Previously, we shared too much of the code with COPY and ZEROPAGE, so we
manipulated things in various invalid ways:

- Previously, we unconditionally called shmem_inode_acct_block. In the
  continue case, we're looking up an existing page which would have been
  accounted for properly when it was allocated. So doing it twice
  results in double-counting, and eventually leaking.

- Previously, we made the pte writable whenever the VMA was writable.
  However, for continue, consider this case:

  1. A tmpfs file was created
  2. The non-UFFD-registered side mmap()-s with MAP_SHARED
  3. The UFFD-registered side mmap()-s with MAP_PRIVATE

  In this case, even though the UFFD-registered VMA may be writable, we
  still want CoW behavior. So, check for this case and don't make the
  pte writable.

- The initial pgoff / max_off check isn't necessary, so we can skip past
  it. The second one seems likely to be unnecessary too, but keep it
  just in case. Modify both checks to use pgoff, as offset is equivalent
  and not needed.

- Previously, we unconditionally called ClearPageDirty() in the error
  path. In the continue case though, since this is an existing page, it
  might have already been dirty before we started touching it. It's very
  problematic to clear the bit incorrectly, but not a problem to leave
  it - so, just omit the ClearPageDirty() entirely.

- Previously, we unconditionally removed the page from the page cache in
  the error path. But in the continue case, we didn't add it - it was
  already there because the page is present in some second
  (non-UFFD-registered) mapping. So, removing it is invalid.

Because the error handling issues are easy to exercise in the selftest,
make a small modification there to do so.

Finally, refactor shmem_mcopy_atomic_pte a bit. By this point, we've
added a lot of "if (!is_continue)"-s everywhere. It's cleaner to just
check for that mode first thing, and then "goto" down to where the parts
we actually want are. This leaves the code in between cleaner.

Changes since v2:
- Drop the ClearPageDirty() entirely, instead of trying to remember the
  old value.
- Modify both pgoff / max_off checks to use pgoff. It's equivalent to
  offset, but offset wasn't initialized until the first check (which
  we're skipping).
- Keep the second pgoff / max_off check in the continue case.

Changes since v1:
- Refactor to skip ahead with goto, instead of adding several more
  "if (!is_continue)".
- Fix unconditional ClearPageDirty().
- Don't pte_mkwrite() when is_continue && !VM_SHARED.

Fixes: 00da60b9d0a0 ("userfaultfd: support minor fault handling for shmem")
Signed-off-by: Axel Rasmussen <axelrasmussen@google.com>
---
 mm/shmem.c                               | 60 +++++++++++++-----------
 tools/testing/selftests/vm/userfaultfd.c | 12 +++++
 2 files changed, 44 insertions(+), 28 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index d2e0e81b7d2e..fbcce850a16e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c

@@ -2377,18 +2377,22 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	struct page *page;
 	pte_t _dst_pte, *dst_pte;
 	int ret;
-	pgoff_t offset, max_off;
-
-	ret = -ENOMEM;
-	if (!shmem_inode_acct_block(inode, 1))
-		goto out;
+	pgoff_t max_off;
+	int writable;

+	if (writable)
 		_dst_pte = pte_mkwrite(pte_mkdirty(_dst_pte));
 	else {
 		/*

@@ -2455,7 +2458,7 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 
 	ret = -EFAULT;
 	max_off = DIV_ROUND_UP(i_size_read(inode), PAGE_SIZE);
-	if (unlikely(offset >= max_off))
+	if (unlikely(pgoff >= max_off))
 		goto out_release_unlock;
 
 	ret = -EEXIST;

@@ -2485,13 +2488,14 @@ int shmem_mcopy_atomic_pte(struct mm_struct *dst_mm, pmd_t *dst_pmd,
 	return ret;
 out_release_unlock:
 	pte_unmap_unlock(dst_pte, ptl);
-	ClearPageDirty(page);
-	delete_from_page_cache(page);
+	if (!is_continue)
+		delete_from_page_cache(page);
 out_release:
 	unlock_page(page);
 	put_page(page);
 out_unacct_blocks:
-	shmem_inode_unacct_blocks(inode, 1);
+	if (!is_continue)
+		shmem_inode_unacct_blocks(inode, 1);