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

[David Herrmann]

Hi

On Mon, Jun 2, 2014 at 11:14 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Mon 02-06-14 13:42:59, Minchan Kim wrote:
>> On Mon, May 19, 2014 at 01:44:25PM +0200, David Herrmann wrote:
>> > 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.
>   Well, his concern are pinned pages (and also pages used for direct IO and
> similar) but not because they are pinned but because they can be modified
> while someone holds reference to them. So I'm not sure Peter's patches will
> help here.

Correct, the problem is not accounting for pinned-pages, but waiting
for them to get released. Furthermore, Peter's patches make VM_PINNED
an optional feature, so we'd still miss all the short-term GUP users.
Sadly, that means we cannot even use it to test for pending GUP users.

Thanks
David
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