Messages in this thread | | | Date | Wed, 25 Mar 2020 23:28:35 -0700 | From | Minchan Kim <> | Subject | Re: [PATCH 2/2] mm/madvise: skip MADV_PAGEOUT on shared swap cache pages |
| |
On Mon, Mar 23, 2020 at 04:41:51PM -0700, Dave Hansen wrote: > > From: Dave Hansen <dave.hansen@linux.intel.com> > > 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@linux.intel.com> > Cc: Michal Hocko <mhocko@suse.com> > Cc: Jann Horn <jannh@google.com> > Cc: Vlastimil Babka <vbabka@suse.cz> > Cc: Minchan Kim <minchan@kernel.org> > Cc: Daniel Colascione <dancol@google.com> > Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org> > Cc: Andrew Morton <akpm@linux-foundation.org> > --- > > 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@kernel.org>
Thanks, Dave!
_
| |