Re: [PATCH v9 7/8] PCI/P2PDMA: Allow userspace VMA allocations through sysfs

[Logan Gunthorpe <logang@deltatee.com>]

On 2022-09-01 12:36, Greg Kroah-Hartman wrote:
> On Thu, Sep 01, 2022 at 12:14:25PM -0600, Logan Gunthorpe wrote:
>> Well we haven't plugged in a remove call into p2pdma, that would be more
>> work and more interfaces touching the PCI code. Note: this code isn't a
>> driver but a set of PCI helpers available to other PCI drivers.
>> Everything that's setup is using the devm interfaces and gets torn down
>> with the same. So I don't really see the benefit of making the change
>> you propose.
> 
> The issue is the classic one with the devm helpers.  They do not lend
> themselves to resource management issues that require ordering or other
> sort of dependencies.  Please do not use them here, just put in a remove
> callback as you eventually will need it anyway, as you have a strong
> requirement for what gets freed when, and the devm api does not provide
> for that well.

This surprises me. Can you elaborate on this classic issue?

I've definitely seen uses of devm that expect the calls will be torn
down in reverse order they are added. The existing p2pdma code will
certainly fail quite significantly if a devm_kzalloc() releases its
memory before the devm_memmap_pages() cleans up. There's also already an
action that is used to cleanup before the last devm_kzalloc() call
happens. If ordering is not guaranteed, then devm seems fairly broken
and unusable and I'd have to drop all uses from this code and go back to
the error prone method. Also what's the point of
devm_add_action_or_reset() if it doesn't guarantee the ordering or the
release?

But if it's that important I can make the change to these patches for v10.

Logan