linux-kernel - Re: [PATCH] mm: remove unnecessary __get_user_pages

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Date:   Wed, 26 Oct 2016 11:12:09 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Lorenzo Stoakes <lstoakes@...il.com>
Cc:     linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Hugh Dickins <hughd@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Rik van Riel <riel@...hat.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: remove unnecessary __get_user_pages_unlocked() calls

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@...il.com>

Acked-by: Michal Hocko <mhocko@...e.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