Re: [PATCH v1] mm: fix PageAnonExclusive clearing racing with concurrent RCU GUP-fast

[Andrew Morton]

On Thu,  1 Sep 2022 10:35:59 +0200 David Hildenbrand <david@xxxxxxxxxx> wrote:

> The possible issues due to reordering are of theoretical nature so far
> and attempts to reproduce the race failed.
> 
> Especially the "no PTE change" case isn't the common case, because we'd
> need an exclusive anonymous page that's mapped R/O and the PTE is clean
> in KSM code -- and using KSM with page pinning isn't extremely common.
> Further, the clear+TLB flush we used for now implies a memory barrier.
> So the problematic missing part should be the missing memory barrier
> after pinning but before checking if the PTE changed.

Obscure bug, large and tricky patch.  Is a -stable backport really
justifiable?