[PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

Sumitra Sharma
2023-06-27T13:51:28Z

kmap() has been deprecated in favor of the kmap_local_page() due to high
cost, restricted mapping space, the overhead of a global lock for
synchronization, and making the process sleep in the absence of free
slots.

kmap_local_{page, folio}() is faster than kmap() and offers thread-local
and CPU-local mappings, can take pagefaults in a local kmap region and
preserves preemption by saving the mappings of outgoing tasks and
restoring those of the incoming one during a context switch.

The difference between kmap_local_page() and kmap_local_folio() consist
only in the first taking a pointer to a page and the second taking two
arguments, a pointer to a folio and the byte offset within the folio which
identifies the page.

The mappings are kept thread local in the functions 'vboxsf_read_folio',
'vboxsf_writepage', 'vboxsf_write_end' in file.c

Suggested-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Sumitra Sharma <sumitraartsy@gmail.com>
---
fs/vboxsf/file.c | 12 ++++++------
1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/fs/vboxsf/file.c b/fs/vboxsf/file.c
index 572aa1c43b37..5190619bc3c5 100644
--- a/fs/vboxsf/file.c
+++ b/fs/vboxsf/file.c
@@ -234,7 +234,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
u8 *buf;
int err;

- buf = kmap(page);
+ buf = kmap_local_folio(folio, off);

err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
if (err == 0) {
@@ -245,7 +245,7 @@ static int vboxsf_read_folio(struct file *file, struct folio *folio)
SetPageError(page);
}

- kunmap(page);
+ kunmap_local(buf);
unlock_page(page);
return err;
}
@@ -286,10 +286,10 @@ static int vboxsf_writepage(struct page *page, struct writeback_control *wbc)
if (!sf_handle)
return -EBADF;

- buf = kmap(page);
+ buf = kmap_local_page(page);
err = vboxsf_write(sf_handle->root, sf_handle->handle,
off, &nwrite, buf);
- kunmap(page);
+ kunmap_local(buf);

kref_put(&sf_handle->refcount, vboxsf_handle_release);

@@ -320,10 +320,10 @@ static int vboxsf_write_end(struct file *file, struct address_space *mapping,
if (!PageUptodate(page) && copied < len)
zero_user(page, from + copied, len - copied);

- buf = kmap(page);
+ buf = kmap_local_page(page);
err = vboxsf_write(sf_handle->root, sf_handle->handle,
pos, &nwritten, buf + from);
- kunmap(page);
+ kunmap_local(buf);

if (err) {
nwritten = 0;
--
2.25.1