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        <naoya.horiguchi@nec.com>
To:     Aili Yao <yaoaili@kingsoft.com>
CC:     "Luck, Tony" <tony.luck@intel.com>,
        Oscar Salvador <osalvador@suse.de>,
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Subject: Re: [PATCH] mm/memory-failure: Use a mutex to avoid memory_failure()
 races
Thread-Topic: [PATCH] mm/memory-failure: Use a mutex to avoid memory_failure()
 races
Thread-Index: AQHXFG4a6KBUueS0DE2O2dOaNXi1CKp66C2AgABDJQA=
Date:   Tue, 9 Mar 2021 06:04:41 +0000
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On Tue, Mar 09, 2021 at 10:04:21AM +0800, Aili Yao wrote:
> On Mon, 8 Mar 2021 14:55:04 -0800
> "Luck, Tony" <tony.luck@intel.com> wrote:
>=20
> > There can be races when multiple CPUs consume poison from the same
> > page. The first into memory_failure() atomically sets the HWPoison
> > page flag and begins hunting for tasks that map this page. Eventually
> > it invalidates those mappings and may send a SIGBUS to the affected
> > tasks.
> >=20
> > But while all that work is going on, other CPUs see a "success"
> > return code from memory_failure() and so they believe the error
> > has been handled and continue executing.
> >=20
> > Fix by wrapping most of the internal parts of memory_failure() in
> > a mutex.
> >=20
> > Signed-off-by: Tony Luck <tony.luck@intel.com>
> > ---
...
>=20
> If others are OK with this method, then I am OK too.
> But I have two concerns, May you take into account:
>=20
> 1. The memory_failure with 0 return code for race condition, then the kil=
l_me_maybe() goes into branch:
> 	if (!memory_failure(p->mce_addr >> PAGE_SHIFT, flags) &&
> 	    !(p->mce_kflags & MCE_IN_KERNEL_COPYIN)) {
> 		set_mce_nospec(p->mce_addr >> PAGE_SHIFT, p->mce_whole_page);
> 		sync_core();
> 		return;
> 	}
>=20
> while we place set_mce_nospec() here is for a reason, please see commit f=
d0e786d9d09024f67b.
>=20
> 2. When memory_failure return 0 and maybe return to user process, and it =
may re-execute the instruction triggering previous fault, this behavior
> assume an implicit dependence that the related pte has been correctly set=
. or if not correctlily set, it will lead to infinite loop again.

These seem to be separate issues from memory_failure()'s concurrency issue,
so I'm still expecting that your patch is to be merged. Maybe do you want
to update it based on the discussion (if it's concluded)?

Thanks,
Naoya Horiguchi=