Re: [PATCH 03/15] shmem: document how to "persist" data when using shmem_*file_setup

["Darrick J. Wong" <djwong@kernel.org>]

On Wed, Jan 03, 2024 at 08:41:14AM +0000, Christoph Hellwig wrote:
> Add a blurb that simply dirtying the folio will persist data for in-kernel
> shmem files.  This is what most of the callers already do.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  mm/shmem.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 328eb3dbea9f1c..235fac6dc53a0b 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2129,6 +2129,11 @@ static int shmem_get_folio_gfp(struct inode *inode, pgoff_t index,
>   *
>   * Return: The found folio, %NULL if SGP_READ or SGP_NOALLOC was passed in @sgp
>   * and no folio was found at @index, or an ERR_PTR() otherwise.
> + *
> + * If the caller modifies data in the returned folio, it must call
> + * folio_mark_dirty() on the locked folio before dropping the reference to
> + * ensure the folio is not reclaimed.  Unlike for normal file systems there is
> + * no need to reserve space for users of shmem_*file_setup().

/me notes that this matches how /I/ think this is supposed to work, but
I think someone more familiar with tmpfs should review this...

--D

>   */
>  int shmem_get_folio(struct inode *inode, pgoff_t index, struct folio **foliop,
>  		enum sgp_type sgp)
> -- 
> 2.39.2
> 
>