linux-kernel - Re: [PATCH v7 16/17] mm: make vma cache SLAB_TYPESAFE_BY

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Message-ID: <5bbc5573-4a10-4c89-bc69-6cf6117be915@suse.cz>
Date: Wed, 8 Jan 2025 19:21:35 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: peterz@...radead.org, willy@...radead.org, liam.howlett@...cle.com,
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Subject: Re: [PATCH v7 16/17] mm: make vma cache SLAB_TYPESAFE_BY_RCU

On 12/26/24 18:07, Suren Baghdasaryan wrote:
> To enable SLAB_TYPESAFE_BY_RCU for vma cache we need to ensure that
> object reuse before RCU grace period is over will be detected by
> lock_vma_under_rcu(). Current checks are sufficient as long as vma
> is detached before it is freed. Implement this guarantee by calling
> vma_ensure_detached() before vma is freed and make vm_area_cachep
> SLAB_TYPESAFE_BY_RCU. This will facilitate vm_area_struct reuse and
> will minimize the number of call_rcu() calls.
> 
> Signed-off-by: Suren Baghdasaryan <surenb@...gle.com>

I've noticed vm_area_dup() went back to the approach of "we memcpy
everything including vma_lock and detached (now the vm_refcnt) followed by a
vma_init_lock(..., true) that does refcount_set(&vma->vm_refcnt, 0);
Is that now safe against a racing lock_vma_under_rcu()? I think it's not?