8bcd3b1998
work. (Moreover, I don't believe that they have ever worked as intended.) The explanation is fairly simple. Both MADV_DONTNEED and MADV_FREE perform vm_page_dontneed() on each page within the range given to madvise(). This function moves the page to the inactive queue. Specifically, if the page is clean, it is moved to the head of the inactive queue where it is first in line for processing by the page daemon. On the other hand, if it is dirty, it is placed at the tail. Let's further examine the case in which the page is clean. Recall that the page is at the head of the line for processing by the page daemon. The expectation of vm_page_dontneed()'s author was that the page would be transferred from the inactive queue to the cache queue by the page daemon. (Once the page is in the cache queue, it is, in effect, free, that is, it can be reallocated to a new vm object by vm_page_alloc() if it isn't reactivated quickly enough by a user of the old vm object.) The trouble is that nowhere in the execution of either MADV_DONTNEED or MADV_FREE is either the machine-independent reference flag (PG_REFERENCED) or the reference bit in any page table entry (PTE) mapping the page cleared. Consequently, the immediate reaction of the page daemon is to reactivate the page because it is referenced. In effect, the madvise() was for naught. The case in which the page was dirty is not too different. Instead of being laundered, the page is reactivated. Note: The essential difference between MADV_DONTNEED and MADV_FREE is that MADV_FREE clears a page's dirty field. So, MADV_FREE is always executing the clean case above. This revision changes vm_page_dontneed() to clear both the machine- independent reference flag (PG_REFERENCED) and the reference bit in all PTEs mapping the page. MFC after: 6 weeks
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