Re: [PATCH RFC v2] mm: use per_vma lock for MADV_DONTNEED

[Lorenzo Stoakes]

On Mon, Jun 09, 2025 at 02:40:09PM +0800, Qi Zheng wrote:
> Hi Lorenzo,
>
> On 6/6/25 6:44 PM, Lorenzo Stoakes wrote:
>
> [snip]
>
> > >
> > > >
> > > > We could in theory always add another callback .pmd_entry_sleep or
> > > > something for this one case and document the requirement...
> > >
> > > Maybe, but the SRCU critical section cannot prevent the PTE page from
> > > being freed via RCU. :(
> >
> > Idea is we'd fall back to non-RCU in this case and take locks... but then
> > ugh we'd race everything RCU and no it's all or nothing isn't it?
>
> So maybe the RCU+refcount method is feasible. We can release the RCU
> lock after incrementing the reference count, which can ensure that the
> page table page is not freed.

Oh interesting... will note down for when I take a look should you not get to it
first :>)

>
> >
> > Overall - I will stash this response somewhere and come back to it if
> > somebody else doesn't in the meantime :)
>
> Thanks!
>
> >
> > >
>
>

Cheers, Lorenzo