Subject: Re: [PATCH v5 3/6] mm/gup: remove vmas parameter from get_user_pages_remote()

From: Anders Roxell <[email-masked]>
Date: Tue, 16 May 2023 11:49:19 +0200

On 2023-05-14 22:26, Lorenzo Stoakes wrote:
> The only instances of get_user_pages_remote() invocations which used the
> vmas parameter were for a single page which can instead simply look up the
> VMA directly. In particular:-
> 
> - __update_ref_ctr() looked up the VMA but did nothing with it so we simply
>   remove it.
> 
> - __access_remote_vm() was already using vma_lookup() when the original
>   lookup failed so by doing the lookup directly this also de-duplicates the
>   code.
> 
> We are able to perform these VMA operations as we already hold the
> mmap_lock in order to be able to call get_user_pages_remote().
> 
> As part of this work we add get_user_page_vma_remote() which abstracts the
> VMA lookup, error handling and decrementing the page reference count should
> the VMA lookup fail.
> 
> This forms part of a broader set of patches intended to eliminate the vmas
> parameter altogether.
> 
> Reviewed-by: Catalin Marinas <[email-masked]> (for arm64)
> Acked-by: David Hildenbrand <[email-masked]>
> Reviewed-by: Janosch Frank <[email-masked]> (for s390)
> Signed-off-by: Lorenzo Stoakes <[email-masked]>
> ---
>  arch/arm64/kernel/mte.c   | 17 +++++++++--------
>  arch/s390/kvm/interrupt.c |  2 +-
>  fs/exec.c                 |  2 +-
>  include/linux/mm.h        | 34 +++++++++++++++++++++++++++++++---
>  kernel/events/uprobes.c   | 13 +++++--------
>  mm/gup.c                  | 12 ++++--------
>  mm/memory.c               | 14 +++++++-------
>  mm/rmap.c                 |  2 +-
>  security/tomoyo/domain.c  |  2 +-
>  virt/kvm/async_pf.c       |  3 +--
>  10 files changed, 61 insertions(+), 40 deletions(-)
> 

[...]

> diff --git a/mm/memory.c b/mm/memory.c
> index 146bb94764f8..63632a5eafc1 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -5590,7 +5590,6 @@ EXPORT_SYMBOL_GPL(generic_access_phys);
>  int __access_remote_vm(struct mm_struct *mm, unsigned long addr, void *buf,
>  		       int len, unsigned int gup_flags)
>  {
> -	struct vm_area_struct *vma;
>  	void *old_buf = buf;
>  	int write = gup_flags & FOLL_WRITE;
>  
> @@ -5599,13 +5598,15 @@ int __access_remote_vm(struct mm_struct *mm, unsigned long addr, void *buf,
>  
>  	/* ignore errors, just check how much was successfully transferred */
>  	while (len) {
> -		int bytes, ret, offset;
> +		int bytes, offset;
>  		void *maddr;
> -		struct page *page = NULL;
> +		struct vm_area_struct *vma;
> +		struct page *page = get_user_page_vma_remote(mm, addr,
> +							     gup_flags, &vma);
> +
> +		if (IS_ERR_OR_NULL(page)) {
> +			int ret = 0;

I see the warning below when building without CONFIG_HAVE_IOREMAP_PROT set.

make --silent --keep-going --jobs=32 \
O=/home/anders/.cache/tuxmake/builds/1244/build ARCH=arm \
CROSS_COMPILE=arm-linux-gnueabihf- /home/anders/src/kernel/next/mm/memory.c: In function '__access_remote_vm':
/home/anders/src/kernel/next/mm/memory.c:5608:29: warning: unused variable 'ret' [-Wunused-variable]
 5608 |                         int ret = 0;
      |                             ^~~

>  
> -		ret = get_user_pages_remote(mm, addr, 1,
> -				gup_flags, &page, &vma, NULL);
> -		if (ret <= 0) {
>  #ifndef CONFIG_HAVE_IOREMAP_PROT
>  			break;
>  #else
> @@ -5613,7 +5614,6 @@ int __access_remote_vm(struct mm_struct *mm, unsigned long addr, void *buf,
>  			 * Check if this is a VM_IO | VM_PFNMAP VMA, which
>  			 * we can access using slightly different code.
>  			 */
> -			vma = vma_lookup(mm, addr);
>  			if (!vma)
>  				break;
>  			if (vma->vm_ops && vma->vm_ops->access)

Cheers,
Anders

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