Re: [PATCH] thp, mm: Fix crash due race in MADV_FREE handling

[Michal Hocko]

On Wed 28-06-17 13:12:49, Kirill A. Shutemov wrote:
> Reinette reported following crash:
> 
>   BUG: Bad page state in process log2exe  pfn:57600
>   page:ffffea00015d8000 count:0 mapcount:0 mapping:          (null) index:0x20200
>   flags: 0x4000000000040019(locked|uptodate|dirty|swapbacked)
>   raw: 4000000000040019 0000000000000000 0000000000020200 00000000ffffffff
>   raw: ffffea00015d8020 ffffea00015d8020 0000000000000000 0000000000000000
>   page dumped because: PAGE_FLAGS_CHECK_AT_FREE flag(s) set
>   bad because of flags: 0x1(locked)
>   Modules linked in: rfcomm 8021q bnep intel_rapl x86_pkg_temp_thermal coretemp efivars btusb btrtl btbcm pwm_lpss_pci snd_hda_codec_hdmi btintel pwm_lpss snd_hda_codec_realtek snd_soc_skl snd_hda_codec_generic snd_soc_skl_ipc spi_pxa2xx_platform snd_soc_sst_ipc snd_soc_sst_dsp i2c_designware_platform i2c_designware_core snd_hda_ext_core snd_soc_sst_match snd_hda_intel snd_hda_codec mei_me snd_hda_core mei snd_soc_rt286 snd_soc_rl6347a snd_soc_core efivarfs
>   CPU: 1 PID: 354 Comm: log2exe Not tainted 4.12.0-rc7-test-test #19
>   Hardware name: Intel corporation NUC6CAYS/NUC6CAYB, BIOS AYAPLCEL.86A.0027.2016.1108.1529 11/08/2016
>   Call Trace:
>    dump_stack+0x95/0xeb
>    bad_page+0x16a/0x1f0
>    free_pages_check_bad+0x117/0x190
>    ? rcu_read_lock_sched_held+0xa8/0x130
>    free_hot_cold_page+0x7b1/0xad0
>    __put_page+0x70/0xa0
>    madvise_free_huge_pmd+0x627/0x7b0
>    madvise_free_pte_range+0x6f8/0x1150
>    ? debug_check_no_locks_freed+0x280/0x280
>    ? swapin_walk_pmd_entry+0x380/0x380
>    __walk_page_range+0x6b5/0xe30
>    walk_page_range+0x13b/0x310
>    madvise_free_page_range.isra.16+0xad/0xd0
>    ? force_swapin_readahead+0x110/0x110
>    ? swapin_walk_pmd_entry+0x380/0x380
>    ? lru_add_drain_cpu+0x160/0x320
>    madvise_free_single_vma+0x2e4/0x470
>    ? madvise_free_page_range.isra.16+0xd0/0xd0
>    ? vmacache_update+0x100/0x130
>    ? find_vma+0x35/0x160
>    SyS_madvise+0x8ce/0x1450
> 
> If somebody frees the page under us and we hold the last reference to
> it, put_page() would attempt to free the page before unlocking it.
> 
> The fix is trivial reorder of operations.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Reported-by: Reinette Chatre <reinette.chatre@xxxxxxxxx>
> Fixes: 9818b8cde622 ("madvise_free, thp: fix madvise_free_huge_pmd return value after splitting")
> Cc: Huang Ying <ying.huang@xxxxxxxxx>
> Cc: Minchan Kim <minchan@xxxxxxxxxx>
> Cc: Dave Hansen <dave.hansen@xxxxxxxxx>

Acked-by: Michal Hocko <mhocko@xxxxxxxx>

> ---
>  mm/huge_memory.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 8624450f7106..25b5965c1130 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1575,8 +1575,8 @@ bool madvise_free_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
>  		get_page(page);
>  		spin_unlock(ptl);
>  		split_huge_page(page);
> -		put_page(page);
>  		unlock_page(page);
> +		put_page(page);
>  		goto out_unlocked;
>  	}

I was about to ask what prevents get_page on an already freed page but
then I've noticed that this is still under pmd_trans_huge_lock which is
released right after that said get_page.

-- 
Michal Hocko
SUSE Labs