mm: streamline COW logic in do_swap_page()

Currently we have a different COW logic when:
* triggering a read-fault to swapin first and then trigger a write-fault
  -> do_swap_page() + do_wp_page()
* triggering a write-fault to swapin
  -> do_swap_page() + do_wp_page() only if we fail reuse in do_swap_page()

The COW logic in do_swap_page() is different than our reuse logic in
do_wp_page().  The COW logic in do_wp_page() -- page_count() == 1 -- makes
currently sure that we certainly don't have a remaining reference, e.g.,
via GUP, on the target page we want to reuse: if there is any unexpected
reference, we have to copy to avoid information leaks.

As do_swap_page() behaves differently, in environments with swap enabled
we can currently have an unintended information leak from the parent to
the child, similar as known from CVE-2020-29374:

	1. Parent writes to anonymous page
	-> Page is mapped writable and modified
	2. Page is swapped out
	-> Page is unmapped and replaced by swap entry
	3. fork()
	-> Swap entries are copied to child
	4. Child pins page R/O
	-> Page is mapped R/O into child
	5. Child unmaps page
	-> Child still holds GUP reference
	6. Parent writes to page
	-> Page is reused in do_swap_page()
	-> Child can observe changes

Exchanging 2. and 3. should have the same effect.

Let's apply the same COW logic as in do_wp_page(), conditionally trying to
remove the page from the swapcache after freeing the swap entry, however,
before actually mapping our page.  We can change the order now that we use
try_to_free_swap(), which doesn't care about the mapcount, instead of
reuse_swap_page().

To handle references from the LRU pagevecs, conditionally drain the local
LRU pagevecs when required, however, don't consider the page_count() when
deciding whether to drain to keep it simple for now.

Link: https://lkml.kernel.org/r/20220131162940.210846-5-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Rientjes <rientjes@google.com>
Cc: Don Dutile <ddutile@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Jann Horn <jannh@google.com>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Liang Zhang <zhangliang5@huawei.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Mike Rapoport <rppt@linux.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Author: davidhildenbrand

mm/memory.c

@@ -3489,6 +3489,25 @@ static vm_fault_t remove_device_exclusive_entry(struct vm_fault *vmf)
 	return 0;
 }
 
+static inline bool should_try_to_free_swap(struct page *page,
+					   struct vm_area_struct *vma,
+					   unsigned int fault_flags)
+{
+	if (!PageSwapCache(page))
+		return false;
+	if (mem_cgroup_swap_full(page) || (vma->vm_flags & VM_LOCKED) ||
+	    PageMlocked(page))
+		return true;
+	/*
+	 * If we want to map a page that's in the swapcache writable, we
+	 * have to detect via the refcount if we're really the exclusive
+	 * user. Try freeing the swapcache to get rid of the swapcache
+	 * reference only in case it's likely that we'll be the exlusive user.
+	 */
+	return (fault_flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+		page_count(page) == 2;
+}
+
 /*
  * We enter with non-exclusive mmap_lock (to exclude vma changes,
  * but allow concurrent faults), and pte mapped but not yet locked.
@@ -3630,6 +3649,16 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 			page = swapcache;
 			goto out_page;
 		}
+
+		/*
+		 * If we want to map a page that's in the swapcache writable, we
+		 * have to detect via the refcount if we're really the exclusive
+		 * owner. Try removing the extra reference from the local LRU
+		 * pagevecs if required.
+		 */
+		if ((vmf->flags & FAULT_FLAG_WRITE) && page == swapcache &&
+		    !PageKsm(page) && !PageLRU(page))
+			lru_add_drain();
 	}
 
 	cgroup_throttle_swaprate(page, GFP_KERNEL);
@@ -3648,19 +3677,25 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	}
 
 	/*
-	 * The page isn't present yet, go ahead with the fault.
-	 *
-	 * Be careful about the sequence of operations here.
-	 * To get its accounting right, reuse_swap_page() must be called
-	 * while the page is counted on swap but not yet in mapcount i.e.
-	 * before page_add_anon_rmap() and swap_free(); try_to_free_swap()
-	 * must be called after the swap_free(), or it will never succeed.
+	 * Remove the swap entry and conditionally try to free up the swapcache.
+	 * We're already holding a reference on the page but haven't mapped it
+	 * yet.
 	 */
+	swap_free(entry);
+	if (should_try_to_free_swap(page, vma, vmf->flags))
+		try_to_free_swap(page);
 
 	inc_mm_counter_fast(vma->vm_mm, MM_ANONPAGES);
 	dec_mm_counter_fast(vma->vm_mm, MM_SWAPENTS);
 	pte = mk_pte(page, vma->vm_page_prot);
-	if ((vmf->flags & FAULT_FLAG_WRITE) && reuse_swap_page(page)) {
+
+	/*
+	 * Same logic as in do_wp_page(); however, optimize for fresh pages
+	 * that are certainly not shared because we just allocated them without
+	 * exposing them to the swapcache.
+	 */
+	if ((vmf->flags & FAULT_FLAG_WRITE) && !PageKsm(page) &&
+	    (page != swapcache || page_count(page) == 1)) {
 		pte = maybe_mkwrite(pte_mkdirty(pte), vma);
 		vmf->flags &= ~FAULT_FLAG_WRITE;
 		ret |= VM_FAULT_WRITE;
@@ -3686,10 +3721,6 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
 	set_pte_at(vma->vm_mm, vmf->address, vmf->pte, pte);
 	arch_do_swap_page(vma->vm_mm, vma, vmf->address, pte, vmf->orig_pte);
 
-	swap_free(entry);
-	if (mem_cgroup_swap_full(page) ||
-	    (vma->vm_flags & VM_LOCKED) || PageMlocked(page))
-		try_to_free_swap(page);
 	unlock_page(page);
 	if (page != swapcache && swapcache) {
 		/*