linux-kernel - Re: open(2) says O_DIRECT works on 512 byte boundries?

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Date:	Tue, 3 Feb 2009 11:55:40 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Greg KH <greg@...ah.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	mtk.manpages@...il.com, linux-man@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: open(2) says O_DIRECT works on 512 byte boundries?

On Tue, 3 Feb 2009 03:31:47 +0100
Andrea Arcangeli <aarcange@...hat.com> wrote:

> On Tue, Feb 03, 2009 at 10:29:20AM +0900, KAMEZAWA Hiroyuki wrote:
> > On Mon, 2 Feb 2009 23:08:56 +0100
> > Andrea Arcangeli <aarcange@...hat.com> wrote:
> > 
> > > Hi Greg!
> > > 
> > > > Thanks for the pointers, I'll go read the thread and follow up there.
> > > 
> > > If you also run into this final fix is attached below. Porting to
> > > mainline is a bit hard because of gup-fast... Perhaps we can use mmu
> > > notifiers to fix gup-fast... need to think more about it then I'll
> > > post something.
> > > 
> > > Please help testing the below on pre-gup-fast kernels, thanks!
> > > 
> > I commented in FJ-Redhat Path but not forwared from unknown reason ;)
> > I comment again.
> > 
> > 1. Why TestSetLockPage() is necessary ?
> >    It seems not necesary.
> 
> To avoid the VM to remove or add the page from/to swapcache and change
> page_count/mapcount from under us. This most certainly wasn't the
> reason of the slowdown (the slowdown were the false positives
> generated by pagevec pinning) and removing it was more intrusive than
> I wanted.

My point is.
  - If TestSetLockPage() failes, force_cow=1.
  - If count/mapcount check fails, force_cow=1.

So, lock_page() here seems meaningless. If you consider lock_page() is important,
just use lock_page() seems better.

> 
> > 2. This patch doesn't cover HugeTLB.
> 
> There's no need to change hugetlb with my approach. I'm not touching
> the cow path, I'm addressing the real source of the problem (i.e. when
> fork pretends to mark the child pte readonly and pointing to the
> shared parent page, same as ksm: while the pte wrprotect + tlb flush
> stops the _cpu_ it can't stop any get_user_pages(write=1) user, hence
> we need to pre-cow the child page in fork instead of marking the child
> pte readonly to avoid the parent to lose writes if post-fork the
> parent cows and the child doesn't cow).
> 
No need to make a patch for copy_hugetlb_page_range() ?
IMHO, HugeTLB can be write-protected at fork().

> > 3. Why "follow_page() successfully finds a page" case only ?
> >  not necessary to insert SetPageGUP() in following path ?
> > 
> >  - handle_mm_fault()
> >            => do_anonymos/swap/wp_page()
> >            or some.
> 
> No need to change that either, all we need to know are the pages whose
> count vs mapcount has a discrepancy that could have been caused by
> get_user_pages. So only follow_page has to set it. More precisely
> FOLL_GET|FOLL_WRITE is the only path we care about there.
> 

Assume 3 threads in a process.
==
 Thread1 (DIO-Read)                        Thread2           Thread3
 get_user_page()
 => handle_mm_fault().
    => map a page with no-write-protect.
                                            fork()
                                      (write-protect here)
                                                              Copy-On-Write
 endio.

pre-cow-at-fork will never happen becasue PageGUP is not set. 
After the end of READ, this process will see a broken page.

Thanks,
-Kame

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