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

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Date:	Fri, 18 Jan 2008 19:57:17 +0100
From:	Miklos Szeredi <miklos@...redi.hu>
To:	torvalds@...ux-foundation.org
CC:	miklos@...redi.hu, peterz@...radead.org, salikhmetov@...il.com,
	linux-mm@...ck.org, jakob@...hought.net,
	linux-kernel@...r.kernel.org, valdis.kletnieks@...edu,
	riel@...hat.com, 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

> > That would need a new page flag (PG_mmap_dirty?).  Do we have one
> > available?
> 
> Yeah, that would be bad. We probably have flags free, but those page flags 
> are always a pain. Scratch that.
> 
> How about just setting a per-vma dirty flag, and then instead of updating 
> the mtime when taking the dirty-page fault, we just set that flag?
> 
> Then, on unmap and msync, we just do
> 
> 	if (vma->dirty-flag) {
> 		vma->dirty_flag = 0;
> 		update_file_times(vma->vm_file);
> 	}
> 
> and be done with it? 

But then background writeout, sync(2), etc, wouldn't update the times.
Dunno.  I don't think actual _physical_ writeout matters much, so it's
not worse to be 30s early with the timestamp, than to be 30s or more
late.

Miklos
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