linux-kernel - Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY

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Message-ID: <k26pa6fhn2j6bgfwtcdp6u5vk25mkclitzvqqeqvji77k4lqop@yowwrqusmdyp>
Date: Wed, 13 Nov 2024 10:22:49 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Matthew Wilcox <willy@...radead.org>
Cc: Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
        akpm@...ux-foundation.org, lorenzo.stoakes@...cle.com, mhocko@...e.com,
        hannes@...xchg.org, mjguzik@...il.com, oliver.sang@...el.com,
        mgorman@...hsingularity.net, david@...hat.com, peterx@...hat.com,
        oleg@...hat.com, dave@...olabs.net, paulmck@...nel.org,
        brauner@...nel.org, dhowells@...hat.com, hdanton@...a.com,
        hughd@...gle.com, minchan@...gle.com, jannh@...gle.com,
        shakeel.butt@...ux.dev, souravpanda@...gle.com,
        pasha.tatashin@...een.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...roid.com
Subject: Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

* Matthew Wilcox <willy@...radead.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?