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    SubjectRe: [PATCH -mm -v2] mm, swap, frontswap: Fix THP swap if frontswap enabled
    On Wed,  7 Feb 2018 15:00:35 +0800 "Huang, Ying" <ying.huang@intel.com> wrote:

    > From: Huang Ying <huang.ying.caritas@gmail.com>
    >
    > 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;
    }

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