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

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Message-ID: <54394536-da24-d01d-e4a7-2ece22b1ddab@google.com>
Date: Tue, 12 Nov 2024 22:52:24 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
cc: Hugh Dickins <hughd@...gle.com>, akpm@...ux-foundation.org, 
    willy@...radead.org, liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, 
    mhocko@...e.com, vbabka@...e.cz, 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, 
    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

On Tue, 12 Nov 2024, Suren Baghdasaryan wrote:
> On Tue, Nov 12, 2024 at 9:08 PM Hugh Dickins <hughd@...gle.com> wrote:
> > On Tue, 12 Nov 2024, Suren Baghdasaryan wrote:
> > >
> > > Thinking about this some more, I don't think this works. I'm relying
> > > on vma_start_read() to stabilize the vma, however the lock I'm taking
> > > is part of the vma which can be reused from under us. So, the lock I'm
> > > taking might be reinitialized after I take the lock...
> > > I need to figure out a way to stabilize the vma in some other manner
> > > before taking this lock.
> >
> > (I'm not paying attention and following the patches, I just happened
> > to notice this remark: forgive me if I'm out of context and have
> > misunderstood, but hope this might help:)
> >
> > But this is exactly the problem SLAB_TYPESAFE_BY_RCU was invented for.
> > You just have to be careful that the locks are initialized only when the
> > slab is first created (allocated from buddy), not reinitialized whenever
> > a new object is allocated from that slab.
> 
> Hi Hugh!
> I'm looking into SLAB_TYPESAFE_BY_RCU implementation and trying to
> figure out if initializing the lock in the ctor() of the cache as
> mentioned in the comment here:
> https://elixir.bootlin.com/linux/v6.12-rc7/source/include/linux/slab.h#L127
> would help my case. I assume that's what you are hinting here?

Yes, if I'm "hinting", it's because offhand I forget the right names:
"ctor", yes, that sounds right.

Just grep around for examples of how it is used: there must be plenty
now. but anon_vma is what it was first used for.

But given the title of this patch, I'm surprised it's new to you.

Hugh