Re: [PATCH] mm: remove unnecessary __get_user_pages_unlocked() calls

Michal Hocko
2016-10-26T09:12:18Z

On Wed 26-10-16 00:36:09, Lorenzo Stoakes wrote:
> In hva_to_pfn_slow() we are able to replace __get_user_pages_unlocked() with
> get_user_pages_unlocked() since we can now pass gup_flags.
>
> In async_pf_execute() we need to pass different tsk, mm arguments so
> get_user_pages_remote() is the sane replacement here (having added manual
> acquisition and release of mmap_sem.)

please also add a note about the FOLL_TOUCH reintroduced after it has
been dropped by 1e9877902dc7e silently

> Since we pass a NULL pages parameter the subsequent call to
> __get_user_pages_locked() will have previously bailed any attempt at
> VM_FAULT_RETRY, so we do not change this behaviour by using
> get_user_pages_remote() which does not invoke VM_FAULT_RETRY logic at all.
>
> Signed-off-by: Lorenzo Stoakes <lstoakes@gmail.com>

Acked-by: Michal Hocko <mhocko@suse.com>

> ---
> virt/kvm/async_pf.c | 7 ++++---
> virt/kvm/kvm_main.c | 5 ++---
> 2 files changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/virt/kvm/async_pf.c b/virt/kvm/async_pf.c
> index 8035cc1..e8c832c 100644
> --- a/virt/kvm/async_pf.c
> +++ b/virt/kvm/async_pf.c
> @@ -82,10 +82,11 @@ static void async_pf_execute(struct work_struct *work)
> /*
> * This work is run asynchromously to the task which owns
> * mm and might be done in another context, so we must
> - * use FOLL_REMOTE.
> + * access remotely.
> */
> - __get_user_pages_unlocked(NULL, mm, addr, 1, NULL,
> - FOLL_WRITE | FOLL_REMOTE);
> + down_read(&mm->mmap_sem);
> + get_user_pages_remote(NULL, mm, addr, 1, FOLL_WRITE, NULL, NULL);
> + up_read(&mm->mmap_sem);
>
> kvm_async_page_present_sync(vcpu, apf);
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 2907b7b..c45d951 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1415,13 +1415,12 @@ static int hva_to_pfn_slow(unsigned long addr, bool *async, bool write_fault,
> npages = get_user_page_nowait(addr, write_fault, page);
> up_read(&current->mm->mmap_sem);
> } else {
> - unsigned int flags = FOLL_TOUCH | FOLL_HWPOISON;
> + unsigned int flags = FOLL_HWPOISON;
>
> if (write_fault)
> flags |= FOLL_WRITE;
>
> - npages = __get_user_pages_unlocked(current, current->mm, addr, 1,
> - page, flags);
> + npages = get_user_pages_unlocked(addr, 1, page, flags);
> }
> if (npages != 1)
> return npages;
> --
> 2.10.1

--
Michal Hocko
SUSE Labs