Re: [PATCH 17/21] filemap: add FGP_CREAT_ONLY

[Sean Christopherson <seanjc@google.com>]

On Tue, Feb 27, 2024, Paolo Bonzini wrote:

This needs a changelog, and also needs to be Cc'd to someone(s) that can give it
a thumbs up.

> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  include/linux/pagemap.h | 2 ++
>  mm/filemap.c            | 4 ++++
>  2 files changed, 6 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index 2df35e65557d..e8ac0b32f84d 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -586,6 +586,7 @@ pgoff_t page_cache_prev_miss(struct address_space *mapping,
>   * * %FGP_CREAT - If no folio is present then a new folio is allocated,
>   *   added to the page cache and the VM's LRU list.  The folio is
>   *   returned locked.
> + * * %FGP_CREAT_ONLY - Fail if a folio is not present
>   * * %FGP_FOR_MMAP - The caller wants to do its own locking dance if the
>   *   folio is already in cache.  If the folio was allocated, unlock it
>   *   before returning so the caller can do the same dance.
> @@ -606,6 +607,7 @@ typedef unsigned int __bitwise fgf_t;
>  #define FGP_NOWAIT		((__force fgf_t)0x00000020)
>  #define FGP_FOR_MMAP		((__force fgf_t)0x00000040)
>  #define FGP_STABLE		((__force fgf_t)0x00000080)
> +#define FGP_CREAT_ONLY		((__force fgf_t)0x00000100)
>  #define FGF_GET_ORDER(fgf)	(((__force unsigned)fgf) >> 26)	/* top 6 bits */
>  
>  #define FGP_WRITEBEGIN		(FGP_LOCK | FGP_WRITE | FGP_CREAT | FGP_STABLE)
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 750e779c23db..d5107bd0cd09 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1854,6 +1854,10 @@ struct folio *__filemap_get_folio(struct address_space *mapping, pgoff_t index,
>  		folio = NULL;
>  	if (!folio)
>  		goto no_page;
> +	if (fgp_flags & FGP_CREAT_ONLY) {
> +		folio_put(folio);
> +		return ERR_PTR(-EEXIST);
> +	}
>  
>  	if (fgp_flags & FGP_LOCK) {
>  		if (fgp_flags & FGP_NOWAIT) {
> -- 
> 2.39.0
> 
>