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

Hi Huang,

On Thu, Feb 08, 2018 at 11:27:50PM +0800, huang ying wrote:

quoted

On Wed, Feb 7, 2018 at 3:00 PM, Huang, Ying [off-list ref] wrote:

quoted

From: Huang Ying <redacted>

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 <redacted>
Signed-off-by: "Huang, Ying" <redacted>
Cc: Konrad Rzeszutek Wilk <redacted>
Cc: Dan Streetman <redacted>
Cc: Seth Jennings <redacted>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: Shaohua Li <shli@kernel.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <redacted>
Cc: Shakeel Butt <redacted>
Cc: stable@vger.kernel.org # 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.

quoted

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;