Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled

[Minchan Kim]

Hi Huang,

On Thu, Feb 08, 2018 at 11:27:50PM +0800, huang ying wrote:
> On Wed, Feb 7, 2018 at 3:00 PM, Huang, Ying <ying.huang@xxxxxxxxx> wrote:
> > From: Huang Ying <huang.ying.caritas@xxxxxxxxx>
> >
> > It was reported by Sergey Senozhatsky that if THP (Transparent Huge
> > Page) and frontswap (via zswap) are both enabled, when memory goes low
> > so that swap is triggered, segfault and memory corruption will occur
> > in random user space applications as follow,
> >
> > kernel: urxvt[338]: segfault at 20 ip 00007fc08889ae0d sp 00007ffc73a7fc40 error 6 in libc-2.26.so[7fc08881a000+1ae000]
> >  #0  0x00007fc08889ae0d _int_malloc (libc.so.6)
> >  #1  0x00007fc08889c2f3 malloc (libc.so.6)
> >  #2  0x0000560e6004bff7 _Z14rxvt_wcstoutf8PKwi (urxvt)
> >  #3  0x0000560e6005e75c n/a (urxvt)
> >  #4  0x0000560e6007d9f1 _ZN16rxvt_perl_interp6invokeEP9rxvt_term9hook_typez (urxvt)
> >  #5  0x0000560e6003d988 _ZN9rxvt_term9cmd_parseEv (urxvt)
> >  #6  0x0000560e60042804 _ZN9rxvt_term6pty_cbERN2ev2ioEi (urxvt)
> >  #7  0x0000560e6005c10f _Z17ev_invoke_pendingv (urxvt)
> >  #8  0x0000560e6005cb55 ev_run (urxvt)
> >  #9  0x0000560e6003b9b9 main (urxvt)
> >  #10 0x00007fc08883af4a __libc_start_main (libc.so.6)
> >  #11 0x0000560e6003f9da _start (urxvt)
> >
> > After bisection, it was found the first bad commit is
> > bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped
> > out").
> >
> > The root cause is as follow.
> >
> > When the pages are written to swap device during swapping out in
> > swap_writepage(), zswap (fontswap) is tried to compress the pages
> > instead to improve the performance.  But zswap (frontswap) will treat
> > THP as normal page, so only the head page is saved.  After swapping
> > in, tail pages will not be restored to its original contents, so cause
> > the memory corruption in the applications.
> >
> > This is fixed via splitting THP before writing the page to swap device
> > if frontswap is enabled.  To deal with the situation where frontswap
> > is enabled at runtime, whether the page is THP is checked before using
> > frontswap during swapping out too.
> >
> > Reported-and-tested-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx>
> > Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: Dan Streetman <ddstreet@xxxxxxxx>
> > Cc: Seth Jennings <sjenning@xxxxxxxxxx>
> > Cc: Minchan Kim <minchan@xxxxxxxxxx>
> > Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Cc: Shaohua Li <shli@xxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx # 4.14
> > Fixes: bd4c82c22c367e068 ("mm, THP, swap: delay splitting THP after swapped out")
> >
> > Changelog:
> >
> > v2:
> >
> > - Move frontswap check into swapfile.c to avoid to make vmscan.c
> >   depends on frontswap.
> > ---
> >  mm/page_io.c  | 2 +-
> >  mm/swapfile.c | 3 +++
> >  2 files changed, 4 insertions(+), 1 deletion(-)
> >
> > diff --git a/mm/page_io.c b/mm/page_io.c
> > index b41cf9644585..6dca817ae7a0 100644
> > --- a/mm/page_io.c
> > +++ b/mm/page_io.c
> > @@ -250,7 +250,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> >                 unlock_page(page);
> >                 goto out;
> >         }
> > -       if (frontswap_store(page) == 0) {
> > +       if (!PageTransHuge(page) && frontswap_store(page) == 0) {
> >                 set_page_writeback(page);
> >                 unlock_page(page);
> >                 end_page_writeback(page);
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index 006047b16814..0b7c7883ce64 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -934,6 +934,9 @@ int get_swap_pages(int n_goal, bool cluster, swp_entry_t swp_entries[])
> >
> >         /* Only single cluster request supported */
> >         WARN_ON_ONCE(n_goal > 1 && cluster);
> > +       /* Frontswap doesn't support THP */
> > +       if (frontswap_enabled() && cluster)
> > +               goto noswap;
> 
> I found this will cause THP swap optimization be turned off forever if
> CONFIG_ZSWAP=y (which cannot =m).  Because frontswap is enabled quite
> statically instead of dynamically.  If frontswap_ops is registered, it
> will be enabled unconditionally and forever.  And zswap will register
> frontswap_ops during initialize regardless whether zswap is enabled or
> not.

Indeed.

> 
> So I think it will be better to remove swapfile.c changes in this
> patch, just keep page_io.c changes.  Because THP is more dynamic, it

Then, I think it should be done by frontswap backend rather than generic
swap layer. Because there are two backends now and one of them can support
first.

diff --git a/drivers/xen/tmem.c b/drivers/xen/tmem.c
index bf13d1ec51f3..bdaf309aeea6 100644
--- a/drivers/xen/tmem.c
+++ b/drivers/xen/tmem.c
@@ -284,6 +284,9 @@ static int tmem_frontswap_store(unsigned type, pgoff_t offset,
        int pool = tmem_frontswap_poolid;
        int ret;
 
+       if (PageTransHuge(page))
+               return -EINVAL;
+
        if (pool < 0)
                return -1;
        if (ind64 != ind)
diff --git a/mm/zswap.c b/mm/zswap.c
index c004aa4fd3f4..e343534d2892 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -1007,6 +1007,9 @@ static int zswap_frontswap_store(unsigned type, pgoff_t offset,
        u8 *src, *dst;
        struct zswap_header zhdr = { .swpentry = swp_entry(type, offset) };
 
+       if (PageTransHuge(page))
+               return -EINVAL;
+
        if (!zswap_enabled || !tree) {
                ret = -ENODEV;
                goto reject;