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Date: Tue, 12 Apr 2022 20:40:56 +0800
From: Chao Peng <chao.p.peng@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
linux-api@...r.kernel.org, qemu-devel@...gnu.org,
Paolo Bonzini <pbonzini@...hat.com>,
Jonathan Corbet <corbet@....net>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Wanpeng Li <wanpengli@...cent.com>,
Jim Mattson <jmattson@...gle.com>,
Joerg Roedel <joro@...tes.org>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
Hugh Dickins <hughd@...gle.com>,
Jeff Layton <jlayton@...nel.org>,
"J . Bruce Fields" <bfields@...ldses.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Mike Rapoport <rppt@...nel.org>,
Steven Price <steven.price@....com>,
"Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
Vlastimil Babka <vbabka@...e.cz>,
Vishal Annapurve <vannapurve@...gle.com>,
Yu Zhang <yu.c.zhang@...ux.intel.com>,
"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
ak@...ux.intel.com, david@...hat.com
Subject: Re: [PATCH v5 10/13] KVM: Register private memslot to memory backing
store
On Tue, Mar 29, 2022 at 07:01:52PM +0000, Sean Christopherson wrote:
> On Thu, Mar 10, 2022, Chao Peng wrote:
> > Add 'notifier' to memslot to make it a memfile_notifier node and then
> > register it to memory backing store via memfile_register_notifier() when
> > memslot gets created. When memslot is deleted, do the reverse with
> > memfile_unregister_notifier(). Note each KVM memslot can be registered
> > to different memory backing stores (or the same backing store but at
> > different offset) independently.
> >
> > Signed-off-by: Yu Zhang <yu.c.zhang@...ux.intel.com>
> > Signed-off-by: Chao Peng <chao.p.peng@...ux.intel.com>
> > ---
> > include/linux/kvm_host.h | 1 +
> > virt/kvm/kvm_main.c | 75 ++++++++++++++++++++++++++++++++++++----
> > 2 files changed, 70 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 6e1d770d6bf8..9b175aeca63f 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -567,6 +567,7 @@ struct kvm_memory_slot {
> > struct file *private_file;
> > loff_t private_offset;
> > struct memfile_pfn_ops *pfn_ops;
> > + struct memfile_notifier notifier;
> > };
> >
> > static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index d11a2628b548..67349421eae3 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -840,6 +840,37 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >
> > #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
> >
> > +#ifdef CONFIG_MEMFILE_NOTIFIER
> > +static inline int kvm_memfile_register(struct kvm_memory_slot *slot)
>
> This is a good oppurtunity to hide away the memfile details a bit. Maybe
> kvm_private_mem_{,un}register()?
Happy to change.
>
> > +{
> > + return memfile_register_notifier(file_inode(slot->private_file),
> > + &slot->notifier,
> > + &slot->pfn_ops);
> > +}
> > +
> > +static inline void kvm_memfile_unregister(struct kvm_memory_slot *slot)
> > +{
> > + if (slot->private_file) {
> > + memfile_unregister_notifier(file_inode(slot->private_file),
> > + &slot->notifier);
> > + fput(slot->private_file);
>
> This should not do fput(), it makes the helper imbalanced with respect to the
> register path and will likely lead to double fput(). Indeed, if preparing the
> region fails, __kvm_set_memory_region() will double up on fput() due to checking
> its local "file" for null, not slot->private for null.
Right.
>
> > + slot->private_file = NULL;
> > + }
> > +}
> > +
> > +#else /* !CONFIG_MEMFILE_NOTIFIER */
> > +
> > +static inline int kvm_memfile_register(struct kvm_memory_slot *slot)
> > +{
>
> This should WARN_ON_ONCE(). Ditto for unregister.
>
> > + return -EOPNOTSUPP;
> > +}
> > +
> > +static inline void kvm_memfile_unregister(struct kvm_memory_slot *slot)
> > +{
> > +}
> > +
> > +#endif /* CONFIG_MEMFILE_NOTIFIER */
> > +
> > #ifdef CONFIG_HAVE_KVM_PM_NOTIFIER
> > static int kvm_pm_notifier_call(struct notifier_block *bl,
> > unsigned long state,
> > @@ -884,6 +915,9 @@ static void kvm_destroy_dirty_bitmap(struct kvm_memory_slot *memslot)
> > /* This does not remove the slot from struct kvm_memslots data structures */
> > static void kvm_free_memslot(struct kvm *kvm, struct kvm_memory_slot *slot)
> > {
> > + if (slot->flags & KVM_MEM_PRIVATE)
> > + kvm_memfile_unregister(slot);
>
> With fput() move out of unregister, this needs to be:
Agreed.
>
> if (slot->flags & KVM_MEM_PRIVATE) {
> kvm_private_mem_unregister(slot);
> fput(slot->private_file);
> }
> > +
> > kvm_destroy_dirty_bitmap(slot);
> >
> > kvm_arch_free_memslot(kvm, slot);
> > @@ -1738,6 +1772,12 @@ static int kvm_set_memslot(struct kvm *kvm,
> > kvm_invalidate_memslot(kvm, old, invalid_slot);
> > }
> >
> > + if (new->flags & KVM_MEM_PRIVATE && change == KVM_MR_CREATE) {
> > + r = kvm_memfile_register(new);
> > + if (r)
> > + return r;
> > + }
>
> This belongs in kvm_prepare_memory_region(). The shenanigans for DELETE and MOVE
> are special.
Sure.
>
> > +
> > r = kvm_prepare_memory_region(kvm, old, new, change);
> > if (r) {
> > /*
> > @@ -1752,6 +1792,10 @@ static int kvm_set_memslot(struct kvm *kvm,
> > } else {
> > mutex_unlock(&kvm->slots_arch_lock);
> > }
> > +
> > + if (new->flags & KVM_MEM_PRIVATE && change == KVM_MR_CREATE)
> > + kvm_memfile_unregister(new);
> > +
> > return r;
> > }
> >
> > @@ -1817,6 +1861,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > enum kvm_mr_change change;
> > unsigned long npages;
> > gfn_t base_gfn;
> > + struct file *file = NULL;
>
> Nit, naming this private_file would help understand its use. Though I think it's
> easier to not have a local variable. More below.
>
> > int as_id, id;
> > int r;
> >
> > @@ -1890,14 +1935,24 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > return 0;
> > }
> >
> > + if (mem->flags & KVM_MEM_PRIVATE) {
> > + file = fdget(region_ext->private_fd).file;
>
> This can use fget() instead of fdget().
>
> > + if (!file)
> > + return -EINVAL;
> > + }
> > +
> > if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
> > - kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
> > - return -EEXIST;
> > + kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages)) {
> > + r = -EEXIST;
> > + goto out;
> > + }
> >
> > /* Allocate a slot that will persist in the memslot. */
> > new = kzalloc(sizeof(*new), GFP_KERNEL_ACCOUNT);
> > - if (!new)
> > - return -ENOMEM;
> > + if (!new) {
> > + r = -ENOMEM;
> > + goto out;
> > + }
> >
> > new->as_id = as_id;
> > new->id = id;
> > @@ -1905,10 +1960,18 @@ int __kvm_set_memory_region(struct kvm *kvm,
> > new->npages = npages;
> > new->flags = mem->flags;
> > new->userspace_addr = mem->userspace_addr;
> > + new->private_file = file;
> > + new->private_offset = mem->flags & KVM_MEM_PRIVATE ?
> > + region_ext->private_offset : 0;
>
> "new" is zero-allocated, so all the private stuff, including the fget(), can be
> wrapped in a single KVM_MEM_PRIVATE check. Moving fget() eliminates the number
> of gotos needed (the above -EEXIST and -ENOMEM paths don't need to be modified).
>
> > r = kvm_set_memslot(kvm, old, new, change);
> > - if (r)
> > - kfree(new);
> > + if (!r)
> > + return r;
>
> Use goto, e.g.
>
> if (r)
> goto out;
>
> return 0;
>
> Burying the happy path in a taken if-statement is confusing and error prone,
> mostly because it breaks well-established kernel patterns. Note, there's no need
> for a separate out_free since new->private_file will be NULL in either case. I
> don't have a strong preference, I just find it easier to read code that's more
> explicit, but I'm a-ok collapsing them into a single label.
Will follow this, thanks for the detailed suggestion.
Chao
>
> if ((change == KVM_MR_CREATE || change == KVM_MR_MOVE) &&
> kvm_check_memslot_overlap(slots, id, base_gfn, base_gfn + npages))
> return -EEXIST;
>
> /* Allocate a slot that will persist in the memslot. */
> new = kzalloc(sizeof(*new), GFP_KERNEL_ACCOUNT);
> if (!new)
> return -ENOMEM;
>
> new->as_id = as_id;
> new->id = id;
> new->base_gfn = base_gfn;
> new->npages = npages;
> new->flags = mem->flags;
> new->userspace_addr = mem->userspace_addr;
>
> if (mem->flags & KVM_MEM_PRIVATE) {
> new->private_file = fget(mem->private_fd);
> if (!new->private_file) {
> r = -EINVAL;
> goto out_free;
> }
> new->private_offset = mem->private_offset;
> }
>
> r = kvm_set_memslot(kvm, old, new, change);
> if (r)
> goto out;
>
> return 0;
>
> out:
> if (new->private_file)
> fput(new->private_file);
>
> out_free:
> kfree(new);
> return r;
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