Re: [PATCH v4 2/6] KVM: s390: pci: Fix memory accounting for pinned/unpinned pages

[Matthew Rosato <mjrosato@linux.ibm.com>]

On 7/22/26 1:06 PM, Farhan Ali wrote:
> The account_mem() and unaccount_mem() functions call get_uid() which
> increments the reference count of struct user_struct on every invocation.
> But we don't decrement the count by calling free_uid(). It also
> accounted/unaccounted the pages against the current->mm. But its possible
> the unaccount_mem() can be called from a different process context than the
> one that originally pinned the pages.
> 
> Let's fix this by storing the pinning process user_struct and mm_struct
> when accounting for pinned pages, and subsequently free these resources
> when the pages are unpinned.
> 
> Fixes: 3c5a1b6f0a18 ("KVM: s390: pci: provide routines for enabling/disabling interrupt forwarding")
> Signed-off-by: Farhan Ali <alifm@linux.ibm.com>
> ---
>  arch/s390/kvm/pci.c | 38 ++++++++++++++++++++++++++++----------
>  arch/s390/kvm/pci.h |  2 ++
>  2 files changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/s390/kvm/pci.c b/arch/s390/kvm/pci.c
> index d2a11cdf6941..1b3114c7cfbb 100644
> --- a/arch/s390/kvm/pci.c
> +++ b/arch/s390/kvm/pci.c
> @@ -190,33 +190,51 @@ static int kvm_zpci_clear_airq(struct zpci_dev *zdev)
>  	return cc ? -EIO : 0;
>  }
>  
> -static inline void unaccount_mem(unsigned long nr_pages)
> +static inline void unaccount_mem(struct kvm_zdev *kzdev, unsigned long nr_pages)
>  {
> -	struct user_struct *user = get_uid(current_user());
> +	struct user_struct *user = kzdev->user_account;
> +	struct mm_struct *mm_account = kzdev->mm_account;
>  
> -	if (user)
> +	if (user) {
>  		atomic_long_sub(nr_pages, &user->locked_vm);
> -	if (current->mm)
> -		atomic64_sub(nr_pages, &current->mm->pinned_vm);

Previous code handled the case where current->mm could be NULL...

> +		free_uid(user);
> +		kzdev->user_account = NULL;
> +	}
> +
> +	if (mm_account) {

... And you check for kzdev->mm_account being NULL here...

> +		atomic64_sub(nr_pages, &mm_account->pinned_vm);
> +		mmdrop(mm_account);
> +		kzdev->mm_account = NULL;
> +	}
>  }
>  
> -static inline int account_mem(unsigned long nr_pages)
> +static inline int account_mem(struct kvm_zdev *kzdev, unsigned long nr_pages)
>  {
>  	struct user_struct *user = get_uid(current_user());
>  	unsigned long page_limit, cur_pages, new_pages;
> +	int rc = 0;
>  
>  	page_limit = rlimit(RLIMIT_MEMLOCK) >> PAGE_SHIFT;
>  
>  	cur_pages = atomic_long_read(&user->locked_vm);
>  	do {
>  		new_pages = cur_pages + nr_pages;
> -		if (new_pages > page_limit)
> -			return -ENOMEM;
> +		if (new_pages > page_limit) {
> +			rc =  -ENOMEM;
> +			goto out;
> +		}
>  	} while (!atomic_long_try_cmpxchg(&user->locked_vm, &cur_pages, new_pages));
>  
> +	mmgrab(current->mm);

... But here you do not check if current->mm is NULL.  I'm not sure if
it will happen in practice, but we did guard against it before this
patch.  Just add if (!current->mm) around this mmgrab?

>  	atomic64_add(nr_pages, &current->mm->pinned_vm);
> +	kzdev->user_account = user;
> +	kzdev->mm_account = current->mm;

Then if it IS NULL, we will stash a NULL into kzdev->mm_account here and
handle it above as before with the if (mm_account) check.

>  
>  	return 0;
> +
> +out:
> +	free_uid(user);
> +	return rc;
>  }
>  
>  static int kvm_s390_pci_aif_enable(struct zpci_dev *zdev, struct zpci_fib *fib,
> @@ -279,7 +297,7 @@ static int kvm_s390_pci_aif_enable(struct zpci_dev *zdev, struct zpci_fib *fib,
>  	}
>  
>  	/* Account for pinned pages, roll back on failure */
> -	if (account_mem(pcount))
> +	if (account_mem(zdev->kzdev, pcount))
>  		goto unpin2;
>  
>  	/* AISB must be allocated before we can fill in GAITE */
> @@ -400,7 +418,7 @@ static int kvm_s390_pci_aif_disable(struct zpci_dev *zdev, bool force)
>  		pcount++;
>  	}
>  	if (pcount > 0)
> -		unaccount_mem(pcount);
> +		unaccount_mem(kzdev, pcount);
>  out:
>  	mutex_unlock(&aift->aift_lock);
>  
> diff --git a/arch/s390/kvm/pci.h b/arch/s390/kvm/pci.h
> index ff0972dd5e71..544e6aa75e38 100644
> --- a/arch/s390/kvm/pci.h
> +++ b/arch/s390/kvm/pci.h
> @@ -21,6 +21,8 @@ struct kvm_zdev {
>  	struct zpci_dev *zdev;
>  	struct kvm *kvm;
>  	struct zpci_fib fib;
> +	struct user_struct *user_account;
> +	struct mm_struct *mm_account;
>  	struct list_head entry;
>  };
>