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

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Message-ID: <54b8d0b9-a1c7-4c1b-a588-2e5308a977fb@suse.cz>
Date: Wed, 13 Nov 2024 09:58:43 +0100
From: Vlastimil Babka <vbabka@...e.cz>
To: Suren Baghdasaryan <surenb@...gle.com>, akpm@...ux-foundation.org
Cc: willy@...radead.org, liam.howlett@...cle.com, 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

On 11/12/24 20:46, 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 inside
> lock_vma_under_rcu().
> lock_vma_under_rcu() enters RCU read section, finds the vma at the
> given address, locks the vma and checks if it got detached or remapped
> to cover a different address range. These last checks are there
> to ensure that the vma was not modified after we found it but before
> locking it. Vma reuse introduces a possibility that in between those
> events of finding and locking the vma, it can get detached, reused,
> added into a tree and be marked as attached. Current checks will help
> detecting cases when:
> - vma was reused but not yet added into the tree (detached check)
> - vma was reused at a different address range (address check)
> If vma is covering a new address range which still includes the address
> we were looking for, it's not a problem unless the reused vma was added
> into a different address space. Therefore checking that vma->vm_mm is
> still the same is the the only missing check to detect vma reuse.

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.