Re: [PATCH 02/10] mm: remove write/force parameters from __get_user_pages_unlocked()

From: Paolo Bonzini
Date: Thu Oct 13 2016 - 02:55:00 EST



On 13/10/2016 02:20, Lorenzo Stoakes wrote:
> This patch removes the write and force parameters from
> __get_user_pages_unlocked() to make the use of FOLL_FORCE explicit in callers as
> use of this flag can result in surprising behaviour (and hence bugs) within the
> mm subsystem.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> ---
> include/linux/mm.h | 3 +--
> mm/gup.c | 17 +++++++++--------
> mm/nommu.c | 12 +++++++++---
> mm/process_vm_access.c | 7 +++++--
> virt/kvm/async_pf.c | 3 ++-
> virt/kvm/kvm_main.c | 11 ++++++++---
> 6 files changed, 34 insertions(+), 19 deletions(-)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index e9caec6..2db98b6 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1285,8 +1285,7 @@ long get_user_pages_locked(unsigned long start, unsigned long nr_pages,
> int write, int force, struct page **pages, int *locked);
> long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
> unsigned long start, unsigned long nr_pages,
> - int write, int force, struct page **pages,
> - unsigned int gup_flags);
> + struct page **pages, unsigned int gup_flags);
> long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> int write, int force, struct page **pages);
> int get_user_pages_fast(unsigned long start, int nr_pages, int write,
> diff --git a/mm/gup.c b/mm/gup.c
> index ba83942..3d620dd 100644
> --- a/mm/gup.c
> +++ b/mm/gup.c
> @@ -865,17 +865,11 @@ EXPORT_SYMBOL(get_user_pages_locked);
> */
> __always_inline long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
> unsigned long start, unsigned long nr_pages,
> - int write, int force, struct page **pages,
> - unsigned int gup_flags)
> + struct page **pages, unsigned int gup_flags)
> {
> long ret;
> int locked = 1;
>
> - if (write)
> - gup_flags |= FOLL_WRITE;
> - if (force)
> - gup_flags |= FOLL_FORCE;
> -
> down_read(&mm->mmap_sem);
> ret = __get_user_pages_locked(tsk, mm, start, nr_pages, pages, NULL,
> &locked, false, gup_flags);
> @@ -905,8 +899,15 @@ EXPORT_SYMBOL(__get_user_pages_unlocked);
> long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> int write, int force, struct page **pages)
> {
> + unsigned int flags = FOLL_TOUCH;
> +
> + if (write)
> + flags |= FOLL_WRITE;
> + if (force)
> + flags |= FOLL_FORCE;
> +
> return __get_user_pages_unlocked(current, current->mm, start, nr_pages,
> - write, force, pages, FOLL_TOUCH);
> + pages, flags);
> }
> EXPORT_SYMBOL(get_user_pages_unlocked);
>
> diff --git a/mm/nommu.c b/mm/nommu.c
> index 95daf81..925dcc1 100644
> --- a/mm/nommu.c
> +++ b/mm/nommu.c
> @@ -185,8 +185,7 @@ EXPORT_SYMBOL(get_user_pages_locked);
>
> long __get_user_pages_unlocked(struct task_struct *tsk, struct mm_struct *mm,
> unsigned long start, unsigned long nr_pages,
> - int write, int force, struct page **pages,
> - unsigned int gup_flags)
> + struct page **pages, unsigned int gup_flags)
> {
> long ret;
> down_read(&mm->mmap_sem);
> @@ -200,8 +199,15 @@ EXPORT_SYMBOL(__get_user_pages_unlocked);
> long get_user_pages_unlocked(unsigned long start, unsigned long nr_pages,
> int write, int force, struct page **pages)
> {
> + unsigned int flags = 0;
> +
> + if (write)
> + flags |= FOLL_WRITE;
> + if (force)
> + flags |= FOLL_FORCE;
> +
> return __get_user_pages_unlocked(current, current->mm, start, nr_pages,
> - write, force, pages, 0);
> + pages, flags);
> }
> EXPORT_SYMBOL(get_user_pages_unlocked);
>
> diff --git a/mm/process_vm_access.c b/mm/process_vm_access.c
> index 07514d4..be8dc8d 100644
> --- a/mm/process_vm_access.c
> +++ b/mm/process_vm_access.c
> @@ -88,12 +88,16 @@ static int process_vm_rw_single_vec(unsigned long addr,
> ssize_t rc = 0;
> unsigned long max_pages_per_loop = PVM_MAX_KMALLOC_PAGES
> / sizeof(struct pages *);
> + unsigned int flags = FOLL_REMOTE;
>
> /* Work out address and page range required */
> if (len == 0)
> return 0;
> nr_pages = (addr + len - 1) / PAGE_SIZE - addr / PAGE_SIZE + 1;
>
> + if (vm_write)
> + flags |= FOLL_WRITE;
> +
> while (!rc && nr_pages && iov_iter_count(iter)) {
> int pages = min(nr_pages, max_pages_per_loop);
> size_t bytes;
> @@ -104,8 +108,7 @@ static int process_vm_rw_single_vec(unsigned long addr,
> * current/current->mm
> */
> pages = __get_user_pages_unlocked(task, mm, pa, pages,
> - vm_write, 0, process_pages,
> - FOLL_REMOTE);
> + process_pages, flags);
> if (pages <= 0)
> return -EFAULT;
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index db96688..8035cc1 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -84,7 +84,8 @@ static void async_pf_execute(struct work_struct *work)
> * mm and might be done in another context, so we must
> * use FOLL_REMOTE.
> */
> - __get_user_pages_unlocked(NULL, mm, addr, 1, 1, 0, NULL, FOLL_REMOTE);
> + __get_user_pages_unlocked(NULL, mm, addr, 1, NULL,
> + FOLL_WRITE | FOLL_REMOTE);
>
> kvm_async_page_present_sync(vcpu, apf);
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 81dfc73..28510e7 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1416,10 +1416,15 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
> down_read(&current->mm->mmap_sem);
> npages = get_user_page_nowait(addr, write_fault, page);
> up_read(&current->mm->mmap_sem);
> - } else
> + } else {
> + unsigned int flags = FOLL_TOUCH | FOLL_HWPOISON;
> +
> + if (write_fault)
> + flags |= FOLL_WRITE;
> +
> npages = __get_user_pages_unlocked(current, current->mm, addr, 1,
> - write_fault, 0, page,
> - FOLL_TOUCH|FOLL_HWPOISON);
> + page, flags);
> + }
> if (npages != 1)
> return npages;
>
>

Acked-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>