Re: [PATCH v5 03/13] mm/shmem: Support memfile_notifier

[Dave Chinner <david@fromorbit.com>]

On Thu, Mar 10, 2022 at 10:09:01PM +0800, Chao Peng wrote:
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> 
> It maintains a memfile_notifier list in shmem_inode_info structure and
> implements memfile_pfn_ops callbacks defined by memfile_notifier. It
> then exposes them to memfile_notifier via
> shmem_get_memfile_notifier_info.
> 
> We use SGP_NOALLOC in shmem_get_lock_pfn since the pages should be
> allocated by userspace for private memory. If there is no pages
> allocated at the offset then error should be returned so KVM knows that
> the memory is not private memory.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> ---
>  include/linux/shmem_fs.h |  4 +++
>  mm/shmem.c               | 76 ++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 80 insertions(+)
> 
> diff --git a/include/linux/shmem_fs.h b/include/linux/shmem_fs.h
> index 2dde843f28ef..7bb16f2d2825 100644
> --- a/include/linux/shmem_fs.h
> +++ b/include/linux/shmem_fs.h
> @@ -9,6 +9,7 @@
>  #include <linux/percpu_counter.h>
>  #include <linux/xattr.h>
>  #include <linux/fs_parser.h>
> +#include <linux/memfile_notifier.h>
>  
>  /* inode in-kernel data */
>  
> @@ -28,6 +29,9 @@ struct shmem_inode_info {
>  	struct simple_xattrs	xattrs;		/* list of xattrs */
>  	atomic_t		stop_eviction;	/* hold when working on inode */
>  	unsigned int		xflags;		/* shmem extended flags */
> +#ifdef CONFIG_MEMFILE_NOTIFIER
> +	struct memfile_notifier_list memfile_notifiers;
> +#endif
>  	struct inode		vfs_inode;
>  };
>  
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 9b31a7056009..7b43e274c9a2 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -903,6 +903,28 @@ static struct folio *shmem_get_partial_folio(struct inode *inode, pgoff_t index)
>  	return page ? page_folio(page) : NULL;
>  }
>  
> +static void notify_fallocate(struct inode *inode, pgoff_t start, pgoff_t end)
> +{
> +#ifdef CONFIG_MEMFILE_NOTIFIER
> +	struct shmem_inode_info *info = SHMEM_I(inode);
> +
> +	memfile_notifier_fallocate(&info->memfile_notifiers, start, end);
> +#endif
> +}

*notify_populate(), not fallocate.  This is a notification that a
range has been populated, not that the fallocate() syscall was run
to populate the backing store of a file.

i.e.  fallocate is the name of a userspace filesystem API that can
be used to manipulate the backing store of a file in various ways.
It can both populate and punch away the backing store of a file, and
some operations that fallocate() can run will do both (e.g.
FALLOC_FL_ZERO_RANGE) and so could generate both
notify_invalidate() and a notify_populate() events.

Hence "fallocate" as an internal mm namespace or operation does not
belong anywhere in core MM infrastructure - it should never get used
anywhere other than the VFS/filesystem layers that implement the
fallocate() syscall or use it directly.

Cheers,

Dave.

-- 
Dave Chinner
david@fromorbit.com