On Thu, Jun 4, 2026 at 12:14 AM Lorenzo Stoakes <[email protected]> wrote:
>
> On Wed, Jun 03, 2026 at 04:49:48PM -0700, Suren Baghdasaryan wrote:
> > On Thu, May 28, 2026 at 8:17 AM Lorenzo Stoakes <[email protected]> wrote:
> > >
> > > On Sat, Apr 25, 2026 at 11:27:16PM -0700, Suren Baghdasaryan wrote:
> > > > proc/pid/{smaps|numa_maps} can be read using the combination of RCU and
> > > > VMA read locks, similar to proc/pid/maps. RCU is required to safely
> > > > traverse the VMA tree and VMA lock stabilizes the VMA being processed
> > > > and the pagetable walk.
> > > >
> > > > Signed-off-by: Suren Baghdasaryan <[email protected]>
> > > > Reviewed-by: Liam R. Howlett <[email protected]>
> > >
> > > The logic LGTM, some nitty things below but in general:
> > >
> > > Reviewed-by: Lorenzo Stoakes <[email protected]>
> >
> > Thanks!
> >
> > >
> > > > ---
> > > > fs/proc/task_mmu.c | 195 ++++++++++++++++++++++++++++++++++++---------
> > > > 1 file changed, 156 insertions(+), 39 deletions(-)
> > > >
> > > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> > > > index 751b9ba160fb..1e3a15bf46f4 100644
> > > > --- a/fs/proc/task_mmu.c
> > > > +++ b/fs/proc/task_mmu.c
> > > > @@ -132,6 +132,22 @@ static void release_task_mempolicy(struct
> > > > proc_maps_private *priv)
> > > >
> > > > #ifdef CONFIG_PER_VMA_LOCK
> > > >
> > > > +static inline int lock_ctx_mm(struct proc_maps_locking_ctx *lock_ctx)
> > >
> > > NIT: But these inlines are unnecessary in a C file. At the kernel
> > > optimisation
> > > level the compiler already decides whether to inline or not.
> > >
> > > However in the kernel we redefine this so we lose function tracing and
> > > also we
> > > don't get warnings about unused functions [0], so this is actually a bit
> > > harmful
> > > - you can end up with dead code and not know abou tit.
> > >
> > > Same goes for all the other static inline's in task_mmu.c.
> >
> > I see. There are a bunch of these small inline functions. Once the
> > dust settles, I can clean them up. Dave's series and my folloup patch
> > to change smaps_rollup will be changing this code. Maybe after all
> > that is done?
>
> Yeah that's fine this obviously isn't urgent! :) tiny nits really.
>
> >
> > >
> > > [0]:
> > > https://elixir.bootlin.com/linux/v7.0.10/source/include/linux/compiler_types.h#L235
> > >
> > > > +{
> > > > + int ret = mmap_read_lock_killable(lock_ctx->mm);
> > > > +
> > > > + if (!ret)
> > > > + lock_ctx->mmap_locked = true;
> > > > +
> > > > + return ret;
> > > > +}
> > > > +
> > > > +static inline void unlock_ctx_mm(struct proc_maps_locking_ctx
> > > > *lock_ctx)
> > > > +{
> > > > + mmap_read_unlock(lock_ctx->mm);
> > > > + lock_ctx->mmap_locked = false;
> > > > +}
> > > > +
> > > > static void reset_lock_ctx(struct proc_maps_locking_ctx *lock_ctx)
> > > > {
> > > > lock_ctx->locked_vma = NULL;
> > > > @@ -146,25 +162,11 @@ static void unlock_ctx_vma(struct
> > > > proc_maps_locking_ctx *lock_ctx)
> > > > }
> > > > }
> > > >
> > > > -static const struct seq_operations proc_pid_maps_op;
> > > > -
> > > > static inline bool lock_vma_range(struct seq_file *m,
> > > > struct proc_maps_locking_ctx *lock_ctx)
> > > > {
> > > > - /*
> > > > - * smaps and numa_maps perform page table walk, therefore require
> > > > - * mmap_lock but maps can be read with locking just the vma and
> > > > - * walking the vma tree under rcu read protection.
> > > > - */
> > > > - if (m->op != &proc_pid_maps_op) {
> > > > - if (mmap_read_lock_killable(lock_ctx->mm))
> > > > - return false;
> > > > -
> > > > - lock_ctx->mmap_locked = true;
> > > > - } else {
> > > > - rcu_read_lock();
> > > > - reset_lock_ctx(lock_ctx);
> > > > - }
> > > > + rcu_read_lock();
> > > > + reset_lock_ctx(lock_ctx);
> > > >
> > > > return true;
> > > > }
> > > > @@ -172,7 +174,7 @@ static inline bool lock_vma_range(struct seq_file
> > > > *m,
> > > > static inline void unlock_vma_range(struct proc_maps_locking_ctx
> > > > *lock_ctx)
> > > > {
> > > > if (lock_ctx->mmap_locked) {
> > > > - mmap_read_unlock(lock_ctx->mm);
> > > > + unlock_ctx_mm(lock_ctx);
> > > > } else {
> > > > unlock_ctx_vma(lock_ctx);
> > > > rcu_read_unlock();
> > > > @@ -213,17 +215,45 @@ static inline bool fallback_to_mmap_lock(struct
> > > > proc_maps_private *priv,
> > > > return true;
> > > > }
> > > >
> > > > +static inline void drop_rcu(struct proc_maps_private *priv)
> > > > +{
> > > > + if (priv->lock_ctx.mmap_locked)
> > > > + return;
> > > > +
> > > > + rcu_read_unlock();
> > > > +}
> > > > +
> > > > +static inline void reacquire_rcu(struct proc_maps_private *priv)
> > > > +{
> > > > + if (priv->lock_ctx.mmap_locked)
> > > > + return;
> > > > +
> > > > + rcu_read_lock();
> > > > + /* Reinitialize the iterator. */
> > > > + vma_iter_set(&priv->iter, priv->lock_ctx.locked_vma->vm_end);
> > >
> > > OK good that we reset this :)
> > >
> > > > +}
> > > > +
> > > > #else /* CONFIG_PER_VMA_LOCK */
> > > >
> > >
> > > We need to pull out the patches from Dave's series and remove this option
> > > and
> > > default VMA locks on by default :)
> >
> > I didn't see any followup after Dave posted his v1. I agree it would
> > make things much simpler but I don't want to delay these changes too
> > much.
> > I'll ask Dave if he plans to continue his cleanup. If not, I'll pick it up.
>
> Great thanks!
I checked with Dave and he will post the next version in a week or so.
>
> >
> >
> > >
> > > > +static inline int lock_ctx_mm(struct proc_maps_locking_ctx *lock_ctx)
> > > > +{
> > > > + return mmap_read_lock_killable(lock_ctx->mm);
> > > > +}
> > > > +
> > > > +static inline void unlock_ctx_mm(struct proc_maps_locking_ctx
> > > > *lock_ctx)
> > > > +{
> > > > + mmap_read_unlock(lock_ctx->mm);
> > > > +}
> > > > +
> > > > static inline bool lock_vma_range(struct seq_file *m,
> > > > struct proc_maps_locking_ctx *lock_ctx)
> > > > {
> > > > - return mmap_read_lock_killable(lock_ctx->mm) == 0;
> > > > + return lock_ctx_mm(lock_ctx) == 0;
> > > > }
> > > >
> > > > static inline void unlock_vma_range(struct proc_maps_locking_ctx
> > > > *lock_ctx)
> > > > {
> > > > - mmap_read_unlock(lock_ctx->mm);
> > > > + unlock_ctx_mm(lock_ctx);
> > > > }
> > > >
> > > > static struct vm_area_struct *get_next_vma(struct proc_maps_private
> > > > *priv,
> > > > @@ -238,6 +268,9 @@ static inline bool fallback_to_mmap_lock(struct
> > > > proc_maps_private *priv,
> > > > return false;
> > > > }
> > > >
> > > > +static inline void drop_rcu(struct proc_maps_private *priv) {}
> > > > +static inline void reacquire_rcu(struct proc_maps_private *priv) {}
> > > > +
> > > > #endif /* CONFIG_PER_VMA_LOCK */
> > > >
> > > > static struct vm_area_struct *proc_get_vma(struct seq_file *m, loff_t
> > > > *ppos)
> > > > @@ -538,12 +571,10 @@ static int query_vma_setup(struct
> > > > proc_maps_locking_ctx *lock_ctx)
> > > >
> > > > static void query_vma_teardown(struct proc_maps_locking_ctx *lock_ctx)
> > > > {
> > > > - if (lock_ctx->mmap_locked) {
> > > > - mmap_read_unlock(lock_ctx->mm);
> > > > - lock_ctx->mmap_locked = false;
> > > > - } else {
> > > > + if (lock_ctx->mmap_locked)
> > > > + unlock_ctx_mm(lock_ctx);
> > > > + else
> > > > unlock_ctx_vma(lock_ctx);
> > > > - }
> > > > }
> > > >
> > > > static struct vm_area_struct *query_vma_find_by_addr(struct
> > > > proc_maps_locking_ctx *lock_ctx,
> > > > @@ -1280,21 +1311,75 @@ static const struct mm_walk_ops
> > > > smaps_shmem_walk_ops = {
> > > > .walk_lock = PGWALK_RDLOCK,
> > > > };
> > > >
> > > > +#ifdef CONFIG_PER_VMA_LOCK
> > > > +
> > > > +static const struct mm_walk_ops smaps_walk_vma_lock_ops = {
> > > > + .pmd_entry = smaps_pte_range,
> > > > + .hugetlb_entry = smaps_hugetlb_range,
> > > > + .walk_lock = PGWALK_VMA_RDLOCK_VERIFY,
> > > > +};
> > > > +
> > > > +static const struct mm_walk_ops smaps_shmem_walk_vma_lock_ops = {
> > > > + .pmd_entry = smaps_pte_range,
> > > > + .hugetlb_entry = smaps_hugetlb_range,
> > > > + .pte_hole = smaps_pte_hole,
> > > > + .walk_lock = PGWALK_VMA_RDLOCK_VERIFY,
> > > > +};
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_smaps_walk_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + if (priv->lock_ctx.mmap_locked)
> > > > + return &smaps_walk_ops;
> > > > + return &smaps_walk_vma_lock_ops;
> > > > +}
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_smaps_shmem_walk_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + if (priv->lock_ctx.mmap_locked)
> > > > + return &smaps_shmem_walk_ops;
> > > > + return &smaps_shmem_walk_vma_lock_ops;
> > > > +}
> > > > +
> > > > +#else /* CONFIG_PER_VMA_LOCK */
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_smaps_walk_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + return &smaps_walk_ops;
> > > > +}
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_smaps_shmem_walk_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + return &smaps_shmem_walk_ops;
> > > > +}
> > > > +
> > > > +#endif /* CONFIG_PER_VMA_LOCK */
> > > > +
> > > > /*
> > > > * Gather mem stats from @vma with the indicated beginning
> > > > * address @start, and keep them in @mss.
> > > > *
> > > > * Use vm_start of @vma as the beginning address if @start is 0.
> > > > */
> > > > -static void smap_gather_stats(struct vm_area_struct *vma,
> > > > - struct mem_size_stats *mss, unsigned long start)
> > > > +static void smap_gather_stats(struct proc_maps_private *priv,
> > > > + struct vm_area_struct *vma,
> > > > + struct mem_size_stats *mss, unsigned long
> > > > start)
> > > > {
> > > > - const struct mm_walk_ops *ops = &smaps_walk_ops;
> > > > + const struct mm_walk_ops *ops = get_smaps_walk_ops(priv);
> > > >
> > > > /* Invalid start */
> > > > if (start >= vma->vm_end)
> > > > return;
> > > >
> > > > + if (vma == get_gate_vma(priv->lock_ctx.mm))
> > > > + return;
> > > > +
> > > > + /* Might sleep. Drop RCU read lock but keep the VMA locked. */
> > > > + drop_rcu(priv);
> > > > +
> > > > if (vma->vm_file && shmem_mapping(vma->vm_file->f_mapping)) {
> > > > /*
> > > > * For shared or readonly shmem mappings we know that all
> > > > @@ -1312,15 +1397,16 @@ static void smap_gather_stats(struct
> > > > vm_area_struct *vma,
> > > > !(vma->vm_flags & VM_WRITE))) {
> > > > mss->swap += shmem_swapped;
> > > > } else {
> > > > - ops = &smaps_shmem_walk_ops;
> > > > + ops = get_smaps_shmem_walk_ops(priv);
> > > > }
> > > > }
> > > >
> > > > - /* mmap_lock is held in m_start */
> > > > if (!start)
> > > > walk_page_vma(vma, ops, mss);
> > > > else
> > > > walk_page_range(vma->vm_mm, start, vma->vm_end, ops, mss);
> > > > +
> > > > + reacquire_rcu(priv);
> > >
> > > OK I was going to say that this RCU lock dance is a bit iffy, but I see
> > > that we're sharing code such that this is necessary.
> > >
> > > > }
> > > >
> > > > #define SEQ_PUT_DEC(str, val) \
> > > > @@ -1369,10 +1455,11 @@ static void __show_smap(struct seq_file *m,
> > > > const struct mem_size_stats *mss,
> > > >
> > > > static int show_smap(struct seq_file *m, void *v)
> > > > {
> > > > + struct proc_maps_private *priv = m->private;
> > > > struct vm_area_struct *vma = v;
> > > > struct mem_size_stats mss = {};
> > > >
> > > > - smap_gather_stats(vma, &mss, 0);
> > > > + smap_gather_stats(priv, vma, &mss, 0);
> > > >
> > > > show_map_vma(m, vma);
> > > >
> > > > @@ -1413,7 +1500,7 @@ static int show_smaps_rollup(struct seq_file *m,
> > > > void *v)
> > > > goto out_put_task;
> > > > }
> > > >
> > > > - ret = mmap_read_lock_killable(mm);
> > > > + ret = lock_ctx_mm(&priv->lock_ctx);
> > > > if (ret)
> > > > goto out_put_mm;
> > > >
> > > > @@ -1425,7 +1512,7 @@ static int show_smaps_rollup(struct seq_file *m,
> > > > void *v)
> > > >
> > > > vma_start = vma->vm_start;
> > > > do {
> > > > - smap_gather_stats(vma, &mss, 0);
> > > > + smap_gather_stats(priv, vma, &mss, 0);
> > > > last_vma_end = vma->vm_end;
> > > >
> > > > /*
> > > > @@ -1434,8 +1521,8 @@ static int show_smaps_rollup(struct seq_file *m,
> > > > void *v)
> > > > */
> > > > if (mmap_lock_is_contended(mm)) {
> > > > vma_iter_invalidate(&vmi);
> > > > - mmap_read_unlock(mm);
> > > > - ret = mmap_read_lock_killable(mm);
> > > > + unlock_ctx_mm(&priv->lock_ctx);
> > > > + ret = lock_ctx_mm(&priv->lock_ctx);
> > > > if (ret) {
> > > > release_task_mempolicy(priv);
> > > > goto out_put_mm;
> > > > @@ -1484,14 +1571,14 @@ static int show_smaps_rollup(struct seq_file
> > > > *m, void *v)
> > > >
> > > > /* Case 1 and 2 above */
> > > > if (vma->vm_start >= last_vma_end) {
> > > > - smap_gather_stats(vma, &mss, 0);
> > > > + smap_gather_stats(priv, vma, &mss, 0);
> > > > last_vma_end = vma->vm_end;
> > > > continue;
> > > > }
> > > >
> > > > /* Case 4 above */
> > > > if (vma->vm_end > last_vma_end) {
> > > > - smap_gather_stats(vma, &mss,
> > > > last_vma_end);
> > > > + smap_gather_stats(priv, vma, &mss,
> > > > last_vma_end);
> > > > last_vma_end = vma->vm_end;
> > > > }
> > > > }
> > > > @@ -1505,7 +1592,7 @@ static int show_smaps_rollup(struct seq_file *m,
> > > > void *v)
> > > > __show_smap(m, &mss, true);
> > > >
> > > > release_task_mempolicy(priv);
> > > > - mmap_read_unlock(mm);
> > > > + unlock_ctx_mm(&priv->lock_ctx);
> > > >
> > > > out_put_mm:
> > > > mmput(mm);
> > > > @@ -3291,6 +3378,31 @@ static const struct mm_walk_ops show_numa_ops = {
> > > > .walk_lock = PGWALK_RDLOCK,
> > > > };
> > > >
> > > > +#ifdef CONFIG_PER_VMA_LOCK
> > > > +static const struct mm_walk_ops show_numa_vma_lock_ops = {
> > > > + .hugetlb_entry = gather_hugetlb_stats,
> > > > + .pmd_entry = gather_pte_stats,
> > > > + .walk_lock = PGWALK_VMA_RDLOCK_VERIFY,
> > > > +};
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_show_numa_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + if (priv->lock_ctx.mmap_locked)
> > > > + return &show_numa_ops;
> > > > + return &show_numa_vma_lock_ops;
> > > > +}
> > > > +
> > > > +#else /* CONFIG_PER_VMA_LOCK */
> > > > +
> > > > +static inline const struct mm_walk_ops *
> > > > +get_show_numa_ops(struct proc_maps_private *priv)
> > > > +{
> > > > + return &show_numa_ops;
> > > > +}
> > > > +
> > > > +#endif /* CONFIG_PER_VMA_LOCK */
> > > > +
> > > > /*
> > > > * Display pages allocated per node and memory policy via /proc.
> > > > */
> > > > @@ -3335,8 +3447,13 @@ static int show_numa_map(struct seq_file *m,
> > > > void *v)
> > > > if (is_vm_hugetlb_page(vma))
> > > > seq_puts(m, " huge");
> > > >
> > > > - /* mmap_lock is held by m_start */
> > > > - walk_page_vma(vma, &show_numa_ops, md);
> > > > + /* Skip walking pages if gate VMA */
> > > > + if (vma != get_gate_vma(proc_priv->lock_ctx.mm)) {
> > > > + /* Might sleep. Drop RCU read lock but keep the VMA
> > > > locked. */
> > > > + drop_rcu(proc_priv);
> > > > + walk_page_vma(vma, get_show_numa_ops(proc_priv), md);
> > > > + reacquire_rcu(proc_priv);
> > > > + }
> > > >
> > > > if (!md->pages)
> > > > goto out;
> > > > --
> > > > 2.54.0.545.g6539524ca2-goog
> > > >
