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

Matthew Wilcox
2023-06-28T17:15:15Z

Here's a more comprehensive read_folio patch. It's not at all
efficient, but then if we wanted an efficient vboxsf, we'd implement
vboxsf_readahead() and actually do an async call with deferred setting
of the uptodate flag. I can consult with anyone who wants to do all
this work.

I haven't even compiled this, just trying to show the direction this
should take.

diff --git a/fs/vboxsf/file.c b/fs/vboxsf/file.c
index 2307f8037efc..f1af9a7bd3d8 100644
--- a/fs/vboxsf/file.c
+++ b/fs/vboxsf/file.c
@@ -227,26 +227,31 @@ const struct inode_operations vboxsf_reg_iops = {

static int vboxsf_read_folio(struct file *file, struct folio *folio)
{
- struct page *page = &folio->page;
struct vboxsf_handle *sf_handle = file->private_data;
- loff_t off = page_offset(page);
- u32 nread = PAGE_SIZE;
- u8 *buf;
+ loff_t pos = folio_pos(folio);
+ size_t offset = 0;
int err;

- buf = kmap(page);
+ do {
+ u8 *buf = kmap_local_folio(folio, offset);
+ u32 nread = PAGE_SIZE;

- err = vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread, buf);
- if (err == 0) {
- memset(&buf[nread], 0, PAGE_SIZE - nread);
- flush_dcache_page(page);
- SetPageUptodate(page);
- } else {
- SetPageError(page);
- }
+ err = vboxsf_read(sf_handle->root, sf_handle->handle, pos,
+ &nread, buf);
+ if (nread < PAGE_SIZE)
+ memset(&buf[nread], 0, PAGE_SIZE - nread);
+ kunmap_local(buf);
+ if (err)
+ break;
+ offset += PAGE_SIZE;
+ pos += PAGE_SIZE;
+ } while (offset < folio_size(folio);

- kunmap(page);
- unlock_page(page);
+ if (!err) {
+ flush_dcache_folio(folio);
+ folio_mark_uptodate(folio);
+ }
+ folio_unlock(folio);
return err;
}