[dm-devel] Re: [PATCH 1/7] dm: use __GFP_HIGH instead PF_MEMALLOC

[KOSAKI Motohiro]

Hi,

Thank you for give me comment.

> On Tue, Nov 17, 2009 at 04:17:07PM +0900, KOSAKI Motohiro wrote:
> > Non MM subsystem must not use PF_MEMALLOC. Memory reclaim need few
> > memory, anyone must not prevent it. Otherwise the system cause
> > mysterious hang-up and/or OOM Killer invokation.
>  
> This code is also on the critical path, for example, if you are swapping
> onto a dm device.  (There are ways we could reduce its use further as
> not every dm ioctl needs to be on the critical path and the buffer size
> could be limited for the ioctls that do.)

May I ask one additional question?
Original code is here.

	-------------------------------------------------------
        /*
         * Trying to avoid low memory issues when a device is
         * suspended.
         */
        current->flags |= PF_MEMALLOC;

        /*
         * Copy the parameters into kernel space.
         */
        r = copy_params(user, &param);

        current->flags &= ~PF_MEMALLOC;
	-------------------------------------------------------

but PF_MEMALLOC doesn't gurantee allocation successfull. In your case,
mempoll seems better to me. copy_params seems enough small function 
and we can rewrite it. Why didn't you use mempool?

Am I missing something?


> But what situations have been causing you trouble?  The OOM killer must
> generally avoid killing userspace processes that suspend & resume dm
> devices, and there are tight restrictions on what those processes
> can do safely between suspending and resuming.

No. This is theorical issue. but I really want to avoid stress weakness
kernel.