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

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Date:   Wed, 28 Jun 2023 19:23:54 -0700
From:   Ira Weiny <ira.weiny@...el.com>
To:     Matthew Wilcox <willy@...radead.org>,
        Sumitra Sharma <sumitraartsy@...il.com>
CC:     Hans de Goede <hdegoede@...hat.com>,
        <linux-fsdevel@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        Ira Weiny <ira.weiny@...el.com>,
        Fabio <fmdefrancesco@...il.com>, Deepak R Varma <drv@...lo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()

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.

I'm a bit confused.  Is this progressing toward having the folio passed
down to vboxsf_read() or just an expanded vboxsf_read_folio which can
handle larger folios as well as fix the kmap?

Ira

> 
> 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;
>  }
>