:07PM -0700, Dave Hansen wrote:
> One other fun thing.  I have a "victim" thread sitting in a loop doing:
> 
> 	sleep(1)
> 	memcpy(&garbage, buffer, sz);
> 
> The "attacker" is doing
> 
> 	madvise(buffer, sz, MADV_PAGEOUT);
> 
> in a loop.  That, oddly enough doesn't cause the victim to page fault.
> But, if I do:
> 
> 	memcpy(&garbage, buffer, sz);
> 	madvise(buffer, sz, MADV_PAGEOUT);
> 
> It *does* cause the memory to get paged out.  The MADV_PAGEOUT code
> actually has a !pte_present() check.  It will punt on a PTE if it sees
> it.  In other words, if a page is in the swap cache but not mapped by a
> pte_present() PTE, MADV_PAGEOUT won't touch it.
> 
> Shouldn't MADV_PAGEOUT be able to find and reclaim those pages?  Patch
> attached.

> 
> 
> ---
> 
>  b/mm/madvise.c |   38 +++++++++++++++++++++++++++++++-------
>  1 file changed, 31 insertions(+), 7 deletions(-)
> 
> diff -puN mm/madvise.c~madv-pageout-find-swap-cache mm/madvise.c
> --- a/mm/madvise.c~madv-pageout-find-swap-cache	2020-03-12 14:24:45.178775035 -0700
> +++ b/mm/madvise.c	2020-03-12 14:35:49.706773378 -0700
> @@ -248,6 +248,36 @@ static void force_shm_swapin_readahead(s
>  #endif		/* CONFIG_SWAP */
>  
>  /*
> + * Given a PTE, find the corresponding 'struct page'.  Also handles
> + * non-present swap PTEs.
> + */
> +struct page *pte_to_reclaim_page(struct vm_area_struct *vma,
> +				 unsigned long addr, pte_t ptent)
> +{
> +	swp_entry_t entry;
> +
> +	/* Totally empty PTE: */
> +	if (pte_none(ptent))
> +		return NULL;
> +
> +	/* A normal, present page is mapped: */
> +	if (pte_present(ptent))
> +		return vm_normal_page(vma, addr, ptent);
> +

Please check is_swap_pte first.

> +	entry = pte_to_swp_entry(vmf->orig_pte);
> +	/* Is it one of the "swap PTEs" that's not really swap? */
> +	if (non_swap_entry(entry))
> +		return false;
> +
> +	/*
> +	 * The PTE was a true swap entry.  The page may be in the
> +	 * swap cache.  If so, find it and return it so it may be
> +	 * reclaimed.
> +	 */
> +	return lookup_swap_cache(entry, vma, addr);

If we go with handling only exclusived owned page for anon,
I think we should apply the rule to swap cache, too.

Do you mind posting it as formal patch?

Thanks for the explain about vulnerability and the patch, Dave!

> +}
> +
> +/*
>   * Schedule all required I/O operations.  Do not wait for completion.
>   */
>  static long madvise_willneed(struct vm_area_struct *vma,
> @@ -389,13 +419,7 @@ regular_page:
>  	for (; addr < end; pte++, addr += PAGE_SIZE) {
>  		ptent = *pte;
>  
> -		if (pte_none(ptent))
> -			continue;
> -
> -		if (!pte_present(ptent))
> -			continue;
> -
> -		page = vm_normal_page(vma, addr, ptent);
> +		page = pte_to_reclaim_page(vma, addr, ptent);
>  		if (!page)
>  			continue;
>  
> _