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Date:   Mon, 7 Dec 2020 02:34:30 +0000
From:   HORIGUCHI NAOYA(堀口 直也) 
        <naoya.horiguchi@....com>
To:     Oscar Salvador <osalvador@...e.de>
CC:     Vlastimil Babka <vbabka@...e.cz>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "n-horiguchi@...jp.nec.com" <n-horiguchi@...jp.nec.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Dan Williams <dan.j.williams@...el.com>
Subject: Re: [PATCH 3/7] mm,madvise: call soft_offline_page() without
 MF_COUNT_INCREASED

On Sat, Dec 05, 2020 at 04:34:23PM +0100, Oscar Salvador wrote:
> On Fri, Dec 04, 2020 at 06:25:31PM +0100, Vlastimil Babka wrote:
> > OK, so that means we don't introduce this race for MADV_SOFT_OFFLINE, but it's
> > already (and still) there for MADV_HWPOISON since Dan's 23e7b5c2e271 ("mm,
> > madvise_inject_error: Let memory_failure() optionally take a page reference") no?
> 
> What about the following?
> CCing Dan as well.

Hi Oscar, Vlastimil,

Thanks for mentioning this. I agree with that direction.

> 
> From: Oscar Salvador <osalvador@...e.de>
> Date: Sat, 5 Dec 2020 16:14:40 +0100
> Subject: [PATCH] mm,memory_failure: Always pin the page in
>  madvise_inject_error
> 
> madvise_inject_error() uses get_user_pages_fast to get the page
> from the addr we specified.
> After [1], we drop such extra reference for memory_failure() path.
> That commit says that memory_failure wanted to keep the pin in order
> to take the page out of circulation.
> 
> The truth is that we need to keep the page pinned, otherwise the
> page might be re-used after the put_page(), and we can end up messing
> with someone else's memory.
> E.g:
> 
> CPU0
> process X					CPU1
>  madvise_inject_error
>   get_user_pages
>    put_page
> 					page gets reclaimed
> 					process Y allocates the page
>   memory_failure
>    // We mess with process Y memory
> 
> madvise() is meant to operate on a self address space, so messing with
> pages that do not belong to us seems the wrong thing to do.
> To avoid that, let us keep the page pinned for memory_failure as well.
> 
> Pages for DAX mappings will release this extra refcount in
> memory_failure_dev_pagemap.
> 
> [1] ("23e7b5c2e271: mm, madvise_inject_error:
>       Let memory_failure() optionally take a page reference")
> 
> Signed-off-by: Oscar Salvador <osalvador@...e.de>
> Suggested-by: Vlastimil Babka <vbabka@...e.cz>
> Fixes: 23e7b5c2e271 ("mm, madvise_inject_error: Let memory_failure() optionally take a page reference")
> ---
>  mm/madvise.c        | 9 +--------
>  mm/memory-failure.c | 6 ++++++
>  2 files changed, 7 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/madvise.c b/mm/madvise.c
> index c6b5524add58..19edddba196d 100644
> --- a/mm/madvise.c
> +++ b/mm/madvise.c
> @@ -907,14 +907,7 @@ static int madvise_inject_error(int behavior,
>  		} else {
>  			pr_info("Injecting memory failure for pfn %#lx at process virtual address %#lx\n",
>  				 pfn, start);
> -			/*
> -			 * Drop the page reference taken by get_user_pages_fast(). In
> -			 * the absence of MF_COUNT_INCREASED the memory_failure()
> -			 * routine is responsible for pinning the page to prevent it
> -			 * from being released back to the page allocator.
> -			 */
> -			put_page(page);
> -			ret = memory_failure(pfn, 0);
> +			ret = memory_failure(pfn, MF_COUNT_INCREASED);
>  		}
>  
>  		if (ret)
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index 869ece2a1de2..ba861169c9ae 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -1269,6 +1269,12 @@ static int memory_failure_dev_pagemap(unsigned long pfn, int flags,
>  	if (!cookie)
>  		goto out;
>  
> +	if (flags & MF_COUNT_INCREASED)
> +		/*
> +		 * Drop the extra refcount in case we come from madvise().
> +		 */
> +		put_page(page);
> +

Should this if-block come before dax_lock_page() block?
It seems that if dax_lock_page returns NULL, memory_failure_dev_pagemap()
returns without releasing the refcount.
memory_failure() on dev_pagemap doesn't use page refcount (unlike other
type of memory), so we can release it unconditionally.

Thanks,
Naoya Horiguchi

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