Re: [Qemu-devel] [PATCH 1/2] hw/9pfs: Add new virtfs option cache=writethrough to skip host page cache

[Stefan Hajnoczi]

On Mon, Oct 10, 2011 at 10:06 AM, Aneesh Kumar K.V
<address@hidden> wrote:
> diff --git a/hw/9pfs/virtio-9p-handle.c b/hw/9pfs/virtio-9p-handle.c
> index 5c8b5ed..441a37f 100644
> --- a/hw/9pfs/virtio-9p-handle.c
> +++ b/hw/9pfs/virtio-9p-handle.c
> @@ -202,6 +202,15 @@ static ssize_t handle_pwritev(FsContext *ctx, int fd, 
> const struct iovec *iov,
>         return writev(fd, iov, iovcnt);

The sync_file_range(2) call below is dead-code since we'll return
immediately after writev(2) completes.  The writev(2) return value
needs to be saved temporarily and then returned after
sync_file_range(2).

>     }
>  #endif
> +    if (ctx->cache_flags & V9FS_WRITETHROUGH_CACHE) {

-drive cache=writethrough means something different from 9pfs
"writethrough".  This is confusing so I wonder if there is a better
name like immediate write-out.

> +        /*
> +         * Initiate a writeback. This is not a data integrity sync.
> +         * We want to ensure that we don't leave dirty pages in the cache
> +         * after write when cache=writethrough is sepcified.
> +         */
> +        sync_file_range(fd, offset, 0,
> +                        SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE);
> +    }

I'm not sure whether SYNC_FILE_RANGE_WAIT_BEFORE is necessary.  As a
best-effort mechanism just SYNC_FILE_RANGE_WRITE does the job although
a client that rapidly rewrites may be able to leave dirty pages in the
host page cache.  SYNC_FILE_RANGE_WAIT_BEFORE ensures that dirty pages
get written out but it is no longer asynchronous because it blocks.

Stefan