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    Subjectzram corruption due to uninitialized do_swap_page fault
    Hello,

    We're looking into using zram, but unfortunately we ran into some
    corruption issues. We've seen rocksdb complaining about "Corruption:
    bad entry in block", and we've also seen some coredumps that point at
    memory being zeroed out. One of our Rust processes coredumps contains
    a non-null pointer pointing at zero, among other things:

    * core::ptr::non_null::NonNull<u8> {pointer: 0x0}

    In fact, a whole bunch of memory around this pointer was all zeros.

    Disabling zram resolves all issues, and we can't reproduce any of
    these issues with other swap setups. I've tried adding crc32
    checksumming for pages that are compressed, but it didn't catch the
    issue either, even though userspace facing symptoms were present. My
    crc32 code doesn't touch ZRAM_SAME pages, though.

    Unfortunately, this isn't trivial to replicate, and I believe that it
    depends on zram used for swap specifically, not for zram as a block
    device. Specifically, swap_slot_free_notify looks suspicious.

    Here's a patch that I have to catch the issue in the act:

    diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
    index 438ce34ee760..fea46a70a3c9 100644
    --- a/drivers/block/zram/zram_drv.c
    +++ b/drivers/block/zram/zram_drv.c
    @@ -1265,6 +1265,9 @@ static int __zram_bvec_read(struct zram *zram,
    struct page *page, u32 index,
    unsigned long value;
    void *mem;

    + if (WARN_ON(!handle && !zram_test_flag(zram, index, ZRAM_SAME)))
    + pr_warn("Page %u read from zram without previous write\n", index);
    +
    value = handle ? zram_get_element(zram, index) : 0;
    mem = kmap_atomic(page);
    zram_fill_page(mem, PAGE_SIZE, value);
    In essence, it warns whenever a page is read from zram that was not
    previously written to. To make this work, one needs to zero out zram
    prior to running mkswap on it.

    I have prepared a GitHub repo with my observations and a reproduction:

    * https://github.com/bobrik/zram-corruptor

    I'm able to trigger the following in an aarch64 VM with two threads
    reading the same memory out of swap:

    [ 512.651752][ T7285] ------------[ cut here ]------------
    [ 512.652279][ T7285] WARNING: CPU: 0 PID: 7285 at
    drivers/block/zram/zram_drv.c:1285 __zram_bvec_read+0x28c/0x2e8 [zram]
    [ 512.653923][ T7285] Modules linked in: zram zsmalloc kheaders nfsv3
    nfs lockd grace sunrpc xt_conntrack nft_chain_nat xt_MASQUERADE nf_nat
    nf_conntrack_netlink nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4
    nft_counter xt_addrtype nft_compat nf_tables nfnetlink bridge stp llc
    overlay xfs libcrc32c zstd zstd_compress af_packet aes_ce_blk
    aes_ce_cipher ghash_ce gf128mul virtio_net sha3_ce net_failover
    sha3_generic failover sha512_ce sha512_arm64 sha2_ce sha256_arm64
    virtio_mmio virtio_ring qemu_fw_cfg rtc_pl031 virtio fuse ip_tables
    x_tables ext4 mbcache crc16 jbd2 nvme nvme_core pci_host_generic
    pci_host_common unix [last unloaded: zsmalloc]
    [ 512.659238][ T7285] CPU: 0 PID: 7285 Comm: zram-corruptor Tainted: G
    W 5.16.0-ivan #1 0877d306c6dc0716835d43cafe4399473d09e406
    [ 512.660413][ T7285] Hardware name: linux,dummy-virt (DT)
    [ 512.661077][ T7285] pstate: 80400005 (Nzcv daif +PAN -UAO -TCO -DIT
    -SSBS BTYPE=--)
    [ 512.661788][ T7285] pc : __zram_bvec_read+0x28c/0x2e8 [zram]
    [ 512.662099][ T7285] lr : zram_bvec_rw+0x70/0x204 [zram]
    [ 512.662422][ T7285] sp : ffffffc01018bac0
    [ 512.662720][ T7285] x29: ffffffc01018bae0 x28: ffffff9e4e725280 x27:
    ffffff9e4e725280
    [ 512.663122][ T7285] x26: ffffff9e4e725280 x25: 00000000000001f6 x24:
    0000000100033e6c
    [ 512.663601][ T7285] x23: 00000000000001f6 x22: 0000000000000000 x21:
    fffffffe7a36d840
    [ 512.664252][ T7285] x20: 00000000000001f6 x19: ffffff9e69423c00 x18:
    ffffffc010711068
    [ 512.664812][ T7285] x17: 0000000000000008 x16: ffffffd34aed51bc x15:
    0000000000000000
    [ 512.665507][ T7285] x14: 0000000000000a88 x13: 0000000000000000 x12:
    0000000000000000
    [ 512.666183][ T7285] x11: 0000000100033e6c x10: ffffffc01091d000 x9 :
    0000000001000000
    [ 512.666627][ T7285] x8 : 0000000000002f10 x7 : 80b75f8fb90b52c4 x6 :
    051609fe50833de3
    [ 512.667276][ T7285] x5 : 0000000000000000 x4 : 0000000000000000 x3 :
    0000000000000000
    [ 512.667875][ T7285] x2 : 00000000000001f6 x1 : 00000000000001f6 x0 :
    ffffffd305b746af
    [ 512.668483][ T7285] Call trace:
    [ 512.668682][ T7285] __zram_bvec_read+0x28c/0x2e8 [zram
    745969ed35ea0fb382bfd518d6f70e13966e9b52]
    [ 512.669405][ T7285] zram_bvec_rw+0x70/0x204 [zram
    745969ed35ea0fb382bfd518d6f70e13966e9b52]
    [ 512.670066][ T7285] zram_rw_page+0xb4/0x16c [zram
    745969ed35ea0fb382bfd518d6f70e13966e9b52]
    [ 512.670584][ T7285] bdev_read_page+0x74/0xac
    [ 512.670843][ T7285] swap_readpage+0x5c/0x2e4
    [ 512.671243][ T7285] do_swap_page+0x2f4/0x988
    [ 512.671560][ T7285] handle_pte_fault+0xcc/0x1fc
    [ 512.671935][ T7285] handle_mm_fault+0x284/0x4a8
    [ 512.672412][ T7285] do_page_fault+0x274/0x428
    [ 512.672704][ T7285] do_translation_fault+0x5c/0xf8
    [ 512.673083][ T7285] do_mem_abort+0x50/0xc8
    [ 512.673293][ T7285] el0_da+0x3c/0x74
    [ 512.673549][ T7285] el0t_64_sync_handler+0xc4/0xec
    [ 512.673972][ T7285] el0t_64_sync+0x1a4/0x1a8
    [ 512.674495][ T7285] ---[ end trace cf983b7507c20343 ]---
    [ 512.675359][ T7285] zram: Page 502 read from zram without previous write
    I can also trace accesses to zram to catch the unfortunate sequence:

    zram_bvec_write index = 502 [cpu = 3, tid = 7286]
    zram_free_page index = 502 [cpu = 3, tid = 7286]
    zram_bvec_read index = 502 [cpu = 3, tid = 7286]
    zram_free_page index = 502 [cpu = 3, tid = 7286] <-- problematic free
    zram_bvec_read index = 502 [cpu = 0, tid = 7285] <-- problematic read

    With stacks for zram_free_page:

    zram_bvec_write index = 502 [cpu = 3, tid = 7286]
    zram_free_page index = 502 [cpu = 3, tid = 7286]

    zram_free_page+0
    $x.97+32
    zram_rw_page+180
    bdev_write_page+124
    __swap_writepage+116
    swap_writepage+160
    pageout+284
    shrink_page_list+2892
    shrink_inactive_list+688
    shrink_lruvec+360
    shrink_node_memcgs+148
    shrink_node+860
    shrink_zones+368
    do_try_to_free_pages+232
    try_to_free_mem_cgroup_pages+292
    try_charge_memcg+608

    zram_bvec_read index = 502 [cpu = 3, tid = 7286]
    zram_free_page index = 502 [cpu = 3, tid = 7286] <-- problematic free

    zram_free_page+0
    swap_range_free+220
    swap_entry_free+244
    swapcache_free_entries+152
    free_swap_slot+288
    __swap_entry_free+216
    swap_free+108
    do_swap_page+1776
    handle_pte_fault+204
    handle_mm_fault+644
    do_page_fault+628
    do_translation_fault+92
    do_mem_abort+80
    el0_da+60
    el0t_64_sync_handler+196
    el0t_64_sync+420

    zram_bvec_read index = 502 [cpu = 0, tid = 7285] <-- problematic read

    The very last read is the same one that triggered the warning from my
    patch in dmesg. You can see that the slot is freed before reading by
    swapcache_free_entries. As far as I can see, only zram implements
    swap_slot_free_notify. Swapping in an uninitialized zram page results
    in all zeroes copied, which matches the symptoms.

    The issue doesn't reproduce if I pin both threads to the same CPU. It
    also doesn't reproduce with a single thread. All of this seems to
    point at some sort of race condition.

    I was able to reproduce this on x86_64 bare metal server as well.

    I'm happy to try out mitigation approaches for this. If my
    understanding here is incorrect, I'm also happy to try out patches
    that could help me catch the issue in the wild.

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      Last update: 2022-03-11 20:53    [W:5.902 / U:0.008 seconds]
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