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

Sumitra Sharma
2023-06-29T09:30:19Z

On Wed, Jun 28, 2023 at 06:15:04PM +0100, Matthew Wilcox wrote:
> 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;
> }
>

Hi

So, after reading the comments, I understood that the problem presented
by Hans and Matthew is as follows:

1) In the current code, the buffers used by vboxsf_write()/vboxsf_read() are
translated to PAGELIST-s before passing to the hypervisor,
but inefficiently— it first maps a page in vboxsf_read_folio() and then
calls page_to_phys(virt_to_page()) in the function hgcm_call_init_linaddr().

The inefficiency in the current implementation arises due to the unnecessary
mapping of a page in vboxsf_read_folio() because the mapping output, i.e. the
linear address, is used deep down in file 'drivers/virt/vboxguest/vboxguest_utils.c'.
Hence, the mapping must be done in this file; to do so, the folio must be passed
until this point. It can be done by adding a new member, 'struct folio *folio',
in the 'struct vmmdev_hgcm_function_parameter64'.

The unused member 'phys_addr' in this struct can also be removed.

2) Expanding the vboxsf_read_folio so that it can handle larger folios.
Matthew already has suggested the changes, I have to read more on this.

Parallelly I will be setting up the testing environment to test the changes.

Please let me know if I am wrong anywhere.

Thanks & regards
Sumitra