mm/memory: don't require head page for do_set_pmd()

The requirement that the head page be passed to do_set_pmd() was added in
commit ef37b2e ("mm/memory: page_add_file_rmap() ->
folio_add_file_rmap_[pte|pmd]()") and prevents pmd-mapping in the
finish_fault() and filemap_map_pages() paths if the page to be inserted is
anything but the head page for an otherwise suitable vma and pmd-sized
page.

Matthew said:

: We're going to stop using PMDs to map large folios unless the fault is
: within the first 4KiB of the PMD.  No idea how many workloads that
: affects, but it only needs to be backported as far as v6.8, so we may
: as well backport it.

Link: https://lkml.kernel.org/r/20240611153216.2794513-1-abrestic@rivosinc.com
Fixes: ef37b2e ("mm/memory: page_add_file_rmap() -> folio_add_file_rmap_[pte|pmd]()")
Signed-off-by: Andrew Bresticker <abrestic@rivosinc.com>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: abrestic-rivos

mm/memory.c

@@ -4608,8 +4608,9 @@ vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
 	if (!thp_vma_suitable_order(vma, haddr, PMD_ORDER))
 		return ret;
 
-	if (page != &folio->page || folio_order(folio) != HPAGE_PMD_ORDER)
+	if (folio_order(folio) != HPAGE_PMD_ORDER)
 		return ret;
+	page = &folio->page;
 
 	/*
 	 * Just backoff if any subpage of a THP is corrupted otherwise