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From:   "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
To:     Matthew Wilcox <willy@infradead.org>
Cc:     Sumitra Sharma <sumitraartsy@gmail.com>,
        Hans de Goede <hdegoede@redhat.com>,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        Ira Weiny <ira.weiny@intel.com>, Deepak R Varma <drv@mailo.com>
Subject: Re: [PATCH] fs/vboxsf: Replace kmap() with kmap_local_{page, folio}()
Date:   Thu, 29 Jun 2023 00:23:54 +0200
Message-ID: <6924669.18pcnM708K@suse>
In-Reply-To: <ZJxqmEVKoxxftfXM@casper.infradead.org>
References: <20230627135115.GA452832@sumitra.com> <ZJxqmEVKoxxftfXM@casper.infradead.org>
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On mercoled=EC 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...
=20
> I haven't even compiled this, just trying to show the direction this
> should take.
>=20
> 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 =3D {
>=20
>  static int vboxsf_read_folio(struct file *file, struct folio *folio)
>  {
> -	struct page *page =3D &folio->page;
>  	struct vboxsf_handle *sf_handle =3D file->private_data;
> -	loff_t off =3D page_offset(page);
> -	u32 nread =3D PAGE_SIZE;
> -	u8 *buf;
> +	loff_t pos =3D folio_pos(folio);
> +	size_t offset =3D 0;
>  	int err;
>=20
> -	buf =3D kmap(page);
> +	do {

Please let me understand why you are calling vboxsf_read() in a loop, a=20
PAGE_SIZE at a time.

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

If I understand the current code it reads a single page at offset zero of a=
=20
folio and then memset() with zeros from &buf[nread] up to the end of the pa=
ge.=20
Then it seems that this function currently assume that the folio doesn't ne=
ed=20
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 sure=
ly=20
missing some basics... =20

> +		u8 *buf =3D kmap_local_folio(folio, offset);
> +		u32 nread =3D PAGE_SIZE;
>=20
> -	err =3D vboxsf_read(sf_handle->root, sf_handle->handle, off, &nread,=20
buf);
> -	if (err =3D=3D 0) {
> -		memset(&buf[nread], 0, PAGE_SIZE - nread);
> -		flush_dcache_page(page);
> -		SetPageUptodate(page);
> -	} else {
> -		SetPageError(page);
> -	}
> +		err =3D 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 +=3D PAGE_SIZE;
> +		pos +=3D PAGE_SIZE;
> +	} while (offset < folio_size(folio);
>=20
> -	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=
=20
folio_unlock()?
=20
If so, I can't find any neither a folio_lock() or a page_lock() in this=20
filesystem.=20

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

>  	return err;
>  }

Thanks,

=46abio