Re: [PATCH 04/11] filemap: add FGP_CREAT_ONLY

[Paolo Bonzini <pbonzini@redhat.com>]

On 4/4/24 20:50, Paolo Bonzini wrote:
> KVM would like to add a ioctl to encrypt and install a page into private
> memory (i.e. into a guest_memfd), in preparation for launching an
> encrypted guest.
> 
> This API should be used only once per page (unless there are failures),
> so we want to rule out the possibility of operating on a page that is
> already in the guest_memfd's filemap.  Overwriting the page is almost
> certainly a sign of a bug, so we might as well forbid it.
> 
> Therefore, introduce a new flag for __filemap_get_folio (to be passed
> together with FGP_CREAT) that allows *adding* a new page to the filemap
> but not returning an existing one.
> 
> An alternative possibility would be to force KVM users to initialize
> the whole filemap in one go, but that is complicated by the fact that
> the filemap includes pages of different kinds, including some that are
> per-vCPU rather than per-VM.  Basically the result would be closer to
> a system call that multiplexes multiple ioctls, than to something
> cleaner like readv/writev.
> 
> Races between callers that pass FGP_CREAT_ONLY are uninteresting to
> the filemap code: one of the racers wins and one fails with EEXIST,
> similar to calling open(2) with O_CREAT|O_EXCL.  It doesn't matter to
> filemap.c if the missing synchronization is in the kernel or in userspace,
> and in fact it could even be intentional.  (In the case of KVM it turns
> out that a mutex is taken around these calls for unrelated reasons,
> so there can be no races.)
> 
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Yosry Ahmed <yosryahmed@google.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Matthew, are your objections still valid or could I have your ack?

Thanks,

Paolo

> ---
>   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 f879c1d54da7..a8c0685e8c08 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -587,6 +587,7 @@ 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 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.
> @@ -607,6 +608,7 @@ 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 7437b2bd75c1..e7440e189ebd 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -1863,6 +1863,10 @@ 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) {