Re: [PATCH] fuse: add a new flag to allow shared mmap in FOPEN_DIRECT_IO mode

[Hao Xu <hao.xu@linux.dev>]

ping...

On 5/5/23 16:16, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
> 
> FOPEN_DIRECT_IO is usually set by fuse daemon to indicate need of strong
> coherency, e.g. network filesystems. Thus shared mmap is disabled since
> it leverages page cache and may write to it, which may cause
> inconsistence. But FOPEN_DIRECT_IO can be used not for coherency but to
> reduce memory footprint as well, e.g. reduce guest memory usage with
> virtiofs. Therefore, add a new flag FOPEN_DIRECT_IO_SHARED_MMAP to allow
> shared mmap for these cases.
> 
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
>   fs/fuse/file.c            | 11 ++++++++---
>   include/uapi/linux/fuse.h |  2 ++
>   2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/fuse/file.c b/fs/fuse/file.c
> index 89d97f6188e0..655896bdb0d5 100644
> --- a/fs/fuse/file.c
> +++ b/fs/fuse/file.c
> @@ -161,7 +161,8 @@ struct fuse_file *fuse_file_open(struct fuse_mount *fm, u64 nodeid,
>   	}
>   
>   	if (isdir)
> -		ff->open_flags &= ~FOPEN_DIRECT_IO;
> +		ff->open_flags &=
> +			~(FOPEN_DIRECT_IO | FOPEN_DIRECT_IO_SHARED_MMAP);
>   
>   	ff->nodeid = nodeid;
>   
> @@ -2509,8 +2510,12 @@ static int fuse_file_mmap(struct file *file, struct vm_area_struct *vma)
>   		return fuse_dax_mmap(file, vma);
>   
>   	if (ff->open_flags & FOPEN_DIRECT_IO) {
> -		/* Can't provide the coherency needed for MAP_SHARED */
> -		if (vma->vm_flags & VM_MAYSHARE)
> +		/* Can't provide the coherency needed for MAP_SHARED.
> +		 * So disable it if FOPEN_DIRECT_IO_SHARED_MMAP is not
> +		 * set, which means we do need strong coherency.
> +		 */
> +		if (!(ff->open_flags & FOPEN_DIRECT_IO_SHARED_MMAP) &&
> +		    vma->vm_flags & VM_MAYSHARE)
>   			return -ENODEV;
>   
>   		invalidate_inode_pages2(file->f_mapping);
> diff --git a/include/uapi/linux/fuse.h b/include/uapi/linux/fuse.h
> index 1b9d0dfae72d..003dcf42e8c2 100644
> --- a/include/uapi/linux/fuse.h
> +++ b/include/uapi/linux/fuse.h
> @@ -314,6 +314,7 @@ struct fuse_file_lock {
>    * FOPEN_STREAM: the file is stream-like (no file position at all)
>    * FOPEN_NOFLUSH: don't flush data cache on close (unless FUSE_WRITEBACK_CACHE)
>    * FOPEN_PARALLEL_DIRECT_WRITES: Allow concurrent direct writes on the same inode
> + * FOPEN_DIRECT_IO_SHARED_MMAP: allow shared mmap when FOPEN_DIRECT_IO is set
>    */
>   #define FOPEN_DIRECT_IO		(1 << 0)
>   #define FOPEN_KEEP_CACHE	(1 << 1)
> @@ -322,6 +323,7 @@ struct fuse_file_lock {
>   #define FOPEN_STREAM		(1 << 4)
>   #define FOPEN_NOFLUSH		(1 << 5)
>   #define FOPEN_PARALLEL_DIRECT_WRITES	(1 << 6)
> +#define FOPEN_DIRECT_IO_SHARED_MMAP	(1 << 7)
>   
>   /**
>    * INIT request/reply flags