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Fri, 10 Jan 2025 22:26:21 +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 YuFfKY2egWeeegAAD6G6ig (envelope-from ); Fri, 10 Jan 2025 22:26:21 +0000 Message-ID: Date: Fri, 10 Jan 2025 23:26:21 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v8 11/16] mm: replace vm_lock and detached flag with a reference count Content-Language: en-US To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, peterz@infradead.org, 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, lokeshgidra@google.com, minchan@google.com, jannh@google.com, shakeel.butt@linux.dev, souravpanda@google.com, pasha.tatashin@soleen.com, klarasmodin@gmail.com, richard.weiyang@gmail.com, corbet@lwn.net, linux-doc@vger.kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, kernel-team@android.com References: <20250109023025.2242447-1-surenb@google.com> <20250109023025.2242447-12-surenb@google.com> <95e9d80e-6c19-4a1f-9c21-307006858dff@suse.cz> From: Vlastimil Babka Autocrypt: addr=vbabka@suse.cz; 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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam10 X-Rspamd-Queue-Id: C1AA118000A X-Stat-Signature: scgoy5axkapqdou3jqcgdbhkih97td3o X-Rspam-User: X-HE-Tag: 1736547983-317838 X-HE-Meta: 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 +RAR6gt/ rhofyB3Keo5N0vWqVjC+5NptCWoKQ4zy1IZ6SaWnYvUacTsDOtbnH8GMQH2T69g8K/l1C31V0mYkAMr/Jc1ZvPnNhSfyplkm4BYlh8br+sTJqxtN0hh3d/HnjOCEpxMNVWXhsHYj9ga2CiPKThEH19rl8aq7CWBGVMqzIxy48gYx5yArWaa3/8aYZDDVZ0jRYga09twLCYHoVa9sLLOm19hgJP9zWOVl8hBoM9jSFyMZ3J6t5Tsq6unMICcY7v9bZKuFn6QeSLKQxvbpCSxmYHugtvXSWKV+o1C1wwtpp5zpXV4wLuIjHrJyyr94b4wxHU8v41l8W4POK6oZnDvQVFrlygi2Jv6fx7L8+48F4NMcYyMWugjSQBT0hEKqu7FPZax8bjODDk5EOBaDcIyJKpPzfmA== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 1/10/25 16:56, Suren Baghdasaryan wrote: >> > --- a/mm/memory.c >> > +++ b/mm/memory.c >> > @@ -6370,9 +6370,41 @@ struct vm_area_struct *lock_mm_and_find_vma(struct mm_struct *mm, >> > #endif >> > >> > #ifdef CONFIG_PER_VMA_LOCK >> > +static inline bool __vma_enter_locked(struct vm_area_struct *vma, unsigned int tgt_refcnt) >> > +{ >> > + /* >> > + * If vma is detached then only vma_mark_attached() can raise the >> > + * vm_refcnt. mmap_write_lock prevents racing with vma_mark_attached(). >> > + */ >> > + if (!refcount_add_not_zero(VMA_LOCK_OFFSET, &vma->vm_refcnt)) >> > + return false; >> > + >> > + rwsem_acquire(&vma->vmlock_dep_map, 0, 0, _RET_IP_); >> > + rcuwait_wait_event(&vma->vm_mm->vma_writer_wait, >> > + refcount_read(&vma->vm_refcnt) == tgt_refcnt, >> > + TASK_UNINTERRUPTIBLE); >> > + lock_acquired(&vma->vmlock_dep_map, _RET_IP_); >> > + >> > + return true; >> > +} >> > + >> > +static inline void __vma_exit_locked(struct vm_area_struct *vma, bool *detached) >> > +{ >> > + *detached = refcount_sub_and_test(VMA_LOCK_OFFSET, &vma->vm_refcnt); >> > + rwsem_release(&vma->vmlock_dep_map, _RET_IP_); >> > +} >> > + >> > void __vma_start_write(struct vm_area_struct *vma, unsigned int mm_lock_seq) >> > { >> > - down_write(&vma->vm_lock.lock); >> > + bool locked; >> > + >> > + /* >> > + * __vma_enter_locked() returns false immediately if the vma is not >> > + * attached, otherwise it waits until refcnt is (VMA_LOCK_OFFSET + 1) >> > + * indicating that vma is attached with no readers. >> > + */ >> > + locked = __vma_enter_locked(vma, VMA_LOCK_OFFSET + 1); >> >> Wonder if it would be slightly better if tgt_refcount was just 1 (or 0 >> below in vma_mark_detached()) and the VMA_LOCK_OFFSET added to it in >> __vma_enter_locked() itself as it's the one adding it in the first place. > > Well, it won't be called tgt_refcount then. Maybe "bool vma_attached" > and inside __vma_enter_locked() we do: > > unsigned int tgt_refcnt = VMA_LOCK_OFFSET + vma_attached ? 1 : 0; > > Is that better? Yeah I think so as it centralizes the target refcount logic into a single place __vma_enter_locked(). Hm but then it's weird that __vma_start_write() would set vma_attached to true and yet it handles also a case where it's not attached. Maybe call the parameter "detaching" and switch the 0 and 1?