Messages in this thread | | | Date | Tue, 16 Oct 2012 13:19:33 -0700 | From | Andrew Morton <> | Subject | Re: [RFC PATCH v1 1/3] mm: teach mm by current context info to not do I/O during memory allocation |
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On Tue, 16 Oct 2012 23:59:41 +0800 Ming Lei <ming.lei@canonical.com> 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 may happen at least in the below situations: > > - during block device runtime resume, 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) > > - during error handling 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. > > Also the introduced solution can be used by block subsystem or block > drivers too, for example, set the PF_MEMALLOC_NOIO flag before doing > actual I/O transfer.
The patch seems reasonable to me. I'd like to see some examples of these resume-time callsite which are performing the GFP_KERNEL allocations, please. You have found some kernel bugs, so those should be fully described.
> @@ -1848,6 +1849,16 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t * > #define tsk_used_math(p) ((p)->flags & PF_USED_MATH) > #define used_math() tsk_used_math(current) > > +#define memalloc_noio() (current->flags & PF_MEMALLOC_NOIO) > +#define memalloc_noio_save(noio_flag) do { \ > + (noio_flag) = current->flags & PF_MEMALLOC_NOIO; \ > + current->flags |= PF_MEMALLOC_NOIO; \ > +} while (0) > +#define memalloc_noio_restore(noio_flag) do { \ > + if (!(noio_flag)) \ > + current->flags &= ~PF_MEMALLOC_NOIO; \ > +} while (0)
This is just awful. Why oh why do we write code in macros when we have a nice C compiler?
These can all be done as nice, clean, type-safe, documented C functions. And if they can be done that way, they *should* be done that way!
And I suggest that a better name for memalloc_noio_save() is memalloc_noio_set(). So this:
static inline unsigned memalloc_noio(void) { return current->flags & PF_MEMALLOC_NOIO; }
static inline unsigned memalloc_noio_set(unsigned flags) { unsigned ret = memalloc_noio();
current->flags |= PF_MEMALLOC_NOIO; return ret; }
static inline unsigned memalloc_noio_restore(unsigned flags) { current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flags; }
(I think that's correct? It's probably more efficient this way).
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