From mboxrd@z Thu Jan  1 00:00:00 1970
From: Martin Jambor <jamborm@gmail.com>
Subject: Re: Access content of file via inodes
Date: Fri, 27 May 2005 21:13:02 +0200
Message-ID: <8e70aacf0505271213a6834ee@mail.gmail.com>
References: <f8ad67b30504041823568cbbae@mail.gmail.com>
	 <4252E09B.9020606@suse.com>
	 <f8ad67b305040723012246f254@mail.gmail.com>
	 <1112948279.28245.4.camel@imp.csi.cam.ac.uk>
Reply-To: Martin Jambor <jamborm@gmail.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7BIT
Cc: "Kathy KN (HK)" <kathy.kn@gmail.com>,
	Jeff Mahoney <jeffm@suse.com>, linux-fsdevel@vger.kernel.org
Return-path: <linux-fsdevel-owner@vger.kernel.org>
Received: from zproxy.gmail.com ([64.233.162.193]:1544 "EHLO zproxy.gmail.com")
	by vger.kernel.org with ESMTP id S262555AbVE0TND convert rfc822-to-8bit
	(ORCPT <rfc822;linux-fsdevel@vger.kernel.org>);
	Fri, 27 May 2005 15:13:03 -0400
Received: by zproxy.gmail.com with SMTP id 18so1498490nzp
        for <linux-fsdevel@vger.kernel.org>; Fri, 27 May 2005 12:13:02 -0700 (PDT)
To: Anton Altaparmakov <aia21@cam.ac.uk>
In-Reply-To: <1112948279.28245.4.camel@imp.csi.cam.ac.uk>
Content-Disposition: inline
Sender: linux-fsdevel-owner@vger.kernel.org
List-Id: linux-fsdevel.vger.kernel.org

Hi Anton,

On 4/8/05, Anton Altaparmakov <aia21@cam.ac.uk> wrote:
> On Fri, 2005-04-08 at 14:01 +0800, Kathy KN (HK) wrote:
> > Hi Jeff,
> >
> > Is it possible to modify the cached page, and invalidate it back
> > to update the page cache of the new page? I did a recursive grep
> > and could only find functions that let you read or grab pages in the
> > cache.
> 
> Once you have the page (via read_cache_page() or whatever) you can
> simply write to it, then do a flush_dcache_page(page), then
> set_page_dirty(page) and finally do the page_cache_release().  Oh, and
> don't forget to unmap the page.  Usually done straight after the
> flush_dcache_page().  And example from ntfs where we get a page, memset
> it to a value (val), and then mark it dirty for later write out:
> 
>         page = read_cache_page(mapping, idx,
>                         (filler_t*)mapping->a_ops->readpage, NULL);
>         if (IS_ERR(page)) {
>                 ntfs_error(vol->sb, "Failed to read first partial "
>                                 "page (sync error, index 0x%lx).", idx);
>                 return PTR_ERR(page);
>         }
>         wait_on_page_locked(page);
>         if (unlikely(!PageUptodate(page))) {
>                 ntfs_error(vol->sb, "Failed to read first partial page "
>                                 "(async error, index 0x%lx).", idx);
>                 page_cache_release(page);
>                 return PTR_ERR(page);
>         }
>         size = PAGE_CACHE_SIZE;
>         if (idx == end)
>                 size = end_ofs;
>         kaddr = kmap_atomic(page, KM_USER0);
>         memset(kaddr + start_ofs, val, size - start_ofs);
>         flush_dcache_page(page);
>         kunmap_atomic(kaddr, KM_USER0);
>         set_page_dirty(page);
>         page_cache_release(page);
> 
> Of course you need to serialise access in some way so multiple writers
> do not step on each other's toes, etc...

What do I have to do to when I need to append something to a file?
BTW, if that matters, I'll be implementing the adress_space_operations
of that file as well...

Thanks very much in advance,

Martin