On Tue, Jul 23, 2019 at 4:49 PM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
>
> Mmap /dev/dax more than once, then read the poison location using address
> from one of the mappings. The other mappings due to not having the page
> mapped in will cause SIGKILLs delivered to the process. SIGKILL succeeds
> over SIGBUS, so user process looses the opportunity to handle the UE.
>
> Although one may add MAP_POPULATE to mmap(2) to work around the issue,
> MAP_POPULATE makes mapping 128GB of pmem several magnitudes slower, so
> isn't always an option.
>
> Details -
>
> ndctl inject-error --block=10 --count=1 namespace6.0
>
> ./read_poison -x dax6.0 -o 5120 -m 2
> mmaped address 0x7f5bb6600000
> mmaped address 0x7f3cf3600000
> doing local read at address 0x7f3cf3601400
> Killed
>
> Console messages in instrumented kernel -
>
> mce: Uncorrected hardware memory error in user-access at edbe201400
> Memory failure: tk->addr = 7f5bb6601000
> Memory failure: address edbe201: call dev_pagemap_mapping_shift
> dev_pagemap_mapping_shift: page edbe201: no PUD
> Memory failure: tk->size_shift == 0
> Memory failure: Unable to find user space address edbe201 in read_poison
> Memory failure: tk->addr = 7f3cf3601000
> Memory failure: address edbe201: call dev_pagemap_mapping_shift
> Memory failure: tk->size_shift = 21
> Memory failure: 0xedbe201: forcibly killing read_poison:22434 because of failure to unmap corrupted page
> => to deliver SIGKILL
> Memory failure: 0xedbe201: Killing read_poison:22434 due to hardware memory corruption
> => to deliver SIGBUS
>
> Signed-off-by: Jane Chu <jane.chu@xxxxxxxxxx>
> ---
> mm/memory-failure.c | 16 ++++++++++------
> 1 file changed, 10 insertions(+), 6 deletions(-)
>
> diff --git a/mm/memory-failure.c b/mm/memory-failure.c
> index d9cc660..7038abd 100644
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -315,7 +315,6 @@ static void add_to_kill(struct task_struct *tsk, struct page *p,
>
> if (*tkc) {
> tk = *tkc;
> - *tkc = NULL;
> } else {
> tk = kmalloc(sizeof(struct to_kill), GFP_ATOMIC);
> if (!tk) {
> @@ -331,16 +330,21 @@ static void add_to_kill(struct task_struct *tsk, struct page *p,
> tk->size_shift = compound_order(compound_head(p)) + PAGE_SHIFT;
>
> /*
> - * In theory we don't have to kill when the page was
> - * munmaped. But it could be also a mremap. Since that's
> - * likely very rare kill anyways just out of paranoia, but use
> - * a SIGKILL because the error is not contained anymore.
> + * Indeed a page could be mmapped N times within a process. And it's possible
> + * that not all of those N VMAs contain valid mapping for the page. In which
> + * case we don't want to send SIGKILL to the process on behalf of the VMAs
> + * that don't have the valid mapping, because doing so will eclipse the SIGBUS
> + * delivered on behalf of the active VMA.
> */
> if (tk->addr == -EFAULT || tk->size_shift == 0) {
> pr_info("Memory failure: Unable to find user space address %lx in %s\n",
> page_to_pfn(p), tsk->comm);
> - tk->addr_valid = 0;
> + if (tk != *tkc)
> + kfree(tk);
> + return;
> }
> + if (tk == *tkc)
> + *tkc = NULL;
> get_task_struct(tsk);
> tk->tsk = tsk;
> list_add_tail(&tk->nd, to_kill);
Concept and policy looks good to me, and I never did understand what
the mremap() case was trying to protect against.
The patch is a bit difficult to read (not your fault) because of the
odd way that add_to_kill() expects the first 'tk' to be pre-allocated.
May I ask for a lead-in cleanup that moves all the allocation internal
to add_to_kill() and drops the **tk argument?
