linux-kernel - Re: + mm-remove-struct-mm_struct-exe_file-et-al.patch added to -mm tree

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Message-ID: <20090401003223.GH29821@us.ibm.com>
Date:	Tue, 31 Mar 2009 17:32:23 -0700
From:	Matt Helsley <matthltc@...ibm.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	adobriyan@...il.com, dhowells@...hat.com, ebiederm@...ssion.com,
	hch@....de, matthltc@...ibm.com
Subject: Re: + mm-remove-struct-mm_struct-exe_file-et-al.patch added to -mm
	tree

On Tue, Mar 31, 2009 at 04:40:51PM +0200, Oleg Nesterov wrote:
> s/mm-commits/lkml/
> 
> On 03/30, Andrew Morton wrote:
> >
> > From: Alexey Dobriyan <adobriyan@...il.com>
> >
> > Commit 925d1c401fa6cfd0df5d2e37da8981494ccdec07 ("procfs task exe
> > symlink").  introduced struct mm_struct::exe_file and struct
> > mm_struct::num_exe_file_vmas.
> >
> > The rationale is weak: unifying MMU and no-MMU version of /proc/*/exe
> > code.  For this a) struct mm_struct becomes bigger, b) mmap/munmap/exit
> > become slower, c) patch adds more code than removes in fact.
> >
> > ->exe_file maybe well defined, but doesn't make sense always.  After
> > original executable is unmapped, /proc/*/exe will still report it and,
> > more importantly, pin corresponding struct file.
> 
> I never liked the change which introduced mm->exe_file, so I vote for
> this patch.
> 
> But, as a advocatus diaboli... There was anotrher reason for ->exe_file,
> iirc.
> 
> bprm->file->f_op->mmap() can change vma->vm_file, this means proc_exe_link()
> can report the "wrong" path. The original file is not pinned in this case.
> 
> Matt?

That's _my_ reason for it. However no mainline code does that and hence it was
not the reason Andrew accepted it.

I still prefer ->exe_file because I think it's a win not to walk the
VMAs with mmap sem when doing a readlink on /proc/*/exe. It's also less
sensitive to the order in which VMAs appear should that ever change.

Cheers,
	-Matt Helsley
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