Messages in this thread | | | Subject | Re: [PATCH] sys_remap_file_pages: fix ->vm_file accounting | From | Matt Helsley <> | Date | Wed, 06 Feb 2008 16:16:38 -0800 |
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On Wed, 2008-02-06 at 20:33 +0000, Hugh Dickins wrote: > On Sun, 3 Feb 2008, Oleg Nesterov wrote: > > > > So I have to try to find another bug ;) Suppose that ->load_binary() does > > a series of do_mmap(MAP_EXECUTABLE). It is possible that mmap_region() can > > merge 2 vmas. In that case we "leak" ->num_exe_file_vmas. Unless I missed > > something, mmap_region() should do removed_exe_file_vma() when vma_merge() > > succeds (near fput(file)). > > Or there's the complementary case of a VM_EXECUTABLE vma being > split in two, for example by an mprotect of a part of it. > > Sorry, Matt, I don't like your patch at all. It seems to add a fair > amount of ugliness and unmaintainablity, all for a peculiar MVFS case
I thought that getting rid of the separate versions of proc_exe_link() improved maintainability. Do you have any specific details on what you think makes the code introduced by the patch unmaintainable?
> (you've tried to argue other advantages, but not always convinced!).
Yup -- looking at how the VM_EXECUTABLE flag affects the vma walk it's clear one of my arguments was wrong. So I can't blame you for being unconvinced by that. :)
I still think it would help any stacking filesystems that can't use the solution adopted by unionfs.
> And I found it quite hard to see where the crucial difference comes. > I guess it's that MVFS changes vma->vm_file in its ->mmap? Well, if
Yup.
> MVFS does that, maybe something else does that too, but precisely to > rely on the present behaviour of /proc/pid/exe - so in fixing for > MVFS, we'd be breaking that hypothetical other?
I'm not completely certain that I understand your point. Are you suggesting that some hypothetical code would want to use this "quirk" of /proc/pid/exe for a legitimate purpose?
Assuming that is your point, I thought my non-hypothetical java example clearly demonstrated that at least one non-hypothetical program doesn't expect the "quirk" and breaks because of it. Frankly, given /proc/pid/exe's output in the non-stacking case, I can't see how its output in the stacking case we're discussing could be considered anything but buggy.
Cheers, -Matt Helsley
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