linux-kernel - Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local

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Message-ID: <6924669.18pcnM708K@suse>
Date:   Thu, 29 Jun 2023 00:23:54 +0200
From:   "Fabio M. De Francesco" <fmdefrancesco@...il.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Sumitra Sharma <sumitraartsy@...il.com>,
        Hans de Goede <hdegoede@...hat.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        Ira Weiny <ira.weiny@...el.com>, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

On mercoledì 28 giugno 2023 19:15:04 CEST 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.

Interesting...
 
> 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 {

Please let me understand why you are calling vboxsf_read() in a loop, a 
PAGE_SIZE at a time.

I have had only few minutes (whereas I'd need more time) to look at this code.

If I understand the current code it reads a single page at offset zero of a 
folio and then memset() with zeros from &buf[nread] up to the end of the page. 
Then it seems that this function currently assume that the folio doesn't need 
to be read until "offset < folio_size(folio)" becomes false.

Does it imply that the folio is always one page sized? Doesn't it? I'm surely 
missing some basics...  

> +		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);

Shouldn't we call folio_lock() to lock the folio to be able to unlock with 
folio_unlock()?
 
If so, I can't find any neither a folio_lock() or a page_lock() in this 
filesystem. 

Again sorry for not understanding, can you please explain it?

>  	return err;
>  }

Thanks,

Fabio