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b=kOZ8+suYI9tE4kDLma/Id4gIZCQraiBxxVBj72UObGWTFLsPQdKqiOUvN9dPgO7v6 wj+ICfXbZ1+gkoV7monhhWSIJK2aUP6Q0FJQFUSq6xmuuU0gKQI4y5Mi/JwwenyKt2 uKcQ/oCiZRWyZAbsoWqv/us2EvZ2aWlHu+YXzqFSBv2jnijERqhH7tB2uEdkyxmBcb cdGIHuo9FMvI85PpMjc3uF2KaL2Sue11k+aBav1hsQ0Dp1yNsH60aXl/X59lYf15Rc +42vYqDTTuFyeLqHnETFPN+66oSpN2IaWkO1yqdUDjlOtdF4U6Mf2HF17brF6WbuAt 3HHRES+JV7Akw== Date: Mon, 3 Aug 2026 12:28:25 +0100 From: "Lorenzo Stoakes (ARM)" To: Suren Baghdasaryan Cc: akpm@linux-foundation.org, dave.hansen@linux.intel.com, Liam.Howlett@oracle.com, 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 Subject: Re: [PATCH v3 3/5] mm: Add RCU-based VMA lookup helper that waits for writers Message-ID: References: <20260802215459.2769283-1-surenb@google.com> <20260802215459.2769283-4-surenb@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260802215459.2769283-4-surenb@google.com> X-Rspamd-Queue-Id: 226AF20005 X-Rspam-User: X-Stat-Signature: zk8ba8w5hdb54ocnf6uxt11m9yp8owrh X-Rspamd-Server: rspam04 X-HE-Tag: 1785756525-388137 X-HE-Meta: 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 ABtjnvmP 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Sun, Aug 02, 2026 at 02:54:57PM -0700, Suren Baghdasaryan wrote: > From: Dave Hansen > > == Background == > > 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. > > == Problem == > > 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 = 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. > > == Solution == > > 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. > Would be nice to have a: Suggested-by: Lorenzo Stoakes (ARM) Here given https://lore.kernel.org/linux-mm/af4Zx0gJIWbdDeY2@lucifer/ :) > 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 Hjønnevåg > 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_struct *vma) > } > > /* > - * Use only while holding mmap read lock which guarantees that locking will not > - * fail (nobody can concurrently write-lock the vma). vma_start_read() should > + * 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 overflow. > * This functionality is used to obtain vma read lock and drop the mmap read lock. > + * VMA can't be detached while we are holding mmap lock, therefore in practice this > + * function can fail only when there are so many readers that vm_refcnt overflows. > */ > 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(struct vm_area_struct *vma, int > } > > /* > - * Use only while holding mmap read lock which guarantees that locking will not > - * fail (nobody can concurrently write-lock the vma). vma_start_read() should > + * 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 overflow. > * This functionality is used to obtain vma read lock and drop the mmap read lock. > + * VMA can't be detached while we are holding mmap lock, therefore in practice this > + * function can fail only when there are so many readers that vm_refcnt overflows. > */ > static inline bool vma_start_read_locked(struct vm_area_struct *vma) > { > return vma_start_read_locked_nested(vma, 0); > } > > +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_struct *mm, > return NULL; > } > > +/* Why not a kdoc comment? > + * Find the VMA covering 'address' and lock it for reading. Waits for writers to > + * finish if the VMA is being modified. Returns NULL if there is no VMA covering > + * 'address'. > + * > + * Use only in code paths where no mmap_lock and no VMA lock is held. Well, a VMA read lock can be held which would make this a noop essentially. If a VMA write lock is held you're also ok as the mmap read lock will preclude an mmap write lock, meaning vma_end_write_all() will have been called and the write lock released. So I think you can just drop this line? > + * > + * 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 = 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 = vma_lookup(mm, address); > + if (vma && !vma_start_read_locked(vma)) This maybe warrants an unlikely() given it can only happen if refcount overflows? Also worth having a comment to that effect here? > + vma = NULL; > + mmap_read_unlock(mm); > + > + return vma; > +} > + > static struct vm_area_struct *lock_next_vma_under_mmap_lock(struct mm_struct *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(struct mm_struct *mm, > * > * Should be called without holding mmap_lock. > * > - * Return: A locked vma containing @address, -ENOENT if no vma is found, or > - * -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. I'm guessing you're fixing this up as part of the patch? But it feels a bit random, I mean fine but you should mention this change in the commit message + explain why. > */ > static struct vm_area_struct *uffd_lock_vma(struct mm_struct *mm, > unsigned long address) > -- > 2.55.0.508.g3f0d502094-goog > -- Cheers, Lorenzo