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    SubjectRe: [PATCH 04/11] filemap: add FGP_CREAT_ONLY
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    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) {


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