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AHgh+RpmZY/Od625YC6ZgrxuAExcTVCQ6UEzAQWkBuRKoVdD319cLvsy3uzn6GEuw/hM+80tra1Rslt9bDM+7sw=@vger.kernel.org X-Gm-Message-State: AOJu0Yx9zgQSImA1TlS6SS6jOC/6ihvzCRzjIky69vuvEiAh1iFOQI9a AbYqK2qrcaAxleep36Cip2kH0vcmjf9ekt/RpEqDLH4x/w9GnYBKm97lGLcwHZDI08+WzUqkzFU iPdQLtQ== X-Received: from dybgk24.prod.google.com ([2002:a05:7301:198:b0:313:cf3c:796]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a20:6a03:b0:398:8870:b58f with SMTP id adf61e73a8af0-3cb85de7ddbmr20267974637.14.1786046757653; Thu, 06 Aug 2026 13:05:57 -0700 (PDT) Date: Thu, 6 Aug 2026 13:05:46 -0700 In-Reply-To: <20260806200548.3124802-1-surenb@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260806200548.3124802-1-surenb@google.com> X-Mailer: git-send-email 2.55.0.654.g21b8a5bc05-goog Message-ID: <20260806200548.3124802-4-surenb@google.com> Subject: [PATCH v4 3/5] mm: Add RCU-based VMA lookup helper that waits for writers From: Suren Baghdasaryan To: akpm@linux-foundation.org Cc: dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, ljs@kernel.org, david@kernel.org, willy@infradead.org, shakeel.butt@linux.dev, vbabka@kernel.org, jannh@google.com, aliceryhl@google.com, arve@android.com, cmllamas@google.com, christian@brauner.io, tkjos@android.com, dsahern@kernel.org, davem@davemloft.net, gregkh@linuxfoundation.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org, netdev@vger.kernel.org, surenb@google.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable From: Dave Hansen =3D=3D Background =3D=3D There are basically two parallel ways to look up a VMA: the traditional way, which is protected by mmap_read_lock, and the RCU-based per-VMA lock way which is based on RCU and refcounts. =3D=3D Problem =3D=3D The mmap_lock one is more straightforward to use but it has a big disadvantage in that it can not be mixed with page faults since those can take mmap_lock for read, which can deadlock when mixed with nested page faults and parallel writers. For example: mmap_read_lock(mm); // Another thread does mmap_write_lock(). // New mmap_lock readers are blocked. vma =3D vma_lookup(mm, address); // This deadlocks on mmap_read_lock() if it faults: copy_from_user(address); mmap_read_unlock(mm); The per-VMA lock can be mixed with faults, but they can fail and need to be able to fall back to the traditional way. =3D=3D Solution =3D=3D Add vma_start_read_unlocked() - a variant of the RCU-based lookup that waits for writers. This is basically the same as the existing RCU-based lookup, but on a failure to lock it temporarily takes mmap_lock for read and waits for writers to finish before locking the VMA, dropping the mmap_lock and returning the locked VMA. This has some advantages: 1. Callers do not need to have a fallback path for when they collide with writers. 2. It can be used in contexts where page faults can happen because it can take the mmap_lock for read but never *holds* it. 3. Its fast path does not require taking mmap_lock for read. Basically, when applied correctly, this approach results in faster *and* simpler code. While at it, fix the comments for vma_start_read_locked(), vma_start_read_locked_nested(), and uffd_lock_vma(). Suggested-by: Lorenzo Stoakes (ARM) Signed-off-by: Dave Hansen Signed-off-by: Suren Baghdasaryan Cc: Suren Baghdasaryan Cc: Andrew Morton Cc: "Liam R. Howlett" Cc: Lorenzo Stoakes Cc: Vlastimil Babka Cc: Shakeel Butt Cc: linux-mm@kvack.org Cc: Greg Kroah-Hartman Cc: Arve Hj=C3=B8nnev=C3=A5g Cc: Todd Kjos Cc: Christian Brauner Cc: Carlos Llamas Cc: Alice Ryhl Cc: "David S. Miller" Cc: David Ahern Cc: netdev@vger.kernel.org --- include/linux/mmap_lock.h | 19 +++++++++++++++---- mm/mmap_lock.c | 35 +++++++++++++++++++++++++++++++++++ mm/userfaultfd.c | 6 ++++-- 3 files changed, 54 insertions(+), 6 deletions(-) diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 7b2bbb09a952..a23fe6cbe301 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -228,10 +228,14 @@ static inline void vma_refcount_put(struct vm_area_st= ruct *vma) } =20 /* - * Use only while holding mmap read lock which guarantees that locking wil= l not - * fail (nobody can concurrently write-lock the vma). vma_start_read() sho= uld + * Use only while holding mmap read lock which guarantees that vma lock is= not + * contended (nobody can concurrently write-lock the vma). vma_start_read(= ) should * not be used in such cases because it might fail due to mm_lock_seq over= flow. * This functionality is used to obtain vma read lock and drop the mmap re= ad lock. + * + * VMA can't be detached while we are holding mmap lock, therefore in prac= tice this + * function can fail only when there are so many readers that vm_refcnt ov= erflows. + * The failure case is very unlikely and is already annotated as such inte= rnally. */ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma= , int subclass) { @@ -247,16 +251,23 @@ static inline bool vma_start_read_locked_nested(struc= t vm_area_struct *vma, int } =20 /* - * Use only while holding mmap read lock which guarantees that locking wil= l not - * fail (nobody can concurrently write-lock the vma). vma_start_read() sho= uld + * Use only while holding mmap read lock which guarantees that vma lock is= not + * contended (nobody can concurrently write-lock the vma). vma_start_read(= ) should * not be used in such cases because it might fail due to mm_lock_seq over= flow. * This functionality is used to obtain vma read lock and drop the mmap re= ad lock. + * + * VMA can't be detached while we are holding mmap lock, therefore in prac= tice this + * function can fail only when there are so many readers that vm_refcnt ov= erflows. + * The failure case is very unlikely and is already annotated as such inte= rnally. */ static inline bool vma_start_read_locked(struct vm_area_struct *vma) { return vma_start_read_locked_nested(vma, 0); } =20 +struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm, + unsigned long address); + static inline void vma_end_read(struct vm_area_struct *vma) { vma_refcount_put(vma); diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c index e20d01e8d38f..1c4902131e98 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -338,6 +338,41 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_st= ruct *mm, return NULL; } =20 +/** + * vma_start_read_unlocked() - Find the VMA covering 'address' and read-lo= ck it. + * @mm: the mm_struct of the address space to search + * @address: address that the vma should contain + * + * The fast path does not take mmap_lock. Waits for writers to finish if t= he + * VMA is being modified by taking mmap_lock. + * Use when mmap_lock is not held, otherwise use vma_start_read_locked(). + * Nothing prevents VMAs being unmapped/mapped before or after the VMA is + * looked up, if a stronger guarantee is required, take an mmap_lock. + * + * Return: If a VMA exists which spans @address, return that VMA, read-loc= ked. + * If no VMA is mapped there or, very unlikely, a reference count overflow + * occurred, return NULL. + */ +struct vm_area_struct *vma_start_read_unlocked(struct mm_struct *mm, + unsigned long address) +{ + struct vm_area_struct *vma; + + /* Fast path: return stable VMA covering 'address': */ + vma =3D lock_vma_under_rcu(mm, address); + if (vma) + return vma; + + /* Slow path: preclude VMA writers by temporarily getting mmap read lock.= */ + mmap_read_lock(mm); + vma =3D vma_lookup(mm, address); + if (vma && !vma_start_read_locked(vma)) + vma =3D NULL; + mmap_read_unlock(mm); + + return vma; +} + static struct vm_area_struct *lock_next_vma_under_mmap_lock(struct mm_stru= ct *mm, struct vma_iterator *vmi, unsigned long from_addr) diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c index edd90892f8cc..c3a0c38a3dc3 100644 --- a/mm/userfaultfd.c +++ b/mm/userfaultfd.c @@ -129,8 +129,10 @@ struct vm_area_struct *find_vma_and_prepare_anon(struc= t mm_struct *mm, * * Should be called without holding mmap_lock. * - * Return: A locked vma containing @address, -ENOENT if no vma is found, o= r - * -ENOMEM if anon_vma couldn't be allocated. + * Return: A locked vma containing @address, -ENOENT if no vma is found, + * -ENOMEM if anon_vma couldn't be allocated, or -EAGAIN if vma refcount + * overflow happened due to high number of readers and the caller should + * retry later. */ static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm, unsigned long address) --=20 2.55.0.654.g21b8a5bc05-goog