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

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Message-ID: <6d0c5c2d-2963-489a-2376-8edaeb064de3@google.com>
Date: Tue, 12 Nov 2024 21:08:07 -0800 (PST)
From: Hugh Dickins <hughd@...gle.com>
To: Suren Baghdasaryan <surenb@...gle.com>
cc: akpm@...ux-foundation.org, willy@...radead.org, liam.howlett@...cle.com, 
    lorenzo.stoakes@...cle.com, mhocko@...e.com, vbabka@...e.cz, 
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Subject: Re: [PATCH v2 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU

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.

Hugh