One other fun thing. I have a "victim" thread sitting in a loop doing:
sleep(1)
memcpy(&garbage, buffer, sz);
The "attacker" is doing
madvise(buffer, sz, MADV_PAGEOUT);
in a loop. That, oddly enough doesn't cause the victim to page fault.
But, if I do:
memcpy(&garbage, buffer, sz);
madvise(buffer, sz, MADV_PAGEOUT);
It *does* cause the memory to get paged out. The MADV_PAGEOUT code
actually has a !pte_present() check. It will punt on a PTE if it sees
it. In other words, if a page is in the swap cache but not mapped by a
pte_present() PTE, MADV_PAGEOUT won't touch it.
Shouldn't MADV_PAGEOUT be able to find and reclaim those pages? Patch
attached.
---
b/mm/madvise.c | 38 +++++++++++++++++++++++++++++++-------
1 file changed, 31 insertions(+), 7 deletions(-)
diff -puN mm/madvise.c~madv-pageout-find-swap-cache mm/madvise.c
--- a/mm/madvise.c~madv-pageout-find-swap-cache 2020-03-12 14:24:45.178775035 -0700
+++ b/mm/madvise.c 2020-03-12 14:35:49.706773378 -0700
@@ -248,6 +248,36 @@ static void force_shm_swapin_readahead(s
#endif /* CONFIG_SWAP */
/*
+ * Given a PTE, find the corresponding 'struct page'. Also handles
+ * non-present swap PTEs.
+ */
+struct page *pte_to_reclaim_page(struct vm_area_struct *vma,
+ unsigned long addr, pte_t ptent)
+{
+ swp_entry_t entry;
+
+ /* Totally empty PTE: */
+ if (pte_none(ptent))
+ return NULL;
+
+ /* A normal, present page is mapped: */
+ if (pte_present(ptent))
+ return vm_normal_page(vma, addr, ptent);
+
+ entry = pte_to_swp_entry(vmf->orig_pte);
+ /* Is it one of the "swap PTEs" that's not really swap? */
+ if (non_swap_entry(entry))
+ return false;
+
+ /*
+ * The PTE was a true swap entry. The page may be in the
+ * swap cache. If so, find it and return it so it may be
+ * reclaimed.
+ */
+ return lookup_swap_cache(entry, vma, addr);
+}
+
+/*
* Schedule all required I/O operations. Do not wait for completion.
*/
static long madvise_willneed(struct vm_area_struct *vma,
@@ -389,13 +419,7 @@ regular_page:
for (; addr < end; pte++, addr += PAGE_SIZE) {
ptent = *pte;
- if (pte_none(ptent))
- continue;
-
- if (!pte_present(ptent))
- continue;
-
- page = vm_normal_page(vma, addr, ptent);
+ page = pte_to_reclaim_page(vma, addr, ptent);
if (!page)
continue;
_
