| Message ID | 20190607160557.GA335@ziepe.ca (mailing list archive) |
|---|---|
| State | Superseded |
| Delegated to: | Jason Gunthorpe |
| Headers | show |
| Series | Various revisions from a locking/code review | expand |
On 6/7/19 9:05 AM, Jason Gunthorpe wrote: > If the trylock on the hmm->mirrors_sem fails the function will return > without decrementing the notifiers that were previously incremented. Since > the caller will not call invalidate_range_end() on EAGAIN this will result > in notifiers becoming permanently incremented and deadlock. > > If the sync_cpu_device_pagetables() required blocking the function will > not return EAGAIN even though the device continues to touch the > pages. This is a violation of the mmu notifier contract. > > Switch, and rename, the ranges_lock to a spin lock so we can reliably > obtain it without blocking during error unwind. > > The error unwind is necessary since the notifiers count must be held > incremented across the call to sync_cpu_device_pagetables() as we cannot > allow the range to become marked valid by a parallel > invalidate_start/end() pair while doing sync_cpu_device_pagetables(). > > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com> Reviewed-by: Ralph Campbell <rcampbell@nvidia.com> > --- > include/linux/hmm.h | 2 +- > mm/hmm.c | 77 +++++++++++++++++++++++++++------------------ > 2 files changed, 48 insertions(+), 31 deletions(-) > > I almost lost this patch - it is part of the series, hasn't been > posted before, and wasn't sent with the rest, sorry. > > diff --git a/include/linux/hmm.h b/include/linux/hmm.h > index bf013e96525771..0fa8ea34ccef6d 100644 > --- a/include/linux/hmm.h > +++ b/include/linux/hmm.h > @@ -86,7 +86,7 @@ > struct hmm { > struct mm_struct *mm; > struct kref kref; > - struct mutex lock; > + spinlock_t ranges_lock; > struct list_head ranges; > struct list_head mirrors; > struct mmu_notifier mmu_notifier; > diff --git a/mm/hmm.c b/mm/hmm.c > index 4215edf737ef5b..10103a24e9b7b3 100644 > --- a/mm/hmm.c > +++ b/mm/hmm.c > @@ -68,7 +68,7 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm) > init_rwsem(&hmm->mirrors_sem); > hmm->mmu_notifier.ops = NULL; > INIT_LIST_HEAD(&hmm->ranges); > - mutex_init(&hmm->lock); > + spin_lock_init(&hmm->ranges_lock); > kref_init(&hmm->kref); > hmm->notifiers = 0; > hmm->mm = mm; > @@ -114,18 +114,19 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) > { > struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier); > struct hmm_mirror *mirror; > + unsigned long flags; > > /* Bail out if hmm is in the process of being freed */ > if (!kref_get_unless_zero(&hmm->kref)) > return; > > - mutex_lock(&hmm->lock); > + spin_lock_irqsave(&hmm->ranges_lock, flags); > /* > * Since hmm_range_register() holds the mmget() lock hmm_release() is > * prevented as long as a range exists. > */ > WARN_ON(!list_empty(&hmm->ranges)); > - mutex_unlock(&hmm->lock); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > > down_read(&hmm->mirrors_sem); > list_for_each_entry(mirror, &hmm->mirrors, list) { > @@ -141,6 +142,23 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) > hmm_put(hmm); > } > > +static void notifiers_decrement(struct hmm *hmm) > +{ > + lockdep_assert_held(&hmm->ranges_lock); > + > + hmm->notifiers--; > + if (!hmm->notifiers) { > + struct hmm_range *range; > + > + list_for_each_entry(range, &hmm->ranges, list) { > + if (range->valid) > + continue; > + range->valid = true; > + } This just effectively sets all ranges to valid. I'm not sure that is best. Shouldn't hmm_range_register() start with range.valid = true and then hmm_invalidate_range_start() set affected ranges to false? Then this becomes just wake_up_all() if --notifiers == 0 and hmm_range_wait_until_valid() should wait for notifiers == 0. Otherwise, range.valid doesn't really mean it's valid. > + wake_up_all(&hmm->wq); > + } > +} > + > static int hmm_invalidate_range_start(struct mmu_notifier *mn, > const struct mmu_notifier_range *nrange) > { > @@ -148,6 +166,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, > struct hmm_mirror *mirror; > struct hmm_update update; > struct hmm_range *range; > + unsigned long flags; > int ret = 0; > > if (!kref_get_unless_zero(&hmm->kref)) > @@ -158,12 +177,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, > update.event = HMM_UPDATE_INVALIDATE; > update.blockable = mmu_notifier_range_blockable(nrange); > > - if (mmu_notifier_range_blockable(nrange)) > - mutex_lock(&hmm->lock); > - else if (!mutex_trylock(&hmm->lock)) { > - ret = -EAGAIN; > - goto out; > - } > + spin_lock_irqsave(&hmm->ranges_lock, flags); > hmm->notifiers++; > list_for_each_entry(range, &hmm->ranges, list) { > if (update.end < range->start || update.start >= range->end) > @@ -171,7 +185,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, > > range->valid = false; > } > - mutex_unlock(&hmm->lock); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > > if (mmu_notifier_range_blockable(nrange)) > down_read(&hmm->mirrors_sem); > @@ -179,16 +193,26 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, > ret = -EAGAIN; > goto out; > } > + > list_for_each_entry(mirror, &hmm->mirrors, list) { > - int ret; > + int rc; > > - ret = mirror->ops->sync_cpu_device_pagetables(mirror, &update); > - if (!update.blockable && ret == -EAGAIN) > + rc = mirror->ops->sync_cpu_device_pagetables(mirror, &update); > + if (rc) { > + if (WARN_ON(update.blockable || rc != -EAGAIN)) > + continue; > + ret = -EAGAIN; > break; > + } > } > up_read(&hmm->mirrors_sem); > > out: > + if (ret) { > + spin_lock_irqsave(&hmm->ranges_lock, flags); > + notifiers_decrement(hmm); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > + } > hmm_put(hmm); > return ret; > } > @@ -197,23 +221,14 @@ static void hmm_invalidate_range_end(struct mmu_notifier *mn, > const struct mmu_notifier_range *nrange) > { > struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier); > + unsigned long flags; > > if (!kref_get_unless_zero(&hmm->kref)) > return; > > - mutex_lock(&hmm->lock); > - hmm->notifiers--; > - if (!hmm->notifiers) { > - struct hmm_range *range; > - > - list_for_each_entry(range, &hmm->ranges, list) { > - if (range->valid) > - continue; > - range->valid = true; > - } > - wake_up_all(&hmm->wq); > - } > - mutex_unlock(&hmm->lock); > + spin_lock_irqsave(&hmm->ranges_lock, flags); > + notifiers_decrement(hmm); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > > hmm_put(hmm); > } > @@ -866,6 +881,7 @@ int hmm_range_register(struct hmm_range *range, > { > unsigned long mask = ((1UL << page_shift) - 1UL); > struct hmm *hmm = mirror->hmm; > + unsigned long flags; > > range->valid = false; > range->hmm = NULL; > @@ -887,7 +903,7 @@ int hmm_range_register(struct hmm_range *range, > kref_get(&hmm->kref); > > /* Initialize range to track CPU page table updates. */ > - mutex_lock(&hmm->lock); > + spin_lock_irqsave(&hmm->ranges_lock, flags); > > range->hmm = hmm; > list_add(&range->list, &hmm->ranges); > @@ -898,7 +914,7 @@ int hmm_range_register(struct hmm_range *range, > */ > if (!hmm->notifiers) > range->valid = true; > - mutex_unlock(&hmm->lock); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > > return 0; > } > @@ -914,13 +930,14 @@ EXPORT_SYMBOL(hmm_range_register); > void hmm_range_unregister(struct hmm_range *range) > { > struct hmm *hmm = range->hmm; > + unsigned long flags; > > if (WARN_ON(range->end <= range->start)) > return; > > - mutex_lock(&hmm->lock); > + spin_lock_irqsave(&hmm->ranges_lock, flags); > list_del(&range->list); > - mutex_unlock(&hmm->lock); > + spin_unlock_irqrestore(&hmm->ranges_lock, flags); > > /* Drop reference taken by hmm_range_register() */ > range->valid = false; >
On Fri, Jun 07, 2019 at 04:52:58PM -0700, Ralph Campbell wrote: > > @@ -141,6 +142,23 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) > > hmm_put(hmm); > > } > > +static void notifiers_decrement(struct hmm *hmm) > > +{ > > + lockdep_assert_held(&hmm->ranges_lock); > > + > > + hmm->notifiers--; > > + if (!hmm->notifiers) { > > + struct hmm_range *range; > > + > > + list_for_each_entry(range, &hmm->ranges, list) { > > + if (range->valid) > > + continue; > > + range->valid = true; > > + } > > This just effectively sets all ranges to valid. > I'm not sure that is best. This is a trade off, it would be much more expensive to have a precise 'valid = true' - instead this algorithm is precise about 'valid = false' and lazy about 'valid = true' which is much less costly to calculate. > Shouldn't hmm_range_register() start with range.valid = true and > then hmm_invalidate_range_start() set affected ranges to false? It kind of does, expect when it doesn't, right? :) > Then this becomes just wake_up_all() if --notifiers == 0 and > hmm_range_wait_until_valid() should wait for notifiers == 0. Almost.. but it is more tricky than that. This scheme is a collision-retry algorithm. The pagefault side runs to completion if no parallel invalidate start/end happens. If a parallel invalidation happens then the pagefault retries. Seeing notifiers == 0 means there is absolutely no current parallel invalidation. Seeing range->valid == true (under the device lock) means this range doesn't intersect with a parallel invalidate. So.. hmm_range_wait_until_valid() checks the per-range valid because it doesn't want to sleep if *this range* is not involved in a parallel invalidation - but once it becomes involved, then yes, valid == true implies notifiers == 0. It is easier/safer to use unlocked variable reads if there is only one variable, thus the weird construction. It is unclear to me if this micro optimization is really worthwhile. It is very expensive to manage all this tracking, and no other mmu notifier implementation really does something like this. Eliminating the per-range tracking and using the notifier count as a global lock would be much simpler... > Otherwise, range.valid doesn't really mean it's valid. Right, it doesn't really mean 'valid' It is tracking possible colliding invalidates such that valid == true (under the device lock) means that there was no colliding invalidate. I still think this implementation doesn't quite work, as I described here: https://lore.kernel.org/linux-mm/20190527195829.GB18019@mellanox.com/ But the idea is basically sound and matches what other mmu notifier users do, just using a seqcount like scheme, not a boolean. Jason
diff --git a/include/linux/hmm.h b/include/linux/hmm.h index bf013e96525771..0fa8ea34ccef6d 100644 --- a/include/linux/hmm.h +++ b/include/linux/hmm.h @@ -86,7 +86,7 @@ struct hmm { struct mm_struct *mm; struct kref kref; - struct mutex lock; + spinlock_t ranges_lock; struct list_head ranges; struct list_head mirrors; struct mmu_notifier mmu_notifier; diff --git a/mm/hmm.c b/mm/hmm.c index 4215edf737ef5b..10103a24e9b7b3 100644 --- a/mm/hmm.c +++ b/mm/hmm.c @@ -68,7 +68,7 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm) init_rwsem(&hmm->mirrors_sem); hmm->mmu_notifier.ops = NULL; INIT_LIST_HEAD(&hmm->ranges); - mutex_init(&hmm->lock); + spin_lock_init(&hmm->ranges_lock); kref_init(&hmm->kref); hmm->notifiers = 0; hmm->mm = mm; @@ -114,18 +114,19 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) { struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier); struct hmm_mirror *mirror; + unsigned long flags; /* Bail out if hmm is in the process of being freed */ if (!kref_get_unless_zero(&hmm->kref)) return; - mutex_lock(&hmm->lock); + spin_lock_irqsave(&hmm->ranges_lock, flags); /* * Since hmm_range_register() holds the mmget() lock hmm_release() is * prevented as long as a range exists. */ WARN_ON(!list_empty(&hmm->ranges)); - mutex_unlock(&hmm->lock); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); down_read(&hmm->mirrors_sem); list_for_each_entry(mirror, &hmm->mirrors, list) { @@ -141,6 +142,23 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm) hmm_put(hmm); } +static void notifiers_decrement(struct hmm *hmm) +{ + lockdep_assert_held(&hmm->ranges_lock); + + hmm->notifiers--; + if (!hmm->notifiers) { + struct hmm_range *range; + + list_for_each_entry(range, &hmm->ranges, list) { + if (range->valid) + continue; + range->valid = true; + } + wake_up_all(&hmm->wq); + } +} + static int hmm_invalidate_range_start(struct mmu_notifier *mn, const struct mmu_notifier_range *nrange) { @@ -148,6 +166,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, struct hmm_mirror *mirror; struct hmm_update update; struct hmm_range *range; + unsigned long flags; int ret = 0; if (!kref_get_unless_zero(&hmm->kref)) @@ -158,12 +177,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, update.event = HMM_UPDATE_INVALIDATE; update.blockable = mmu_notifier_range_blockable(nrange); - if (mmu_notifier_range_blockable(nrange)) - mutex_lock(&hmm->lock); - else if (!mutex_trylock(&hmm->lock)) { - ret = -EAGAIN; - goto out; - } + spin_lock_irqsave(&hmm->ranges_lock, flags); hmm->notifiers++; list_for_each_entry(range, &hmm->ranges, list) { if (update.end < range->start || update.start >= range->end) @@ -171,7 +185,7 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, range->valid = false; } - mutex_unlock(&hmm->lock); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); if (mmu_notifier_range_blockable(nrange)) down_read(&hmm->mirrors_sem); @@ -179,16 +193,26 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn, ret = -EAGAIN; goto out; } + list_for_each_entry(mirror, &hmm->mirrors, list) { - int ret; + int rc; - ret = mirror->ops->sync_cpu_device_pagetables(mirror, &update); - if (!update.blockable && ret == -EAGAIN) + rc = mirror->ops->sync_cpu_device_pagetables(mirror, &update); + if (rc) { + if (WARN_ON(update.blockable || rc != -EAGAIN)) + continue; + ret = -EAGAIN; break; + } } up_read(&hmm->mirrors_sem); out: + if (ret) { + spin_lock_irqsave(&hmm->ranges_lock, flags); + notifiers_decrement(hmm); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); + } hmm_put(hmm); return ret; } @@ -197,23 +221,14 @@ static void hmm_invalidate_range_end(struct mmu_notifier *mn, const struct mmu_notifier_range *nrange) { struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier); + unsigned long flags; if (!kref_get_unless_zero(&hmm->kref)) return; - mutex_lock(&hmm->lock); - hmm->notifiers--; - if (!hmm->notifiers) { - struct hmm_range *range; - - list_for_each_entry(range, &hmm->ranges, list) { - if (range->valid) - continue; - range->valid = true; - } - wake_up_all(&hmm->wq); - } - mutex_unlock(&hmm->lock); + spin_lock_irqsave(&hmm->ranges_lock, flags); + notifiers_decrement(hmm); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); hmm_put(hmm); } @@ -866,6 +881,7 @@ int hmm_range_register(struct hmm_range *range, { unsigned long mask = ((1UL << page_shift) - 1UL); struct hmm *hmm = mirror->hmm; + unsigned long flags; range->valid = false; range->hmm = NULL; @@ -887,7 +903,7 @@ int hmm_range_register(struct hmm_range *range, kref_get(&hmm->kref); /* Initialize range to track CPU page table updates. */ - mutex_lock(&hmm->lock); + spin_lock_irqsave(&hmm->ranges_lock, flags); range->hmm = hmm; list_add(&range->list, &hmm->ranges); @@ -898,7 +914,7 @@ int hmm_range_register(struct hmm_range *range, */ if (!hmm->notifiers) range->valid = true; - mutex_unlock(&hmm->lock); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); return 0; } @@ -914,13 +930,14 @@ EXPORT_SYMBOL(hmm_range_register); void hmm_range_unregister(struct hmm_range *range) { struct hmm *hmm = range->hmm; + unsigned long flags; if (WARN_ON(range->end <= range->start)) return; - mutex_lock(&hmm->lock); + spin_lock_irqsave(&hmm->ranges_lock, flags); list_del(&range->list); - mutex_unlock(&hmm->lock); + spin_unlock_irqrestore(&hmm->ranges_lock, flags); /* Drop reference taken by hmm_range_register() */ range->valid = false;
If the trylock on the hmm->mirrors_sem fails the function will return without decrementing the notifiers that were previously incremented. Since the caller will not call invalidate_range_end() on EAGAIN this will result in notifiers becoming permanently incremented and deadlock. If the sync_cpu_device_pagetables() required blocking the function will not return EAGAIN even though the device continues to touch the pages. This is a violation of the mmu notifier contract. Switch, and rename, the ranges_lock to a spin lock so we can reliably obtain it without blocking during error unwind. The error unwind is necessary since the notifiers count must be held incremented across the call to sync_cpu_device_pagetables() as we cannot allow the range to become marked valid by a parallel invalidate_start/end() pair while doing sync_cpu_device_pagetables(). Signed-off-by: Jason Gunthorpe <jgg@mellanox.com> --- include/linux/hmm.h | 2 +- mm/hmm.c | 77 +++++++++++++++++++++++++++------------------ 2 files changed, 48 insertions(+), 31 deletions(-) I almost lost this patch - it is part of the series, hasn't been posted before, and wasn't sent with the rest, sorry.