We have the swap-entry information cached in the page->offset field all
the time after the first swapin. It will stay uptodate also after removing
the page from the swap cache in the do_wp_page fault.
We only need to know when the entry is uptodate and I take care of that
with a per-page bitflag (PG_swap_entry).
Here it is the simple patch against 2.3.12:
diff -ur 2.3.12/include/linux/mm.h 2.3.12-swap_entry/include/linux/mm.h
--- 2.3.12/include/linux/mm.h Wed Aug 4 12:28:17 1999
+++ 2.3.12-swap_entry/include/linux/mm.h Sat Aug 7 16:59:12 1999
@@ -152,6 +152,7 @@
#define PG_Slab 8
#define PG_swap_cache 9
#define PG_skip 10
+#define PG_swap_entry 11
/* bits 21-30 unused */
#define PG_reserved 31
diff -ur 2.3.12/mm/memory.c 2.3.12-swap_entry/mm/memory.c
--- 2.3.12/mm/memory.c Sun Aug 1 18:11:22 1999
+++ 2.3.12-swap_entry/mm/memory.c Sat Aug 7 16:51:49 1999
@@ -990,6 +990,7 @@
pte = mk_pte(page_address(page), vma->vm_page_prot);
+ set_bit(PG_swap_entry, &page->flags);
if (write_access && !is_page_shared(page)) {
delete_from_swap_cache(page);
pte = pte_mkwrite(pte_mkdirty(pte));
diff -ur 2.3.12/mm/swap_state.c 2.3.12-swap_entry/mm/swap_state.c
--- 2.3.12/mm/swap_state.c Tue Jul 13 02:02:10 1999
+++ 2.3.12-swap_entry/mm/swap_state.c Sat Aug 7 16:53:51 1999
@@ -274,6 +274,8 @@
}
UnlockPage(page);
+ clear_bit(PG_swap_entry, &page->flags);
+
__free_page(page);
}
diff -ur 2.3.12/mm/swapfile.c 2.3.12-swap_entry/mm/swapfile.c
--- 2.3.12/mm/swapfile.c Thu Jul 22 01:07:28 1999
+++ 2.3.12-swap_entry/mm/swapfile.c Sat Aug 7 16:55:21 1999
@@ -157,6 +157,35 @@
goto out;
}
+/* needs the big kernel lock */
+int reacquire_swap_entry(unsigned long entry)
+{
+ struct swap_info_struct * p;
+ unsigned long offset, type;
+ int retval = 0;
+
+ if (!entry)
+ goto out;
+ type = SWP_TYPE(entry);
+ if (type & SHM_SWP_TYPE)
+ goto out;
+ if (type >= nr_swapfiles)
+ goto out;
+ p = type + swap_info;
+ if ((p->flags & SWP_WRITEOK) != SWP_WRITEOK)
+ goto out;
+ offset = SWP_OFFSET(entry);
+ if (offset >= p->max)
+ goto out;
+ if (!p->swap_map[offset])
+ {
+ retval = p->swap_map[offset] = 1;
+ nr_swap_pages--;
+ }
+out:
+ return retval;
+}
+
/*
* The swap entry has been read in advance, and we return 1 to indicate
* that the page has been used or is no longer needed.
diff -ur 2.3.12/mm/vmscan.c 2.3.12-swap_entry/mm/vmscan.c
--- 2.3.12/mm/vmscan.c Thu Jul 22 01:07:28 1999
+++ 2.3.12-swap_entry/mm/vmscan.c Sat Aug 7 16:57:00 1999
@@ -153,9 +153,16 @@
* we have the swap cache set up to associate the
* page with that swap entry.
*/
+ if (test_and_clear_bit(PG_swap_entry, &page->flags))
+ {
+ entry = page->offset;
+ if (reacquire_swap_entry(entry))
+ goto swap_entry_reacquired;
+ }
entry = get_swap_page();
if (!entry)
goto out_failed; /* No swap space left */
+ swap_entry_reacquired:
vma->vm_mm->rss--;
tsk->nswap++;
When I benchmarked it on 2.2.x long ago it was a very nice improvement.
Andrea
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