Re: [RFC PATCH 1/3] mm: teach mm by current context info to not doI/O during memory allocation

[Minchan Kim]

On Tue, Oct 16, 2012 at 09:56:48AM +0800, Ming Lei wrote:
> On Mon, Oct 15, 2012 at 11:47 PM, Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > On Mon, Oct 15, 2012 at 01:14:17PM +0800, Ming Lei wrote:
> >> This patch introduces PF_MEMALLOC_NOIO on process flag('flags' field of
> >> 'struct task_struct'), so that the flag can be set by one task
> >> to avoid doing I/O inside memory allocation in the task's context.
> >>
> >> The patch trys to solve one deadlock problem caused by block device,
> >> and the problem can be occured at least in the below situations:
> >>
> >> - during block device runtime resume situation, if memory allocation
> >> with GFP_KERNEL is called inside runtime resume callback of any one
> >> of its ancestors(or the block device itself), the deadlock may be
> >> triggered inside the memory allocation since it might not complete
> >> until the block device becomes active and the involed page I/O finishes.
> >> The situation is pointed out first by Alan Stern. It is not a good
> >> approach to convert all GFP_KERNEL in the path into GFP_NOIO because
> >> several subsystems may be involved(for example, PCI, USB and SCSI may
> >> be involved for usb mass stoarage device)
> >
> > Couldn't we expand pm_restrict_gfp_mask to cover resume path as well as
> > suspend path?
> 
> IMO, we could, but it is not good and might trigger memory allocation problem.
> 
> pm_restrict_gfp_mask uses the global variable of gfp_allowed_mask to
> avoid allocating page with GFP_IOFS in all contexts during system sleep,
> when processes have been frozen.
> 
> But during runtime PM, the whole system is running and all processes are
> runnable. Also runtime PM is per device and the whole system may have
> lots of devices, so taking the global gfp_allowed_mask may keep page
> allocation with ~GFP_IOFS for a considerable proportion of system
> running time, then alloc_page() will return failure easier.
> 
> The above deadlock problem may be fixed by allocating memory with
> ~GFP_IOFS only in the context of calling runtime_resume, and that is
> idea of the patch.

Fair enough but it wouldn't be a good idea that add new unlikely branch
in allocator's fast path. Please move the check into slow path which could
be in __alloc_pages_slowpath.

> 
> >
> >>
> >> - during error handling situation of usb mass storage deivce, USB
> >> bus reset will be put on the device, so there shouldn't have any
> >> memory allocation with GFP_KERNEL during USB bus reset, otherwise
> >> the deadlock similar with above may be triggered. Unfortunately, any
> >> usb device may include one mass storage interface in theory, so it
> >> requires all usb interface drivers to handle the situation. In fact,
> >> most usb drivers don't know how to handle bus reset on the device
> >> and don't provide .pre_set() and .post_reset() callback at all, so
> >> USB core has to unbind and bind driver for these devices. So it
> >> is still not practical to resort to GFP_NOIO for solving the problem.
> >
> > I hope this case could be handled by usb core like usb_restrict_gfp_mask
> > rather than adding new branch on fast path.
> 
> See above, applying the global gfp_allowed_mask is not good.
> 
> 
> Thanks,
> --
> Ming Lei

-- 
Kind Regards,
Minchan Kim
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