Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Matthew Wilcox <willy@infradead.org> [241113 08:57]:
> On Wed, Nov 13, 2024 at 07:38:02AM -0500, Liam R. Howlett wrote:
> > > Hi, I was wondering if we actually need the detached flag. Couldn't
> > > "detached" simply mean vma->vm_mm == NULL and we save 4 bytes? Do we ever
> > > need a vma that's detached but still has a mm pointer? I'd hope the places
> > > that set detached to false have the mm pointer around so it's not inconvenient.
> > 
> > I think the gate vmas ruin this plan.
> 
> But the gate VMAs aren't to be found in the VMA tree.  Used to be that
> was because the VMA tree was the injective RB tree and so VMAs could
> only be in one tree at a time.  We could change that now!

\o/

> 
> Anyway, we could use (void *)1 instead of NULL to indicate a "detached"
> VMA if we need to distinguish between a detached VMA and a gate VMA.

I was thinking a pointer to itself vma->vm_mm = vma, then a check for
this, instead of null like we do today.

Either way, we should make it a function so it's easier to reuse for
whatever we need in the future, wdyt?