From: Huang Ying ying.huang@intel.com Subject: mm, swap: fix swapoff with KSM pages
KSM pages may be mapped to the multiple VMAs that cannot be reached from one anon_vma. So during swapin, a new copy of the page need to be generated if a different anon_vma is needed, please refer to comments of ksm_might_need_to_copy() for details.
During swapoff, unuse_vma() uses anon_vma (if available) to locate VMA and virtual address mapped to the page, so not all mappings to a swapped out KSM page could be found. So in try_to_unuse(), even if the swap count of a swap entry isn't zero, the page needs to be deleted from swap cache, so that, in the next round a new page could be allocated and swapin for the other mappings of the swapped out KSM page.
But this contradicts with the THP swap support. Where the THP could be deleted from swap cache only after the swap count of every swap entry in the huge swap cluster backing the THP has reach 0. So try_to_unuse() is changed in commit e07098294adf ("mm, THP, swap: support to reclaim swap space for THP swapped out") to check that before delete a page from swap cache, but this has broken KSM swapoff too.
Fortunately, KSM is for the normal pages only, so the original behavior for KSM pages could be restored easily via checking PageTransCompound(). That is how this patch works.
The bug is introduced by e07098294adf ("mm, THP, swap: support to reclaim swap space for THP swapped out"), which is merged by v4.14-rc1. So I think we should backport the fix to from 4.14 on. But Hugh thinks it may be rare for the KSM pages being in the swap device when swapoff, so nobody reports the bug so far.
Link: http://lkml.kernel.org/r/20181226051522.28442-1-ying.huang@intel.com Fixes: e07098294adf ("mm, THP, swap: support to reclaim swap space for THP swapped out") Signed-off-by: "Huang, Ying" ying.huang@intel.com Reported-by: Hugh Dickins hughd@google.com Tested-by: Hugh Dickins hughd@google.com Acked-by: Hugh Dickins hughd@google.com Cc: Rik van Riel riel@redhat.com Cc: Johannes Weiner hannes@cmpxchg.org Cc: Minchan Kim minchan@kernel.org Cc: Shaohua Li shli@kernel.org Cc: Daniel Jordan daniel.m.jordan@oracle.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---
--- a/mm/swapfile.c~mm-swap-fix-swapoff-with-ksm-pages +++ a/mm/swapfile.c @@ -2197,7 +2197,8 @@ int try_to_unuse(unsigned int type, bool */ if (PageSwapCache(page) && likely(page_private(page) == entry.val) && - !page_swapped(page)) + (!PageTransCompound(page) || + !swap_page_trans_huge_swapped(si, entry))) delete_from_swap_cache(compound_head(page));
/* _