linux-kernel - Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user

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Date:	Tue, 24 Jun 2008 02:15:06 +0800
From:	"Jeff Chua" <jeff.chua.linux@...il.com>
To:	"Linus Torvalds" <torvalds@...ux-foundation.org>
Cc:	"Hugh Dickins" <hugh@...itas.com>, "Greg KH" <gregkh@...e.de>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	"Justin Forbes" <jmforbes@...uxtx.org>,
	"Zwane Mwaikambo" <zwane@....linux.org.uk>,
	"Theodore Ts'o" <tytso@....edu>,
	"Randy Dunlap" <rdunlap@...otime.net>,
	"Dave Jones" <davej@...hat.com>,
	"Chuck Wolber" <chuckw@...ntumlinux.com>,
	"Chris Wedgwood" <reviews@...cw.f00f.org>,
	"Michael Krufky" <mkrufky@...uxtv.org>,
	"Chuck Ebbert" <cebbert@...hat.com>,
	"Domenico Andreoli" <cavokz@...il.com>, "Willy Tarreau" <w@....eu>,
	"Rodrigo Rubira Branco" <rbranco@...checkpoint.com>,
	akpm@...ux-foundation.org, alan@...rguk.ukuu.org.uk,
	"Oleg Nesterov" <oleg@...sign.ru>, "Nick Piggin" <npiggin@...e.de>,
	"KAMEZAWA Hiroyuki" <kamezawa.hiroyu@...fujitsu.com>,
	"Ingo Molnar" <mingo@...e.hu>, "Roland McGrath" <roland@...hat.com>
Subject: Re: [patch 2/5] Reinstate ZERO_PAGE optimization in get_user_pages() and fix XIP

On Tue, Jun 24, 2008 at 1:27 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:

> On the other hand, if you add a trace to the "use_zero_page()" function to
> print out the vm_flags and other details, that probably would help.

Let me know if you still want me to test this.

> That said, since the previous patch _did_ work, I bet that one that does
> both VM_LOCKED and VM_SHARED works too. There was a reason I wanted > to do that VM_SHARED test. I think the VM_SHARED test is sane, unlike the
> VM_LOCKED test (that is a fairly dubious hack for mlock).
> So here's the final version. I bet it works.

Yeh, it works great! Thank you.

Jeff.

>  mm/memory.c |   23 +++++++++++++++++++++--
>  1 files changed, 21 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memory.c b/mm/memory.c
> index 9aefaae..423e0e7 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1045,6 +1045,26 @@ no_page_table:
>        return page;
>  }
>
> +/* Can we do the FOLL_ANON optimization? */
> +static inline int use_zero_page(struct vm_area_struct *vma)
> +{
> +       /*
> +        * We don't want to optimize FOLL_ANON for make_pages_present()
> +        * when it tries to page in a VM_LOCKED region. As to VM_SHARED,
> +        * we want to get the page from the page tables to make sure
> +        * that we serialize and update with any other user of that
> +        * mapping.
> +        */
> +       if (vma->vm_flags & (VM_LOCKED | VM_SHARED))
> +               return 0;
> +       /*
> +        * And if we have a fault or a nopfn routine, it's not an
> +        * anonymous region.
> +        */
> +       return !vma->vm_ops ||
> +               (!vma->vm_ops->fault && !vma->vm_ops->nopfn);
> +}
> +
>  int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>                unsigned long start, int len, int write, int force,
>                struct page **pages, struct vm_area_struct **vmas)
> @@ -1119,8 +1139,7 @@ int get_user_pages(struct task_struct *tsk, struct mm_struct *mm,
>                foll_flags = FOLL_TOUCH;
>                if (pages)
>                        foll_flags |= FOLL_GET;
> -               if (!write && !(vma->vm_flags & VM_LOCKED) &&
> -                   (!vma->vm_ops || !vma->vm_ops->fault))
> +               if (!write && use_zero_page(vma))
>                        foll_flags |= FOLL_ANON;
>
>                do {
>
--
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