tmpfs: fix data loss from failed fallocate

Fix tmpfs data loss when the fallocate system call is interrupted by a
signal, or fails for some other reason.  The partial folio handling in
shmem_undo_range() forgot to consider this unfalloc case, and was liable
to erase or truncate out data which had already been committed earlier.

It turns out that none of the partial folio handling there is appropriate
for the unfalloc case, which just wants to proceed to removal of whole
folios: which find_get_entries() provides, even when partially covered.

Original patch by Rui Wang.

Link: https://lore.kernel.org/linux-mm/[email protected]/
Link: https://lkml.kernel.org/r/[email protected]
Fixes: b9a8a41 ("truncate,shmem: Handle truncates that split large folios")
Signed-off-by: Hugh Dickins <[email protected]>
Reported-by: Guoqi Chen <[email protected]>
  Link: https://lore.kernel.org/all/[email protected]/
Cc: Rui Wang <[email protected]>
Cc: Huacai Chen <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Vishal Moola (Oracle) <[email protected]>
Cc: <[email protected]>	[5.17+]
Signed-off-by: Andrew Morton <[email protected]>

mm/shmem.c

@@ -948,6 +948,15 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 		index++;
 	}
 
+	/*
+	 * When undoing a failed fallocate, we want none of the partial folio
+	 * zeroing and splitting below, but shall want to truncate the whole
+	 * folio when !uptodate indicates that it was added by this fallocate,
+	 * even when [lstart, lend] covers only a part of the folio.
+	 */
+	if (unfalloc)
+		goto whole_folios;
+
 	same_folio = (lstart >> PAGE_SHIFT) == (lend >> PAGE_SHIFT);
 	folio = shmem_get_partial_folio(inode, lstart >> PAGE_SHIFT);
 	if (folio) {
@@ -973,6 +982,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
 		folio_put(folio);
 	}
 
+whole_folios:
+
 	index = start;
 	while (index < end) {
 		cond_resched();