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charset="UTF-8" X-Stat-Signature: 1emiapum1ki8jzs9z8g8wbee3gfka5mk X-Rspam-User: X-Rspamd-Queue-Id: 29DF7C0019 X-Rspamd-Server: rspam06 X-HE-Tag: 1673930088-431949 X-HE-Meta: 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 5bttjfBo QzGiNVs15fv1A1eU7u4N9vynOT7zDJBvUCo9sD+BCnm61jGlHKTLIpwgalM/7zDWxSPaOnJmj+u/fIHrT/TkFpxixAc+mHO+lnfR0Nsy8URyyRd7O2T8nKrsG8LrxiUUhbn5w X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Mon, Jan 16, 2023 at 8:14 PM Matthew Wilcox wrote: > > On Mon, Jan 16, 2023 at 11:14:38AM +0000, Hyeonggon Yoo wrote: > > > @@ -643,20 +647,28 @@ static inline void vma_write_lock(struct vm_area_struct *vma) > > > static inline bool vma_read_trylock(struct vm_area_struct *vma) > > > { > > > /* Check before locking. A race might cause false locked result. */ > > > - if (vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > > + if (vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq)) > > > return false; > > > > > > - if (unlikely(down_read_trylock(&vma->vm_lock->lock) == 0)) > > > + if (unlikely(!atomic_inc_unless_negative(&vma->vm_lock->count))) > > > return false; > > > > > > + /* If atomic_t overflows, restore and fail to lock. */ > > > + if (unlikely(atomic_read(&vma->vm_lock->count) < 0)) { > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > + return false; > > > + } > > > + > > > /* > > > * Overflow might produce false locked result. > > > * False unlocked result is impossible because we modify and check > > > * vma->vm_lock_seq under vma->vm_lock protection and mm->mm_lock_seq > > > * modification invalidates all existing locks. > > > */ > > > - if (unlikely(vma->vm_lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > > - up_read(&vma->vm_lock->lock); > > > + if (unlikely(vma->vm_lock->lock_seq == READ_ONCE(vma->vm_mm->mm_lock_seq))) { > > > + if (atomic_dec_and_test(&vma->vm_lock->count)) > > > + wake_up(&vma->vm_mm->vma_writer_wait); > > > return false; > > > } > > > > With this change readers can cause writers to starve. > > What about checking waitqueue_active() before or after increasing > > vma->vm_lock->count? > > I don't understand how readers can starve a writer. Readers do > atomic_inc_unless_negative() so a writer can always force readers > to fail. I think the point here was that if page faults keep occuring and they prevent vm_lock->count from reaching 0 then a writer will be blocked and there is no reader throttling mechanism (no max time that writer will be waiting).