Re: [PATCH v2 0/3] File Sealing & memfd_create()

[Minchan Kim]

Hello,

On Mon, May 19, 2014 at 01:44:25PM +0200, David Herrmann wrote:
> Hi
> 
> On Thu, May 15, 2014 at 12:35 AM, Hugh Dickins <hughd@xxxxxxxxxx> wrote:
> > The aspect which really worries me is this: the maintenance burden.
> > This approach would add some peculiar new code, introducing a rare
> > special case: which we might get right today, but will very easily
> > forget tomorrow when making some other changes to mm.  If we compile
> > a list of danger areas in mm, this would surely belong on that list.
> 
> I tried doing the page-replacement in the last 4 days, but honestly,
> it's far more complex than I thought. So if no-one more experienced
> with mm/ comes up with a simple implementation, I'll have to delay
> this for some more weeks.
> 
> However, I still wonder why we try to fix this as part of this
> patchset. Using FUSE, a DIRECT-IO call can be delayed for an arbitrary
> amount of time. Same is true for network block-devices, NFS, iscsi,
> maybe loop-devices, ... This means, _any_ once mapped page can be
> written to after an arbitrary delay. This can break any feature that
> makes FS objects read-only (remounting read-only, setting S_IMMUTABLE,
> sealing, ..).
> 
> Shouldn't we try to fix the _cause_ of this?

I didn't follow this patchset and couldn't find what's your most cocern
but at a first glance, it seems you have troubled with pinned page.
If so, it's really big problem for CMA and I think peterz's approach(ie,
mm_mpin) is really make sense to me.

https://lkml.org/lkml/2014/5/26/340


> 
> Isn't there a simple way to lock/mark/.. affected vmas in
> get_user_pages(_fast)() and release them once done? We could increase
> i_mmap_writable on all affected address_space and decrease it on
> release. This would at least prevent sealing and could be check on
> other operations, too (like setting S_IMMUTABLE).
> This should be as easy as checking page_mapping(page) != NULL and then
> adjusting ->i_mmap_writable in
> get_writable_user_pages/put_writable_user_pages, right?
> 
> Thanks
> David
> 
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-- 
Kind regards,
Minchan Kim
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