mm/filemap: fix filemap_get_folios_contig THP panic

Patch series "memfd-pin huge page fixes".

Fix multiple bugs that occur when using memfd_pin_folios with hugetlb
pages and THP.  The hugetlb bugs only bite when the page is not yet
faulted in when memfd_pin_folios is called.  The THP bug bites when the
starting offset passed to memfd_pin_folios is not huge page aligned.  See
the commit messages for details.


This patch (of 5):

memfd_pin_folios on memory backed by THP panics if the requested start
offset is not huge page aligned:

BUG: kernel NULL pointer dereference, address: 0000000000000036
RIP: 0010:filemap_get_folios_contig+0xdf/0x290
RSP: 0018:ffffc9002092fbe8 EFLAGS: 00010202
RAX: 0000000000000002 RBX: 0000000000000002 RCX: 0000000000000002

The fault occurs here, because xas_load returns a folio with value 2:

    filemap_get_folios_contig()
        for (folio = xas_load(&xas); folio && xas.xa_index <= end;
                        folio = xas_next(&xas)) {
                ...
                if (!folio_try_get(folio))   <-- BOOM

"2" is an xarray sibling entry.  We get it because memfd_pin_folios does
not round the indices passed to filemap_get_folios_contig to huge page
boundaries for THP, so we load from the middle of a huge page range see a
sibling.  (It does round for hugetlbfs, at the is_file_hugepages test).

To fix, if the folio is a sibling, then return the next index as the
starting point for the next call to filemap_get_folios_contig.

Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 89c1905d9c ("mm/gup: introduce memfd_pin_folios() for pinning memfd folios")
Signed-off-by: Steve Sistare <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Jason Gunthorpe <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Vivek Kasireddy <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/filemap.c

@@ -2196,6 +2196,10 @@ unsigned filemap_get_folios_contig(struct address_space *mapping,
 		if (xa_is_value(folio))
 			goto update_start;
 
+		/* If we landed in the middle of a THP, continue at its end. */
+		if (xa_is_sibling(folio))
+			goto update_start;
+
 		if (!folio_try_get(folio))
 			goto retry;