Re: [PATCH 0/2] mm: memory-failure: fix HWPoison flag race with non-atomic page flag ops

[Michael S. Tsirkin]

On Mon, Jun 29, 2026 at 07:04:25PM +0200, David Hildenbrand (Arm) wrote:
> On 6/29/26 18:54, Andi Kleen wrote:
> >> However, this was a basic test, when allocating 4k pages. With 2M hugepages:
> >>
> >>                                    insns/iter    cycles/iter
> >>      -------------------------------------------------------
> >>        base                       20758 +/- 12.5   191208 +/-1946.6
> >>        rcu                        20785 +/-  3.7   197108 +/- 132.1
> >>        atomic                     20727 +/-  6.4   204591 +/- 160.2
> >>
> >>        rcu vs base                  +27 (+0.13%)     +5900 (+3.09%)
> >>        atomic vs base               -31 (-0.15%)    +13383 (+7.00%)
> >>
> >> and even with THP:
> >>
> >>                                    insns/iter    cycles/iter
> >>      -------------------------------------------------------
> >>        base                       27220 +/-  2.8   192151 +/- 483.3
> >>        rcu                        27248 +/- 30.1   194159 +/-2746.6
> >>        atomic                     27186 +/-  3.2   200526 +/- 746.2
> >>
> >>        rcu vs base                  +28 (+0.10%)     +2008 (+1.04%)
> >>        atomic vs base               -34 (-0.12%)     +8374 (+4.36%)
> >>
> >>
> >> needs more thought.
> > 
> > Well the alternative is to not bother with RCU, but just wait a bit and
> > check if the bit stuck and repeat if needed. While that could in theory
> > livelock it is extremely unlikely (especially if you add a bit of randomization 
> > to the sleep)
> 
> We discussed that a bit already. Hypervisors make it fairly unpredictable how
> long you would actually have to spin.


Way I see it, this is not the issue. The issue is it does not fix the
race:


CPU1:

read flags

CPU2:

test and set

test and set #2 - sees it is set

CPU1:

write flags clearing the bit




> -- 
> Cheers,
> 
> David