Re: [PATCH] mm/memory-failure: Poison read receives SIGKILL instead of SIGBUS if mmaped more than once
From: Naoya Horiguchi
Date: Wed Jul 24 2019 - 03:01:54 EST
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@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;
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