[PATCH 5/7] shmem: open code the page cache lookup in shmem_get_folio_gfp

[Christoph Hellwig <hch@xxxxxx>]

Use the very low level filemap_get_entry helper to look up the
entry in the xarray, and then:

 - don't bother locking the folio if only doing a userfault notification
 - open code locking the page and checking for truncation in a related
   code block

This will allow to eventually remove the FGP_ENTRY flag.

Signed-off-by: Christoph Hellwig <hch@xxxxxx>
---
 mm/shmem.c | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index a8371ffeeee54a..23d5cf8182cb1f 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1856,12 +1856,10 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	sbinfo = SHMEM_SB(inode->i_sb);
 	charge_mm = vma ? vma->vm_mm : NULL;
 
-	folio = __filemap_get_folio(mapping, index, FGP_ENTRY | FGP_LOCK, 0);
+	folio = filemap_get_entry(mapping, index);
 	if (folio && vma && userfaultfd_minor(vma)) {
-		if (!xa_is_value(folio)) {
-			folio_unlock(folio);
+		if (!xa_is_value(folio))
 			folio_put(folio);
-		}
 		*fault_type = handle_userfault(vmf, VM_UFFD_MINOR);
 		return 0;
 	}
@@ -1877,6 +1875,14 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
 	}
 
 	if (folio) {
+		folio_lock(folio);
+
+		/* Has the page been truncated? */
+		if (unlikely(folio->mapping != mapping)) {
+			folio_unlock(folio);
+			folio_put(folio);
+			goto repeat;
+		}
 		if (sgp == SGP_WRITE)
 			folio_mark_accessed(folio);
 		if (folio_test_uptodate(folio))
-- 
2.39.0