Re: [PATCH v3 3/3] fs/proc/task_mmu: execute PROCMAP_QUERY ioctl under per-vma locks

On Wed, Aug 06, 2025 at 08:59:04AM -0700, Suren Baghdasaryan wrote:

Utilize per-vma locks to stabilize vma after lookup without taking mmap_lock during PROCMAP_QUERY ioctl execution. If vma lock is contended, we fall back to mmap_lock but take it only momentarily to lock the vma and release the mmap_lock. In a very unlikely case of vm_refcnt overflow, this fall back path will fail and ioctl is done under mmap_lock protection.

This change is designed to reduce mmap_lock contention and prevent PROCMAP_QUERY ioctl calls from blocking address space updates.

Signed-off-by: Suren Baghdasaryan surenb@google.com

A lot of nits but nothing's really standing out as broken, AFAICT...

---

fs/proc/task_mmu.c | 84 +++++++++++++++++++++++++++++++++++++--------- 1 file changed, 68 insertions(+), 16 deletions(-)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index 45134335e086..0396315dfaee 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -517,28 +517,81 @@ static int pid_maps_open(struct inode *inode, struct file *file) PROCMAP_QUERY_VMA_FLAGS \ )

-static int query_vma_setup(struct mm_struct *mm) +#ifdef CONFIG_PER_VMA_LOCK

+static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx) {

• return mmap_read_lock_killable(mm);

• lock_ctx->locked_vma = NULL;
• lock_ctx->mmap_locked = false;

We also do this in lock_vma_range(), seems sensible to factor out? E.g.:

static void ctx_clear_vma(struct proc_maps_locking_ctx *lock_ctx) { lock_ctx->locked_vma = NULL; lock_ctx->mmap_locked = false; }

• return 0;

}

-static void query_vma_teardown(struct mm_struct *mm, struct vm_area_struct *vma) +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx) {

• mmap_read_unlock(mm);

• if (lock_ctx->mmap_locked)

• mmap_read_unlock(lock_ctx->mm);
  

Maybe worth a comment as to why we leave lock_ctx->mmap_locked set here?

• else

• unlock_vma(lock_ctx);
  

Should have said on 2/3, but I wonder if we should prefix with ctx_, as 'unlock_vma()' and 'lock_vma()' seem like core functions... esp. since we have vma_start_read/write() rather than functions that reference locking.

So - ctx_unlock_vma() and ctx_lock_vma() or unlock_ctx_vma() / lock_ctx_vma()?

+}

+static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,

• 				     unsigned long addr)
  

+{

• struct vm_area_struct *vma;
• struct vma_iterator vmi;
• if (lock_ctx->mmap_locked)

• return find_vma(lock_ctx->mm, addr);
  

• unlock_vma(lock_ctx);
• rcu_read_lock();
• vma_iter_init(&vmi, lock_ctx->mm, addr);
• vma = lock_next_vma(lock_ctx->mm, &vmi, addr);
• rcu_read_unlock();

I think a comment at the top of this block would be useful, something like 'We unlock any previously locked VMA and find the next under RCU'.

• if (!IS_ERR_OR_NULL(vma)) {

Is the NULL bit here really necessary? presumably lock_ctx->locked_vma is expected to be NULL already, so we're not overwriting anything here.

In fact we could get rid of the horrid if/else here with a guard clause like:

if (!IS_ERR(vma) || PTR_ERR(vma) != -EAGAIN) return vma;

(the !IS_ERR() bit is probably a bit redundant but makes things clearer vs. just the PTR_ERR() thing)

Then do the rest below.

• lock_ctx->locked_vma = vma;
  

• } else if (PTR_ERR(vma) == -EAGAIN) {

• /* Fallback to mmap_lock on vma->vm_refcnt overflow */
  

• mmap_read_lock(lock_ctx->mm);
  

• vma = find_vma(lock_ctx->mm, addr);
  

• lock_ctx->mmap_locked = true;
  

Kinda sucks we have two separate ways of doing fallback now, this open-coded thing and fallback_to_mmap_lock().

Sort of hard to combine since we have subtly diffrent semantics - the RCU read lock is being held in the /proc/$pid/maps case, but here we've released it already.

• }
• return vma;

+}

+#else /* CONFIG_PER_VMA_LOCK */

+static int query_vma_setup(struct proc_maps_locking_ctx *lock_ctx) +{

• return mmap_read_lock_killable(lock_ctx->mm);

}

-static struct vm_area_struct *query_vma_find_by_addr(struct mm_struct *mm, unsigned long addr) +static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx) {

• return find_vma(mm, addr);

• mmap_read_unlock(lock_ctx->mm);

}

-static struct vm_area_struct *query_matching_vma(struct mm_struct *mm, +static struct vm_area_struct *query_vma_find_by_addr(struct proc_maps_locking_ctx *lock_ctx,

• 				     unsigned long addr)
  

+{

• return find_vma(lock_ctx->mm, addr);

+}

+#endif /* CONFIG_PER_VMA_LOCK */

+static struct vm_area_struct *query_matching_vma(struct proc_maps_locking_ctx *lock_ctx, unsigned long addr, u32 flags) { struct vm_area_struct *vma;

next_vma:

• vma = query_vma_find_by_addr(mm, addr);

• vma = query_vma_find_by_addr(lock_ctx, addr);
• if (IS_ERR(vma))

• return vma;
  

• if (!vma) goto no_vma;

@@ -579,11 +632,11 @@ static struct vm_area_struct *query_matching_vma(struct mm_struct *mm, return ERR_PTR(-ENOENT); }

-static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) +static int do_procmap_query(struct mm_struct *mm, void __user *uarg) {

• struct proc_maps_locking_ctx lock_ctx = { .mm = mm };

struct procmap_query karg; struct vm_area_struct *vma;

• struct mm_struct *mm; const char *name = NULL; char build_id_buf[BUILD_ID_SIZE_MAX], *name_buf = NULL; __u64 usize;

@@ -610,17 +663,16 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) if (!!karg.build_id_size != !!karg.build_id_addr) return -EINVAL;

• mm = priv->lock_ctx.mm; if (!mm || !mmget_not_zero(mm)) return -ESRCH;

• err = query_vma_setup(mm);

• err = query_vma_setup(&lock_ctx); if (err) { mmput(mm); return err; }

• vma = query_matching_vma(mm, karg.query_addr, karg.query_flags);

• vma = query_matching_vma(&lock_ctx, karg.query_addr, karg.query_flags); if (IS_ERR(vma)) { err = PTR_ERR(vma); vma = NULL;

@@ -705,7 +757,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) }

/* unlock vma or mmap_lock, and put mm_struct before copying data to user */

• query_vma_teardown(mm, vma);

• query_vma_teardown(&lock_ctx); mmput(mm);

  if (karg.vma_name_size && copy_to_user(u64_to_user_ptr(karg.vma_name_addr),

@@ -725,7 +777,7 @@ static int do_procmap_query(struct proc_maps_private *priv, void __user *uarg) return 0;

out:

• query_vma_teardown(mm, vma);

• query_vma_teardown(&lock_ctx); mmput(mm); kfree(name_buf); return err;

@@ -738,7 +790,7 @@ static long procfs_procmap_ioctl(struct file *file, unsigned int cmd, unsigned l

switch (cmd) { case PROCMAP_QUERY:

• return do_procmap_query(priv, (void __user *)arg);
  

• return do_procmap_query(priv->lock_ctx.mm, (void __user *)arg);
  

OK this confused me until I worked it through.

We set priv->lock_ctx.mm in:

pid_maps_open() -> do_maps_open() -> proc_maps_open()

Which it gets from proc_mem_open() which figures out the mm.

Maybe one for 2/3, but it'd be nice to have a comment saying something about how this is set, since it being part of lock_ctx makes it seem like it's something that would be set elsewhere.

Since we have fallback stuff and want to thread through this new lokc context type I guess it makes sense to put it here but given that's the case, let's maybe just add a comment here to clarify.

default: return -ENOIOCTLCMD; } -- 2.50.1.565.gc32cd1483b-goog

Cheers, Lorenzo