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

[Paolo Bonzini <pbonzini@redhat.com>]

On Wed, Feb 28, 2024 at 2:15 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Tue, Feb 27, 2024 at 06:17:34PM -0800, Yosry Ahmed wrote:
> > On Tue, Feb 27, 2024 at 6:15 PM Sean Christopherson <seanjc@google.com> wrote:
> > >
> > > 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.
> >
> > +Matthew Wilcox
>
> If only there were an entry in MAINTAINERS for filemap.c ...

Not CCing you (or mm in general) was intentional because I first
wanted a review of the KVM APIs; of course I wouldn't have committed
it without an Acked-by. But yeah, not writing the changelog yet was
pure laziness.

Since you're here: KVM would like to add a ioctl to encrypt and
install a page into guest_memfd, in preparation for launching an
encrypted guest. For this API we want to rule out the possibility of
overwriting a page that is already in the guest_memfd's filemap,
therefore this API would pass FGP_CREAT_ONLY|FGP_CREAT
into__filemap_get_folio. Do you think this is bogus...

> This looks bogus to me, and if it's not bogus, it's incomplete.

... or if not, what incompleteness can you spot?

Thanks,

Paolo

> But it's hard to judge without a commit message that describes what it's
> supposed to mean.
>
> > >
> > > > 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
> > > >
> > > >
> > >
>