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>

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