linux-kernel - Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a reference count

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Message-ID: <CAJuCfpH_4EF6+-cUF9uNrfX7wONWHR8f13EQHN4AeVkALMBWfQ@mail.gmail.com>
Date: Mon, 13 Jan 2025 13:18:48 -0800
From: Suren Baghdasaryan <surenb@...gle.com>
To: Wei Yang <richard.weiyang@...il.com>
Cc: akpm@...ux-foundation.org, peterz@...radead.org, willy@...radead.org, 
	liam.howlett@...cle.com, lorenzo.stoakes@...cle.com, 
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Subject: Re: [PATCH v9 11/17] mm: replace vm_lock and detached flag with a
 reference count

On Mon, Jan 13, 2025 at 1:36 AM Wei Yang <richard.weiyang@...il.com> wrote:
>
> On Fri, Jan 10, 2025 at 08:25:58PM -0800, Suren Baghdasaryan wrote:
> [...]
> >
> >+static inline bool is_vma_writer_only(int refcnt)
> >+{
> >+      /*
> >+       * With a writer and no readers, refcnt is VMA_LOCK_OFFSET if the vma
> >+       * is detached and (VMA_LOCK_OFFSET + 1) if it is attached. Waiting on
> >+       * a detached vma happens only in vma_mark_detached() and is a rare
> >+       * case, therefore most of the time there will be no unnecessary wakeup.
> >+       */
> >+      return refcnt & VMA_LOCK_OFFSET && refcnt <= VMA_LOCK_OFFSET + 1;
>
> It looks equivalent to
>
>         return (refcnt == VMA_LOCK_OFFSET) || (refcnt == VMA_LOCK_OFFSET + 1);
>
> And its generated code looks a little simpler.

Yeah, but I think the original version is a bit more descriptive,
checking that (1) there is a writer and (2) there are no readers.

>
> >+}
> >+
> >+static inline void vma_refcount_put(struct vm_area_struct *vma)
> >+{
> >+      /* Use a copy of vm_mm in case vma is freed after we drop vm_refcnt */
> >+      struct mm_struct *mm = vma->vm_mm;
> >+      int oldcnt;
> >+
> >+      rwsem_release(&vma->vmlock_dep_map, _RET_IP_);
> >+      if (!__refcount_dec_and_test(&vma->vm_refcnt, &oldcnt)) {
> >+
> >+              if (is_vma_writer_only(oldcnt - 1))
> >+                      rcuwait_wake_up(&mm->vma_writer_wait);
> >+      }
> >+}
> >+
>
> --
> Wei Yang
> Help you, Help me