[Virtio-fs] [PATCH V2] virtiofsd: Reset O_DIRECT flag during file open

[Virtio-fs] [PATCH V2] virtiofsd: Reset O_DIRECT flag during file open

[Vivek Goyal]

If an application wants to do direct IO and opens a file with O_DIRECT
in guest, that does not necessarily mean that we need to bypass page
cache on host as well. So reset this flag on host.

If somebody needs to bypass page cache on host as well (and it is safe to
do so), we can add a knob in daemon later to control this behavior.

I check virtio-9p and they do reset O_DIRECT flag.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
---
 contrib/virtiofsd/passthrough_ll.c |   14 ++++++++++++++
 1 file changed, 14 insertions(+)

Index: qemu/contrib/virtiofsd/passthrough_ll.c
===================================================================
--- qemu.orig/contrib/virtiofsd/passthrough_ll.c	2019-08-20 09:33:07.900975145 -0400
+++ qemu/contrib/virtiofsd/passthrough_ll.c	2019-08-20 14:31:39.725781357 -0400
@@ -1752,6 +1752,13 @@ static void lo_create(fuse_req_t req, fu
 	if (err)
 		goto out;
 
+	/*
+	 * O_DIRECT in guest should not necessarily mean bypassing page
+	 * cache on host as well. If somebody needs that behavior, it
+	 * probably should be a configuration knob in daemon.
+	 */
+	fi->flags &= ~O_DIRECT;
+
 	fd = openat(parent_inode->fd, name,
 		    (fi->flags | O_CREAT) & ~O_NOFOLLOW, mode);
 	err = fd == -1 ? errno : 0;
@@ -1967,6 +1974,13 @@ static void lo_open(fuse_req_t req, fuse
 	if (lo->writeback && (fi->flags & O_APPEND))
 		fi->flags &= ~O_APPEND;
 
+	/*
+	 * O_DIRECT in guest should not necessarily mean bypassing page
+	 * cache on host as well. If somebody needs that behavior, it
+	 * probably should be a configuration knob in daemon.
+	 */
+	fi->flags &= ~O_DIRECT;
+
 	sprintf(buf, "%i", lo_fd(req, ino));
 	fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
 	if (fd == -1)

Re: [Virtio-fs] [PATCH V2] virtiofsd: Reset O_DIRECT flag during file open

[Stefan Hajnoczi]

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On Tue, Aug 20, 2019 at 02:37:46PM -0400, Vivek Goyal wrote:
> If an application wants to do direct IO and opens a file with O_DIRECT
> in guest, that does not necessarily mean that we need to bypass page
> cache on host as well. So reset this flag on host.
> 
> If somebody needs to bypass page cache on host as well (and it is safe to
> do so), we can add a knob in daemon later to control this behavior.
> 
> I check virtio-9p and they do reset O_DIRECT flag.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> ---
>  contrib/virtiofsd/passthrough_ll.c |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)

virtio-fs should work even if the host file system doesn't support
O_DIRECT:

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [Virtio-fs] [PATCH V2] virtiofsd: Reset O_DIRECT flag during file open

[Dr. David Alan Gilbert]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> If an application wants to do direct IO and opens a file with O_DIRECT
> in guest, that does not necessarily mean that we need to bypass page
> cache on host as well. So reset this flag on host.
> 
> If somebody needs to bypass page cache on host as well (and it is safe to
> do so), we can add a knob in daemon later to control this behavior.
> 
> I check virtio-9p and they do reset O_DIRECT flag.
> 
> Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

Merged; thanks


> ---
>  contrib/virtiofsd/passthrough_ll.c |   14 ++++++++++++++
>  1 file changed, 14 insertions(+)
> 
> Index: qemu/contrib/virtiofsd/passthrough_ll.c
> ===================================================================
> --- qemu.orig/contrib/virtiofsd/passthrough_ll.c	2019-08-20 09:33:07.900975145 -0400
> +++ qemu/contrib/virtiofsd/passthrough_ll.c	2019-08-20 14:31:39.725781357 -0400
> @@ -1752,6 +1752,13 @@ static void lo_create(fuse_req_t req, fu
>  	if (err)
>  		goto out;
>  
> +	/*
> +	 * O_DIRECT in guest should not necessarily mean bypassing page
> +	 * cache on host as well. If somebody needs that behavior, it
> +	 * probably should be a configuration knob in daemon.
> +	 */
> +	fi->flags &= ~O_DIRECT;
> +
>  	fd = openat(parent_inode->fd, name,
>  		    (fi->flags | O_CREAT) & ~O_NOFOLLOW, mode);
>  	err = fd == -1 ? errno : 0;
> @@ -1967,6 +1974,13 @@ static void lo_open(fuse_req_t req, fuse
>  	if (lo->writeback && (fi->flags & O_APPEND))
>  		fi->flags &= ~O_APPEND;
>  
> +	/*
> +	 * O_DIRECT in guest should not necessarily mean bypassing page
> +	 * cache on host as well. If somebody needs that behavior, it
> +	 * probably should be a configuration knob in daemon.
> +	 */
> +	fi->flags &= ~O_DIRECT;
> +
>  	sprintf(buf, "%i", lo_fd(req, ino));
>  	fd = openat(lo->proc_self_fd, buf, fi->flags & ~O_NOFOLLOW);
>  	if (fd == -1)
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK