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Wed, 7 Feb 2018 16:05:37 -0500 Date: Wed, 7 Feb 2018 13:05:34 -0800 From: Andrew Morton To: "Huang, Ying" Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, Huang Ying , Konrad Rzeszutek Wilk , Dan Streetman , Seth Jennings , Minchan Kim , Tetsuo Handa , Shaohua Li , Michal Hocko , Johannes Weiner , Mel Gorman , Shakeel Butt , stable@vger.kernel.org, Sergey Senozhatsky Subject: Re: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled Message-Id: <20180207130534.259cd71a595c6275b2da38d3@linux-foundation.org> In-Reply-To: <20180207070035.30302-1-ying.huang@intel.com> References: <20180207070035.30302-1-ying.huang@intel.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On Wed, 7 Feb 2018 15:00:35 +0800 "Huang, Ying" wrote: > From: Huang Ying > > 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. > > ... > > --- 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; > hm. This is assuming that "cluster==true" means "this is thp swap". That's presently true, but is it appropriate that get_swap_pages() is peeking at "cluster" to work out why it is being called? Or would it be cleaner to do this in get_swap_page()? Something like --- a/mm/swap_slots.c~a +++ a/mm/swap_slots.c @@ -317,8 +317,11 @@ swp_entry_t get_swap_page(struct page *p entry.val = 0; if (PageTransHuge(page)) { - if (IS_ENABLED(CONFIG_THP_SWAP)) - get_swap_pages(1, true, &entry); + /* Frontswap doesn't support THP */ + if (!frontswap_enabled()) { + if (IS_ENABLED(CONFIG_THP_SWAP)) + get_swap_pages(1, true, &entry); + } return entry; } _