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

Christoph Hellwig
2023-01-21T06:58:30Z

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@lst.de>
---
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