linux-kernel - Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped files

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Message-Id: <1200703803.25782.45.camel@cinder.waste.org>
Date:	Fri, 18 Jan 2008 18:50:03 -0600
From:	Matt Mackall <mpm@...enic.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Oeser <ioe-lkml@...eria.de>,
	Miklos Szeredi <miklos@...redi.hu>, peterz@...radead.org,
	salikhmetov@...il.com, linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu, ksm@...dk,
	staubach@...hat.com, jesper.juhl@...il.com,
	akpm@...ux-foundation.org, protasnb@...il.com,
	r.e.wolff@...wizard.nl, hidave.darkstar@...il.com,
	hch@...radead.org
Subject: Re: [PATCH -v6 2/2] Updating ctime and mtime for memory-mapped
	files


On Fri, 2008-01-18 at 17:54 -0500, Rik van Riel wrote:
> On Fri, 18 Jan 2008 14:47:33 -0800 (PST)
> Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
> >  - keep it simple. Let's face it, Linux has never ever given those 
> >    guarantees before, and it's not is if anybody has really cared. Even 
> >    now, the issue seems to be more about paper standards conformance than 
> >    anything else.
> 
> There is one issue which is way more than just standards conformance.
> 
> When a program changes file data through mmap(), at some point the
> mtime needs to be update so that backup programs know to back up the
> new version of the file.
> 
> Backup programs not seeing an updated mtime is a really big deal.

And that's fixed with the 4-line approach.

Reminds me, I've got a patch here for addressing that problem with loop mounts:

Writes to loop should update the mtime of the underlying file.

Signed-off-by: Matt Mackall <mpm@...enic.com>

Index: l/drivers/block/loop.c
===================================================================
--- l.orig/drivers/block/loop.c	2007-11-05 17:50:07.000000000 -0600
+++ l/drivers/block/loop.c	2007-11-05 19:03:51.000000000 -0600
@@ -221,6 +221,7 @@ static int do_lo_send_aops(struct loop_d
 	offset = pos & ((pgoff_t)PAGE_CACHE_SIZE - 1);
 	bv_offs = bvec->bv_offset;
 	len = bvec->bv_len;
+	file_update_time(file);
 	while (len > 0) {
 		sector_t IV;
 		unsigned size;
@@ -299,6 +300,7 @@ static int __do_lo_send_write(struct fil
 
 	set_fs(get_ds());
 	bw = file->f_op->write(file, buf, len, &pos);
+	file_update_time(file);
 	set_fs(old_fs);
 	if (likely(bw == len))
 		return 0;


-- 
Mathematics is the supreme nostalgia of our time.

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