Re: [PATCH 2/2] mm/madvise: skip MADV_PAGEOUT on shared swap cache pages

[Minchan Kim]

On Mon, Mar 23, 2020 at 04:41:51PM -0700, Dave Hansen wrote:
> 
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> 
> MADV_PAGEOUT might interfere with other processes if it is
> allowed to reclaim pages shared with other processses.  A
> previous patch tried to avoid this for anonymous pages
> which were shared by a fork().  It did this by checking
> page_mapcount().
> 
> That works great for mapped pages.  But, it can not detect
> unmapped swap cache pages.  This has not been a problem,
> until the previous patch which added the ability for
> MADV_PAGEOUT to *find* swap cache pages.
> 
> A process doing MADV_PAGEOUT which finds an unmapped swap
> cache page and evicts it might interfere with another process
> which had the same page mapped.  But, such a page would have
> a page_mapcount() of 1 since the page is only actually mapped
> in the *other* process.  The page_mapcount() test would fail
> to detect the situation.
> 
> Thankfully, there is a reference count for swap entries.
> To fix this, simply consult both page_mapcount() and the swap
> reference count via page_swapcount().
> 
> I rigged up a little test program to try to create these
> situations.  Basically, if the parent "reader" RSS changes
> in response to MADV_PAGEOUT actions in the child, there is
> a problem.
> 
> 	https://www.sr71.net/~dave/intel/madv-pageout.c
> 
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Michal Hocko <mhocko@xxxxxxxx>
> Cc: Jann Horn <jannh@xxxxxxxxxx>
> Cc: Vlastimil Babka <vbabka@xxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Daniel Colascione <dancol@xxxxxxxxxx>
> Cc: "Joel Fernandes (Google)" <joel@xxxxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
> 
>  b/mm/madvise.c |   37 +++++++++++++++++++++++++++++--------
>  1 file changed, 29 insertions(+), 8 deletions(-)
> 
> diff -puN mm/madvise.c~madv-pageout-ignore-shared-swap-cache mm/madvise.c
> --- a/mm/madvise.c~madv-pageout-ignore-shared-swap-cache	2020-03-23 16:30:52.022385888 -0700
> +++ b/mm/madvise.c	2020-03-23 16:41:15.448384333 -0700
> @@ -261,6 +261,7 @@ static struct page *pte_get_reclaim_page
>  {
>  	swp_entry_t entry;
>  	struct page *page;
> +	int nr_page_references = 0;

nit: just 'referenced' would be enough.

Acked-by: Minchan Kim <minchan@xxxxxxxxxx>

Thanks, Dave!

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