Re: [PATCH v3 3/3] mm: introduce mmap_lock_speculate_{try_begin|retry}

[Wei Yang]

On Fri, Nov 22, 2024 at 09:44:16AM -0800, Suren Baghdasaryan 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.
>
>Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>Signed-off-by: Suren Baghdasaryan <surenb@xxxxxxxxxx>
>Reviewed-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
>---
>Changes since v2 [1]
>- Added SOB, per Liam Howlett
>
>[1] https://lore.kernel.org/all/20241121162826.987947-3-surenb@xxxxxxxxxx/
>
> include/linux/mmap_lock.h | 33 +++++++++++++++++++++++++++++++--
> 1 file changed, 31 insertions(+), 2 deletions(-)
>
>diff --git a/include/linux/mmap_lock.h b/include/linux/mmap_lock.h
>index 9715326f5a85..8ac3041df053 100644
>--- a/include/linux/mmap_lock.h
>+++ b/include/linux/mmap_lock.h
>@@ -71,6 +71,7 @@ static inline void mmap_assert_write_locked(const struct mm_struct *mm)
> }
> 
> #ifdef CONFIG_PER_VMA_LOCK
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm)
> {
> 	seqcount_init(&mm->mm_lock_seq);
>@@ -87,11 +88,39 @@ static inline void mm_lock_seqcount_end(struct mm_struct *mm)
> 	do_raw_write_seqcount_end(&mm->mm_lock_seq);
> }
> 
>-#else
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	/*
>+	 * Since mmap_lock is a sleeping lock, and waiting for it to become
>+	 * unlocked is more or less equivalent with taking it ourselves, don't
>+	 * bother with the speculative path if mmap_lock is already write-locked
>+	 * and take the slow path, which takes the lock.
>+	 */
>+	return raw_seqcount_try_begin(&mm->mm_lock_seq, *seq);
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return do_read_seqcount_retry(&mm->mm_lock_seq, seq);

Just curious why we don't use read_seqcount_retry().

Looks this is the only user outside seqlock.h.

>+}
>+
>+#else /* CONFIG_PER_VMA_LOCK */
>+
> static inline void mm_lock_seqcount_init(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_begin(struct mm_struct *mm) {}
> static inline void mm_lock_seqcount_end(struct mm_struct *mm) {}
>-#endif
>+
>+static inline bool mmap_lock_speculate_try_begin(struct mm_struct *mm, unsigned int *seq)
>+{
>+	return false;
>+}
>+
>+static inline bool mmap_lock_speculate_retry(struct mm_struct *mm, unsigned int seq)
>+{
>+	return true;
>+}
>+
>+#endif /* CONFIG_PER_VMA_LOCK */
> 
> static inline void mmap_init_lock(struct mm_struct *mm)
> {
>-- 
>2.47.0.371.ga323438b13-goog

-- 
Wei Yang
Help you, Help me