shmem: open code the page cache lookup in shmem_get_folio_gfp

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.

Link: https://lkml.kernel.org/r/20230307143410.28031-6-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

diff --git a/mm/shmem.c b/mm/shmem.c
index e56b4addc80153bcdde9055c8bb5f108aa031196..e801e978af69d229306161fdf04277368bddaafa 100644 (file)
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1883,12 +1883,10 @@ repeat:
        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;
        }
@@ -1904,6 +1902,14 @@ repeat:
        }
 
        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))