• Johannes Weiner's avatar
    mm: filemap: don't plant shadow entries without radix tree node · 7d5d3b13
    Johannes Weiner authored
    commit d3798ae8c6f3767c726403c2ca6ecc317752c9dd upstream.
    
    When the underflow checks were added to workingset_node_shadow_dec(),
    they triggered immediately:
    
      kernel BUG at ./include/linux/swap.h:276!
      invalid opcode: 0000 [#1] SMP
      Modules linked in: isofs usb_storage fuse xt_CHECKSUM ipt_MASQUERADE nf_nat_masquerade_ipv4 tun nf_conntrack_netbios_ns nf_conntrack_broadcast ip6t_REJECT nf_reject_ipv6
       soundcore wmi acpi_als pinctrl_sunrisepoint kfifo_buf tpm_tis industrialio acpi_pad pinctrl_intel tpm_tis_core tpm nfsd auth_rpcgss nfs_acl lockd grace sunrpc dm_crypt
      CPU: 0 PID: 20929 Comm: blkid Not tainted 4.8.0-rc8-00087-gbe67d60b #1
      Hardware name: System manufacturer System Product Name/Z170-K, BIOS 1803 05/06/2016
      task: ffff8faa93ecd940 task.stack: ffff8faa7f478000
      RIP: page_cache_tree_insert+0xf1/0x100
      Call Trace:
        __add_to_page_cache_locked+0x12e/0x270
        add_to_page_cache_lru+0x4e/0xe0
        mpage_readpages+0x112/0x1d0
        blkdev_readpages+0x1d/0x20
        __do_page_cache_readahead+0x1ad/0x290
        force_page_cache_readahead+0xaa/0x100
        page_cache_sync_readahead+0x3f/0x50
        generic_file_read_iter+0x5af/0x740
        blkdev_read_iter+0x35/0x40
        __vfs_read+0xe1/0x130
        vfs_read+0x96/0x130
        SyS_read+0x55/0xc0
        entry_SYSCALL_64_fastpath+0x13/0x8f
      Code: 03 00 48 8b 5d d8 65 48 33 1c 25 28 00 00 00 44 89 e8 75 19 48 83 c4 18 5b 41 5c 41 5d 41 5e 5d c3 0f 0b 41 bd ef ff ff ff eb d7 <0f> 0b e8 88 68 ef ff 0f 1f 84 00
      RIP  page_cache_tree_insert+0xf1/0x100
    
    This is a long-standing bug in the way shadow entries are accounted in
    the radix tree nodes. The shrinker needs to know when radix tree nodes
    contain only shadow entries, no pages, so node->count is split in half
    to count shadows in the upper bits and pages in the lower bits.
    
    Unfortunately, the radix tree implementation doesn't know of this and
    assumes all entries are in node->count. When there is a shadow entry
    directly in root->rnode and the tree is later extended, the radix tree
    implementation will copy that entry into the new node and and bump its
    node->count, i.e. increases the page count bits. Once the shadow gets
    removed and we subtract from the upper counter, node->count underflows
    and triggers the warning. Afterwards, without node->count reaching 0
    again, the radix tree node is leaked.
    
    Limit shadow entries to when we have actual radix tree nodes and can
    count them properly. That means we lose the ability to detect refaults
    from files that had only the first page faulted in at eviction time.
    
    Fixes: 449dd698 ("mm: keep page cache radix tree nodes in check")
    Signed-off-by: default avatarJohannes Weiner <hannes@cmpxchg.org>
    Reported-and-tested-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Reviewed-by: default avatarJan Kara <jack@suse.cz>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    7d5d3b13
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