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AHgh+RpNBfuqyz5FAhvp4l5b9AzJf5AR14gEqO40b+PEq/yFKK8tgi8f71oV/WLI5oh+AtyD174gfTU=@vger.kernel.org X-Gm-Message-State: AOJu0YyWpXZdl/xU+/Bzf1Wt1bdGqOwBmP/SZA1Ebq7opiG+hhUIIeV5 H9P5jVg7IXR5hpvLGiNDZTAQKLPKFRjjMeG/lhYxz6omg6vyKYxP1xHJa2MX2Q9WmpmjH5KEUMY 3ITtezQ== X-Received: from dlbou23.prod.google.com ([2002:a05:7022:1117:b0:13e:6e68:2a1]) (user=surenb job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:4d0c:b0:37f:c69d:ce69 with SMTP id 98e67ed59e1d1-38fbc432bb3mr6817011a91.10.1785707710250; Sun, 02 Aug 2026 14:55:10 -0700 (PDT) Date: Sun, 2 Aug 2026 14:54:57 -0700 In-Reply-To: <20260802215459.2769283-1-surenb@google.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260802215459.2769283-1-surenb@google.com> X-Mailer: git-send-email 2.55.0.508.g3f0d502094-goog Message-ID: <20260802215459.2769283-4-surenb@google.com> Subject: [PATCH v3 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@redhat.com, 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 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. 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 | 15 +++++++++++---- mm/mmap_lock.c | 29 +++++++++++++++++++++++++++++ mm/userfaultfd.c | 6 ++++-- 3 files changed, 44 insertions(+), 6 deletions(-) diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index eb32b482434e..fdd8f5cf5722 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -228,10 +228,12 @@ 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. */ static inline bool vma_start_read_locked_nested(struct vm_area_struct *vma= , int subclass) { @@ -247,16 +249,21 @@ 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. */ 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..6ff05e68e61b 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -338,6 +338,35 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_st= ruct *mm, return NULL; } =20 +/* + * Find the VMA covering 'address' and lock it for reading. Waits for writ= ers to + * finish if the VMA is being modified. Returns NULL if there is no VMA co= vering + * 'address'. + * + * Use only in code paths where no mmap_lock and no VMA lock is held. + * + * The fast path does not take mmap_lock. + */ +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 2.55.0.508.g3f0d502094-goog