On Thu, Aug 8, 2024 at 2:02 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Thu, Aug 8, 2024 at 8:19 PM Andrii Nakryiko > <andrii.nakryiko@xxxxxxxxx> wrote: > > > > On Wed, Aug 7, 2024 at 11:23 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > > > > > Add helper functions to speculatively perform operations without > > > read-locking mmap_lock, expecting that mmap_lock will not be > > > write-locked and mm is not modified from under us. > > > > > > Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx> > > > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx> > > > Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> > > > --- > > > > This change makes sense and makes mm's seq a bit more useful and > > meaningful. I've also tested it locally with uprobe stress-test, and > > it seems to work great, I haven't run into any problems with a > > multi-hour stress test run so far. Thanks! > > Thanks for testing and feel free to include this patch into your set. Will do! > > I've been thinking about this some more and there is a very unlikely > corner case if between mmap_lock_speculation_start() and > mmap_lock_speculation_end() mmap_lock is write-locked/unlocked so many > times that mm->mm_lock_seq (int) overflows and just happen to reach > the same value as we recorded in mmap_lock_speculation_start(). This > would generate a false positive, which would show up as if the > mmap_lock was never touched. Such overflows are possible for vm_lock > as well (see: https://elixir.bootlin.com/linux/v6.10.3/source/include/linux/mm_types.h#L688) > but they are not critical because a false result would simply lead to > a retry under mmap_lock. However for your case this would be a > critical issue. This is an extremely low probability scenario but > should we still try to handle it? > No, I think it's fine. Similar problems could happen with refcount_t, for instance (it has a logic to have a sticky "has overflown" state, which I believe relies on the fact that we'll never be able to increment refcount 2bln+ times in between some resetting logic). Anyways, I think it's utterly unrealistic and should be considered impossible. > I'm CC'ing several mm folks and Jann Horn to chime in. > > > > > Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx> > > > > > Discussion [1] follow-up. If proves to be useful can be included in that > > > patchset. Based on mm-unstable. > > > > > > [1] https://lore.kernel.org/all/20240730134605.GO33588@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ > > > > > > include/linux/mm_types.h | 3 +++ > > > include/linux/mmap_lock.h | 53 +++++++++++++++++++++++++++++++-------- > > > kernel/fork.c | 3 --- > > > 3 files changed, 46 insertions(+), 13 deletions(-) > > > > > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > > > index 003619fab20e..a426e6ced604 100644 > > > --- a/include/linux/mm_types.h > > > +++ b/include/linux/mm_types.h > > > @@ -887,6 +887,9 @@ struct mm_struct { > > > * Roughly speaking, incrementing the sequence number is > > > * equivalent to releasing locks on VMAs; reading the sequence > > > * number can be part of taking a read lock on a VMA. > > > + * Incremented every time mmap_lock is write-locked/unlocked. > > > + * Initialized to 0, therefore odd values indicate mmap_lock > > > + * is write-locked and even values that it's released. > > > * > > > * Can be modified under write mmap_lock using RELEASE > > > * semantics. > > > diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h > > > index de9dc20b01ba..5410ce741d75 100644 > > > --- a/include/linux/mmap_lock.h > > > +++ b/include/linux/mmap_lock.h > > > @@ -71,15 +71,12 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm) > > > } > > > > > > #ifdef CONFIG_PER_VMA_LOCK > > > -/* > > > - * Drop all currently-held per-VMA locks. > > > - * This is called from the mmap_lock implementation directly before releasing > > > - * a write-locked mmap_lock (or downgrading it to read-locked). > > > - * This should normally NOT be called manually from other places. > > > - * If you want to call this manually anyway, keep in mind that this will release > > > - * *all* VMA write locks, including ones from further up the stack. > > > - */ > > > -static inline void vma_end_write_all(struct mm_struct *mm) > > > +static inline void init_mm_lock_seq(struct mm_struct *mm) > > > +{ > > > + mm->mm_lock_seq = 0; > > > +} > > > + > > > +static inline void inc_mm_lock_seq(struct mm_struct *mm) > > > { > > > mmap_assert_write_locked(mm); > > > /* > > > @@ -91,19 +88,52 @@ static inline void vma_end_write_all(struct mm_struct *mm) > > > */ > > > smp_store_release(&mm->mm_lock_seq, mm->mm_lock_seq + 1); > > > } > > > + > > > +static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq) > > > +{ > > > + /* Pairs with RELEASE semantics in inc_mm_lock_seq(). */ > > > + *seq = smp_load_acquire(&mm->mm_lock_seq); > > > + /* Allow speculation if mmap_lock is not write-locked */ > > > + return (*seq & 1) == 0; > > > +} > > > + > > > +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq) > > > +{ > > > + /* Pairs with RELEASE semantics in inc_mm_lock_seq(). */ > > > + return seq == smp_load_acquire(&mm->mm_lock_seq); > > > +} > > > + > > > #else > > > -static inline void vma_end_write_all(struct mm_struct *mm) {} > > > +static inline void init_mm_lock_seq(struct mm_struct *mm) {} > > > +static inline void inc_mm_lock_seq(struct mm_struct *mm) {} > > > +static inline bool mmap_lock_speculation_start(struct mm_struct *mm, int *seq) { return false; } > > > +static inline bool mmap_lock_speculation_end(struct mm_struct *mm, int seq) { return false; } > > > #endif > > > > > > +/* > > > + * Drop all currently-held per-VMA locks. > > > + * This is called from the mmap_lock implementation directly before releasing > > > + * a write-locked mmap_lock (or downgrading it to read-locked). > > > + * This should normally NOT be called manually from other places. > > > + * If you want to call this manually anyway, keep in mind that this will release > > > + * *all* VMA write locks, including ones from further up the stack. > > > + */ > > > +static inline void vma_end_write_all(struct mm_struct *mm) > > > +{ > > > + inc_mm_lock_seq(mm); > > > +} > > > + > > > static inline void mmap_init_lock(struct mm_struct *mm) > > > { > > > init_rwsem(&mm->mmap_lock); > > > + init_mm_lock_seq(mm); > > > } > > > > > > static inline void mmap_write_lock(struct mm_struct *mm) > > > { > > > __mmap_lock_trace_start_locking(mm, true); > > > down_write(&mm->mmap_lock); > > > + inc_mm_lock_seq(mm); > > > __mmap_lock_trace_acquire_returned(mm, true, true); > > > } > > > > > > @@ -111,6 +141,7 @@ static inline void mmap_write_lock_nested(struct mm_struct *mm, int subclass) > > > { > > > __mmap_lock_trace_start_locking(mm, true); > > > down_write_nested(&mm->mmap_lock, subclass); > > > + inc_mm_lock_seq(mm); > > > __mmap_lock_trace_acquire_returned(mm, true, true); > > > } > > > > > > @@ -120,6 +151,8 @@ static inline int mmap_write_lock_killable(struct mm_struct *mm) > > > > > > __mmap_lock_trace_start_locking(mm, true); > > > ret = down_write_killable(&mm->mmap_lock); > > > + if (!ret) > > > + inc_mm_lock_seq(mm); > > > __mmap_lock_trace_acquire_returned(mm, true, ret == 0); > > > return ret; > > > } > > > diff --git a/kernel/fork.c b/kernel/fork.c > > > index 3d590e51ce84..73e37af8a24d 100644 > > > --- a/kernel/fork.c > > > +++ b/kernel/fork.c > > > @@ -1259,9 +1259,6 @@ static struct mm_struct *mm_init(struct mm_struct *mm, struct task_struct *p, > > > seqcount_init(&mm->write_protect_seq); > > > mmap_init_lock(mm); > > > INIT_LIST_HEAD(&mm->mmlist); > > > -#ifdef CONFIG_PER_VMA_LOCK > > > - mm->mm_lock_seq = 0; > > > -#endif > > > mm_pgtables_bytes_init(mm); > > > mm->map_count = 0; > > > mm->locked_vm = 0; > > > > > > base-commit: 98808d08fc0f78ee638e0c0a88020fbbaf581ec6 > > > -- > > > 2.46.0.rc2.264.g509ed76dc8-goog > > >
