• NonResidentPages

The goals for this implementation of non-resident page bookkeeping:

• minimal space overhead
• SMP scalability
• reasonably fast

Interface

extern int recently_evicted(struct address_space * mapping, unsigned long offset);
extern int remember_page(struct address_space * mapping, unsigned long offset);

The recently_evicted function is queried by the pagein or page cache allocation code, to determine whether the data at the offset offset from the page cache or process object mapping was recently evicted. The function returns 0 if the page was not found, 1 if the page was found.

The remember_page function is called by the pageout code, telling the non-resident page management code to remember that a page at offset offset from mapping was just paged out.

Data structures

When looking up a page, we use page->offset, page->mapping to determine which nr_bucket hash bucket to put the page in, and hash page->offset, page->mapping and page->mapping->host->i_ino to get to the value stored in the nr_bucket.page array. We recycle nr_bucket.page entries in a circular fashion, with hand pointing to the current entry that's being updated.

This means we do not need a lookup list for these pages, we simply look through all the objects in the cacheline, doing simple comparisons. Hopefully the cost of comparing 31 entries is outweighed by only having 1 cache miss per lookup.

/* Number of non-resident pages per hash bucket */
#define NUM_NR ((L1_CACHE_BYTES - sizeof(atomic_t))/sizeof(unsigned long))

struct nr_bucket
{
        atomic_t hand;
        unsigned long page[NUM_NR];
} ____cacheline_aligned;

Uniqueness of mapping

Because the address_space structure underlying mapping can be freed by the kernel and then reallocated for another file, we need to take some measures to prevent the VM from thinking a brand-new page of a newly opened file was recently evicted. The obvious way to do this would be invalidating all entries of a particular mapping when the struct address_space is freed. However, we would need a much larger data structure if we wanted to support efficient invalidation.

The alternative is to simply hash the inode number into the non-resident page value, and hope that the next time a particular struct address_space is allocated to a different file, that file will have a different inode number.

The swap cache is a special case, since we can invalidate entries when a process exits, because we free up swap pages one by one. We can simply call the recently_evicted function from remove_exclusive_swap_page. This also covers swapoff(8) and a subsequent swapon(8), since the non-resident entries will be invalidated at swapoff time.

For other pages in the AdvancedPageReplacement category, please see CategoryAdvancedPageReplacement