On Mon, Jul 7, 2025 at 9:52 AM Lorenzo Stoakes lorenzo.stoakes@oracle.com wrote:
Sorry I know it's annoying, but some petty commit msg nits:
On Thu, Jul 03, 2025 at 11:07:25PM -0700, Suren Baghdasaryan wrote:
With maple_tree supporting vma tree traversal under RCU and per-vma locks, /proc/pid/maps can be read while holding individual vma locks instead of locking the entire address space. Completely lockless approach (walking vma tree under RCU) would be quite
Completely lockless approach -> A completely lockless approach
complex with the main issue being get_vma_name() using callbacks which might not work correctly with a stable vma copy, requiring original (unstable) vma - see special_mapping_name() for an example.
NIT: for an example -> for example
Ack.
When per-vma lock acquisition fails, we take the mmap_lock for reading, lock the vma, release the mmap_lock and continue. This fallback to mmap read lock guarantees the reader to make forward progress even during lock contention. This will interfere with the writer but for a very short time while we are acquiring the per-vma lock and only when there was contention on the vma reader is interested in. We shouldn't see a
Can we separate out into a new paragraph?
Will do.
repeated fallback to mmap read locks in practice, as this require a very unlikely series of lock contentions (for instance due to repeated vma split operations). However even if this did somehow happen, we would still progress. One case requiring special handling is when vma changes between the
when vma changes -> when a vma chnages
Ack.
time it was found and the time it got locked. A problematic case would be if vma got shrunk so that it's start moved higher in the address
vma -> a vma
Ack.
it's start moved higher -> its vm_start moved higher
Ack.
space and a new vma was installed at the beginning:
reader found: |--------VMA A--------| VMA is modified: |-VMA B-|----VMA A----| reader locks modified VMA A reader reports VMA A: | gap |----VMA A----|
This would result in reporting a gap in the address space that does not exist. To prevent this we retry the lookup after locking the vma, however we do that only when we identify a gap and detect that the address space was changed after we found the vma.
Can we separate out into a new paragraph?
Ack.
This change is designed to reduce mmap_lock contention and prevent a process reading /proc/pid/maps files (often a low priority task, such as monitoring/data collection services) from blocking address space updates. Note that this change has a userspace visible disadvantage: it allows for sub-page data tearing as opposed to the previous mechanism where data tearing could happen only between pages of generated output data. Since current userspace considers data tearing between pages to be acceptable, we assume is will be able to handle sub-page data tearing as well.
Signed-off-by: Suren Baghdasaryan surenb@google.com
OK this is looking pretty great now, I make a bunch of points below, but I don't think anything is holding this up from being OK, so with those addressed:
Reviewed-by: Lorenzo Stoakes lorenzo.stoakes@oracle.com
Thanks!
fs/proc/internal.h | 5 ++ fs/proc/task_mmu.c | 118 ++++++++++++++++++++++++++++++++++---- include/linux/mmap_lock.h | 11 ++++ mm/madvise.c | 3 +- mm/mmap_lock.c | 88 ++++++++++++++++++++++++++++ 5 files changed, 214 insertions(+), 11 deletions(-)
diff --git a/fs/proc/internal.h b/fs/proc/internal.h index 3d48ffe72583..7c235451c5ea 100644 --- a/fs/proc/internal.h +++ b/fs/proc/internal.h @@ -384,6 +384,11 @@ struct proc_maps_private { struct task_struct *task; struct mm_struct *mm; struct vma_iterator iter;
loff_t last_pos;+#ifdef CONFIG_PER_VMA_LOCK
bool mmap_locked;struct vm_area_struct *locked_vma;+#endif #ifdef CONFIG_NUMA struct mempolicy *task_mempolicy; #endif diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index b8bc06d05a72..ff3fe488ce51 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -127,15 +127,107 @@ static void release_task_mempolicy(struct proc_maps_private *priv) } #endif
-static struct vm_area_struct *proc_get_vma(struct proc_maps_private *priv,
loff_t *ppos)+#ifdef CONFIG_PER_VMA_LOCK
+static void unlock_vma(struct proc_maps_private *priv) +{
if (priv->locked_vma) {vma_end_read(priv->locked_vma);priv->locked_vma = NULL;}+}
+static const struct seq_operations proc_pid_maps_op;
+static inline bool lock_vma_range(struct seq_file *m,
struct proc_maps_private *priv)OK this is a nice abstraction.
+{
/** smaps and numa_maps perform page table walk, therefore require* mmap_lock but maps can be read with locking just the vma.Probably worth mentioning that you hold the RCU read lock for the operation also.
Ack.
*/if (m->op != &proc_pid_maps_op) {if (mmap_read_lock_killable(priv->mm))return false;priv->mmap_locked = true;} else {rcu_read_lock();priv->locked_vma = NULL;priv->mmap_locked = false;}return true;+}
+static inline void unlock_vma_range(struct proc_maps_private *priv)
I guess the 'range' is either - the whole thing in case of mmap read locked, or single VMA in case of per-VMA locks.
Correct.
+{
if (priv->mmap_locked) {mmap_read_unlock(priv->mm);} else {unlock_vma(priv);rcu_read_unlock();}+}
+static struct vm_area_struct *get_next_vma(struct proc_maps_private *priv,
loff_t last_pos)+{
struct vm_area_struct *vma;if (priv->mmap_locked)return vma_next(&priv->iter);unlock_vma(priv);vma = lock_next_vma(priv->mm, &priv->iter, last_pos);if (!IS_ERR_OR_NULL(vma))priv->locked_vma = vma;return vma;+}
+#else /* CONFIG_PER_VMA_LOCK */
+static inline bool lock_vma_range(struct seq_file *m,
struct proc_maps_private *priv){
struct vm_area_struct *vma = vma_next(&priv->iter);
return mmap_read_lock_killable(priv->mm) == 0;+}
+static inline void unlock_vma_range(struct proc_maps_private *priv) +{
mmap_read_unlock(priv->mm);+}
+static struct vm_area_struct *get_next_vma(struct proc_maps_private *priv,
loff_t last_pos)+{
return vma_next(&priv->iter);+}
+#endif /* CONFIG_PER_VMA_LOCK */
+static struct vm_area_struct *proc_get_vma(struct seq_file *m, loff_t *ppos) +{
struct proc_maps_private *priv = m->private;struct vm_area_struct *vma;vma = get_next_vma(priv, *ppos);/* EINTR is possible */if (IS_ERR(vma))return vma;/* Store previous position to be able to restart if needed */priv->last_pos = *ppos; if (vma) {
*ppos = vma->vm_start;
/** Track the end of the reported vma to ensure position changes* even if previous vma was merged with the next vma and we* found the extended vma with the same vm_start.*/*ppos = vma->vm_end; } else {
*ppos = -2;
*ppos = -2; /* -2 indicates gate vma */ vma = get_gate_vma(priv->mm); }@@ -163,28 +255,34 @@ static void *m_start(struct seq_file *m, loff_t *ppos) return NULL; }
if (mmap_read_lock_killable(mm)) {
if (!lock_vma_range(m, priv)) { mmput(mm); put_task_struct(priv->task); priv->task = NULL; return ERR_PTR(-EINTR); }/** Reset current position if last_addr was set before* and it's not a sentinel.*/if (last_addr > 0)*ppos = last_addr = priv->last_pos; vma_iter_init(&priv->iter, mm, (unsigned long)last_addr); hold_task_mempolicy(priv); if (last_addr == -2) return get_gate_vma(mm);
return proc_get_vma(priv, ppos);
return proc_get_vma(m, ppos);}
static void *m_next(struct seq_file *m, void *v, loff_t *ppos) { if (*ppos == -2) {
*ppos = -1;
*ppos = -1; /* -1 indicates no more vmas */ return NULL; }
return proc_get_vma(m->private, ppos);
return proc_get_vma(m, ppos);}
static void m_stop(struct seq_file *m, void *v) @@ -196,7 +294,7 @@ static void m_stop(struct seq_file *m, void *v) return;
release_task_mempolicy(priv);
mmap_read_unlock(mm);
unlock_vma_range(priv); mmput(mm); put_task_struct(priv->task); priv->task = NULL;diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h index 5da384bd0a26..1f4f44951abe 100644 --- a/include/linux/mmap_lock.h +++ b/include/linux/mmap_lock.h @@ -309,6 +309,17 @@ void vma_mark_detached(struct vm_area_struct *vma); struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, unsigned long address);
+/*
- Locks next vma pointed by the iterator. Confirms the locked vma has not
- been modified and will retry under mmap_lock protection if modification
- was detected. Should be called from read RCU section.
- Returns either a valid locked VMA, NULL if no more VMAs or -EINTR if the
- process was interrupted.
- */
+struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
struct vma_iterator *iter,unsigned long address);#else /* CONFIG_PER_VMA_LOCK */
static inline void mm_lock_seqcount_init(struct mm_struct *mm) {} diff --git a/mm/madvise.c b/mm/madvise.c index a34c2c89a53b..e61e32b2cd91 100644 --- a/mm/madvise.c +++ b/mm/madvise.c @@ -108,7 +108,8 @@ void anon_vma_name_free(struct kref *kref)
struct anon_vma_name *anon_vma_name(struct vm_area_struct *vma) {
mmap_assert_locked(vma->vm_mm);
if (!rwsem_is_locked(&vma->vm_mm->mmap_lock))vma_assert_locked(vma);This looks familiar ;)
Yep, that's your fix which I folded in.
return vma->anon_name;} diff --git a/mm/mmap_lock.c b/mm/mmap_lock.c index 5f725cc67334..ed0e5e2171cd 100644 --- a/mm/mmap_lock.c +++ b/mm/mmap_lock.c @@ -178,6 +178,94 @@ struct vm_area_struct *lock_vma_under_rcu(struct mm_struct *mm, count_vm_vma_lock_event(VMA_LOCK_ABORT); return NULL; }
+static struct vm_area_struct *lock_vma_under_mmap_lock(struct mm_struct *mm,
struct vma_iterator *iter,Nit, but we tend to call this vmi (yes Liam and I are addicted to 3 letter abbreviations, we are evil beings)
Ok, I'll rename it.
unsigned long address)I swear we already had a helper for this? Maybe misremembering
I think you might be confusing it with lock_vma_under_rcu()
+{
struct vm_area_struct *vma;int ret;ret = mmap_read_lock_killable(mm);if (ret)return ERR_PTR(ret);/* Lookup the vma at the last position again under mmap_read_lock */vma_iter_init(iter, mm, address);vma = vma_next(iter);Maybe worth calling this lock_next_under_mmap_lock() as we are grabbing the next VMA here??
Sure. lock_next_vma_under_mmap_lock() ?
if (vma)vma_start_read_locked(vma);mmap_read_unlock(mm);return vma;+}
+struct vm_area_struct *lock_next_vma(struct mm_struct *mm,
struct vma_iterator *iter,unsigned long address)Slightly confusing this, I think last_pos would be better? Or last_address?
Otherwise it's not clear it's the address of the next VMA or the end of the previous.
Ok, last_address it is then.
+{
struct vm_area_struct *vma;unsigned int mm_wr_seq;bool mmap_unlocked;RCU_LOCKDEP_WARN(!rcu_read_lock_held(), "no rcu read lock held");+retry:
/* Start mmap_lock speculation in case we need to verify the vma later */mmap_unlocked = mmap_lock_speculate_try_begin(mm, &mm_wr_seq);vma = vma_next(iter);if (!vma)return NULL;vma = vma_start_read(mm, vma);Nit, but myabe erase this newline.
Ack.
if (IS_ERR_OR_NULL(vma)) {/** Retry immediately if the vma gets detached from under us.* Infinite loop should not happen because the vma we find will* have to be constantly knocked out from under us.*/if (PTR_ERR(vma) == -EAGAIN) {Maybe worth a comment here stating that we intentionally retry getting the next VMA, and therefore must reset to the last visited adress each time.
Ack.
vma_iter_init(iter, mm, address);Maybe Liam can confirm this is the best approach? Seems correct though.
Liam's Reviewed-by confirms correctness now :)
goto retry;}goto out;}/** Verify the vma we locked belongs to the same address space and it's* not behind of the last search position.*/if (unlikely(vma->vm_mm != mm || address >= vma->vm_end))goto out_unlock;/** vma can be ahead of the last search position but we need to verify* it was not shrunk after we found it and another vma has not been* installed ahead of it. Otherwise we might observe a gap that should* not be there.*/if (address < vma->vm_start) {/* Verify only if the address space might have changed since vma lookup. */if (!mmap_unlocked || mmap_lock_speculate_retry(mm, mm_wr_seq)) {vma_iter_init(iter, mm, address);if (vma != vma_next(iter))goto out_unlock;}}return vma;+out_unlock:
vma_end_read(vma);+out:
Maybe these labels should reflect the fact this is a fallback case?
Like fallback_unlock + fallback?
Ack.
rcu_read_unlock();vma = lock_vma_under_mmap_lock(mm, iter, address);rcu_read_lock();OK I guess we hold the RCU lock _the whole time_ as we traverse except when we lock under mmap lock.
Correct.
return vma;+} #endif /* CONFIG_PER_VMA_LOCK */
#ifdef CONFIG_LOCK_MM_AND_FIND_VMA
2.50.0.727.gbf7dc18ff4-goog