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bh=iJ0fhc3nRRLgNyK2OSmovOVzCLYbFdfk+N0sZ8Il+t8=; b=m04/al2j+hYCaywYmw6q52mVuh0CAWEQqjZpQmhdi/gIa7PGS5Rry8elaKw+CqxC+vL5Bg Q7c/7IjnVtcH94AA== Received: from imap1.dmz-prg2.suse.org (localhost [127.0.0.1]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (4096 bits) server-digest SHA256) (No client certificate requested) by imap1.dmz-prg2.suse.org (Postfix) with ESMTPS id 21E12137CF; Wed, 20 Nov 2024 10:16:46 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id Q3rrBw63PWfgYAAAD6G6ig (envelope-from ); Wed, 20 Nov 2024 10:16:46 +0000 Message-ID: Date: Wed, 20 Nov 2024 11:16:45 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 4/5] mm: make vma cache SLAB_TYPESAFE_BY_RCU To: Suren Baghdasaryan , akpm@linux-foundation.org Cc: willy@infradead.org, liam.howlett@oracle.com, lorenzo.stoakes@oracle.com, mhocko@suse.com, hannes@cmpxchg.org, mjguzik@gmail.com, oliver.sang@intel.com, mgorman@techsingularity.net, david@redhat.com, peterx@redhat.com, oleg@redhat.com, dave@stgolabs.net, paulmck@kernel.org, brauner@kernel.org, dhowells@redhat.com, hdanton@sina.com, hughd@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com References: <20241120000826.335387-1-surenb@google.com> <20241120000826.335387-5-surenb@google.com> Content-Language: en-US From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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DKIM_SIGNED(0.00)[suse.cz:s=susede2_rsa,suse.cz:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.cz:+] X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Score: -4.51 X-Spam-Flag: NO On 11/20/24 01:08, 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 several new possibilities: > 1. vma can be reused after it was found but before it is locked; > 2. vma can be reused and reinitialized (including changing its vm_mm) > while being locked in vma_start_read(); > 3. vma can be reused and reinitialized after it was found but before > it is locked, then attached at a new address or to a new mm while being > read-locked; > For case #1 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); > We are missing the check for vm_mm to ensure the reused vma was not > attached to a different mm. This patch adds the missing check. > For case #2, we pass mm to vma_start_read() to prevent access to > unstable vma->vm_mm. So we may now be looking at different mm's mm_lock_seq.sequence and return a false unlocked result, right? I guess the mm validation in lock_vma_under_rcu() handles that, but maybe the comment of vma_start_read() needs updating. > For case #3, we ensure the order in which vma->detached flag and > vm_start/vm_end/vm_mm are set and checked. vma gets attached after > vm_start/vm_end/vm_mm were set and lock_vma_under_rcu() should check > vma->detached before checking vm_start/vm_end/vm_mm. This is required > because attaching vma happens without vma write-lock, as opposed to > vma detaching, which requires vma write-lock. This patch adds memory > barriers inside is_vma_detached() and vma_mark_attached() needed to > order reads and writes to vma->detached vs vm_start/vm_end/vm_mm. > After these provisions, SLAB_TYPESAFE_BY_RCU is added to vm_area_cachep. > This will facilitate vm_area_struct reuse and will minimize the number > of call_rcu() calls. > Adding a freeptr_t into vm_area_struct (unioned with vm_start/vm_end) > could be used to avoids bloating the structure, however currently > custom free pointers are not supported in combination with a ctor > (see the comment for kmem_cache_args.freeptr_offset). I think there's nothing fundamental preventing to support that, there was just no user of it. We can do it later. > Signed-off-by: Suren Baghdasaryan > --- a/kernel/fork.c > +++ b/kernel/fork.c > @@ -436,6 +436,11 @@ static struct kmem_cache *vm_area_cachep; > /* SLAB cache for mm_struct structures (tsk->mm) */ > static struct kmem_cache *mm_cachep; > > +static void vm_area_ctor(void *data) > +{ > + vma_lock_init(data); > +} > + > struct vm_area_struct *vm_area_alloc(struct mm_struct *mm) > { > struct vm_area_struct *vma; > @@ -462,8 +467,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > * orig->shared.rb may be modified concurrently, but the clone > * will be reinitialized. > */ > - data_race(memcpy(new, orig, sizeof(*new))); > - vma_lock_init(new); > + vma_copy(new, orig); > INIT_LIST_HEAD(&new->anon_vma_chain); > #ifdef CONFIG_PER_VMA_LOCK > /* vma is not locked, can't use vma_mark_detached() */ Here we mark it detached but we might have already copied it as attached and confused a reader? I think this will be covered by what you said in reply to willy: "vma_copy() will have to also copy vma members individually." > @@ -475,32 +479,37 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > return new; > } >