[PATCH v8 05/10] filemap: cap PTE range to be created to allowed zero fill in folio_map_range()

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

From: Pankaj Raghav <p.raghav@samsung.com>

Usually the page cache does not extend beyond the size of the inode,
therefore, no PTEs are created for folios that extend beyond the size.

But with LBS support, we might extend page cache beyond the size of the
inode as we need to guarantee folios of minimum order. While doing a
read, do_fault_around() can create PTEs for pages that lie beyond the
EOF leading to incorrect error return when accessing a page beyond the
mapped file.

Cap the PTE range to be created for the page cache up to the end of
file(EOF) in filemap_map_pages() so that return error codes are consistent
with POSIX[1] for LBS configurations.

generic/749(currently in xfstest-dev patches-in-queue branch [0]) has
been created to trigger this edge case. This also fixes generic/749 for
tmpfs with huge=always on systems with 4k base page size.

[0] https://lore.kernel.org/all/20240615002935.1033031-3-mcgrof@kernel.org/
[1](from mmap(2))  SIGBUS
    Attempted access to a page of the buffer that lies beyond the end
    of the mapped file.  For an explanation of the treatment  of  the
    bytes  in  the  page that corresponds to the end of a mapped file
    that is not a multiple of the page size, see NOTES.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Matthew Wilcox (Oracle) <willy@infradead.org>
---
 mm/filemap.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/mm/filemap.c b/mm/filemap.c
index 8eafbd4a4d0c..56ff1d936aa8 100644
--- a/mm/filemap.c
+++ b/mm/filemap.c
@@ -3612,7 +3612,7 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 	struct vm_area_struct *vma = vmf->vma;
 	struct file *file = vma->vm_file;
 	struct address_space *mapping = file->f_mapping;
-	pgoff_t last_pgoff = start_pgoff;
+	pgoff_t file_end, last_pgoff = start_pgoff;
 	unsigned long addr;
 	XA_STATE(xas, &mapping->i_pages, start_pgoff);
 	struct folio *folio;
@@ -3638,6 +3638,10 @@ vm_fault_t filemap_map_pages(struct vm_fault *vmf,
 		goto out;
 	}
 
+	file_end = DIV_ROUND_UP(i_size_read(mapping->host), PAGE_SIZE) - 1;
+	if (end_pgoff > file_end)
+		end_pgoff = file_end;
+
 	folio_type = mm_counter_file(folio);
 	do {
 		unsigned long end;
-- 
2.44.1