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Date: Wed, 24 Jul 2019 15:33:10 -0700
From: jane.chu@...cle.com
To: Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
Dan Williams <dan.j.williams@...el.com>
Cc: Linux MM <linux-mm@...ck.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] mm/memory-failure: Poison read receives SIGKILL instead
of SIGBUS if mmaped more than once
Thank you all for your comments!
I've incorporated them, tested, and have a v2 ready for review.
Thanks!
-jane
On 7/23/19 11:48 PM, Naoya Horiguchi wrote:
> Hi Jane, Dan,
>
> On Tue, Jul 23, 2019 at 06:34:35PM -0700, Dan Williams wrote:
>> On Tue, Jul 23, 2019 at 4:49 PM Jane Chu <jane.chu@...cle.com> 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@...cle.com>
>>> ---
>>> 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;
>
> The immediate return bypasses list_add_tail() below, so we might lose
> the chance of sending SIGBUS to the process.
>
> tk->size_shift is always non-zero for !is_zone_device_page(), so
> "tk->size_shift == 0" effectively checks "no mapping on ZONE_DEVICE" now.
> As you mention above, "no mapping" doesn't means "invalid address"
> so we can drop "tk->size_shift == 0" check from this if-statement.
> Going forward in this direction, "tk->addr_valid == 0" is equivalent to
> "tk->addr == -EFAULT", so we seems to be able to remove ->addr_valid.
> This observation leads me to the following change, does it work for you?
>
> --- a/mm/memory-failure.c
> +++ b/mm/memory-failure.c
> @@ -199,7 +199,6 @@ struct to_kill {
> struct task_struct *tsk;
> unsigned long addr;
> short size_shift;
> - char addr_valid;
> };
>
> /*
> @@ -324,7 +323,6 @@ static void add_to_kill(struct task_struct *tsk, struct page *p,
> }
> }
> tk->addr = page_address_in_vma(p, vma);
> - tk->addr_valid = 1;
> if (is_zone_device_page(p))
> tk->size_shift = dev_pagemap_mapping_shift(p, vma);
> else
> @@ -336,11 +334,9 @@ static void add_to_kill(struct task_struct *tsk, struct page *p,
> * likely very rare kill anyways just out of paranoia, but use
> * a SIGKILL because the error is not contained anymore.
> */
> - if (tk->addr == -EFAULT || tk->size_shift == 0) {
> + if (tk->addr == -EFAULT)
> pr_info("Memory failure: Unable to find user space address %lx in %s\n",
> page_to_pfn(p), tsk->comm);
> - tk->addr_valid = 0;
> - }
> get_task_struct(tsk);
> tk->tsk = tsk;
> list_add_tail(&tk->nd, to_kill);
> @@ -366,7 +362,7 @@ static void kill_procs(struct list_head *to_kill, int forcekill, bool fail,
> * make sure the process doesn't catch the
> * signal and then access the memory. Just kill it.
> */
> - if (fail || tk->addr_valid == 0) {
> + if (fail || tk->addr == -EFAULT) {
> pr_err("Memory failure: %#lx: forcibly killing %s:%d because of failure to unmap corrupted page\n",
> pfn, tk->tsk->comm, tk->tsk->pid);
> do_send_sig_info(SIGKILL, SEND_SIG_PRIV,
>
>>> }
>>> + 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?
>
> I totally agree with this cleanup. Thanks for the comment.
>
> Thanks,
> Naoya Horiguchi
>
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