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SubjectRe: [PATCH v4 1/6] mm: teach mm by current context info to not do I/O during memory allocation
On Sat,  3 Nov 2012 16:35:09 +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[1] 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, network devices involved too
> in the iSCSI case)
>
> - during block device runtime suspend, because runtime resume need
> to wait for completion of concurrent runtime suspend.
>
> - 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.
>
> It is not a good idea to convert all these GFP_KERNEL in the
> affected path into GFP_NOIO because these functions doing that may be
> implemented as library and will be called in many other contexts.
>
> In fact, memalloc_noio() can convert some of current static GFP_NOIO
> allocation into GFP_KERNEL back in other non-affected contexts, at least
> almost all GFP_NOIO in USB subsystem can be converted into GFP_KERNEL
> after applying the approach and make allocation with GFP_IO
> only happen in runtime resume/bus reset/block I/O transfer contexts
> generally.

It's unclear from the description why we're also clearing __GFP_FS in
this situation.

If we can avoid doing this then there will be a very small gain: there
are some situations in which a filesystem can clean pagecache without
performing I/O.


It doesn't appear that the patch will add overhead to the alloc/free
hotpaths, which is good.

>
> ...
>
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1805,6 +1805,7 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
> #define PF_FROZEN 0x00010000 /* frozen for system suspend */
> #define PF_FSTRANS 0x00020000 /* inside a filesystem transaction */
> #define PF_KSWAPD 0x00040000 /* I am kswapd */
> +#define PF_MEMALLOC_NOIO 0x00080000 /* Allocating memory without IO involved */
> #define PF_LESS_THROTTLE 0x00100000 /* Throttle me less: I clean memory */
> #define PF_KTHREAD 0x00200000 /* I am a kernel thread */
> #define PF_RANDOMIZE 0x00400000 /* randomize virtual address space */
> @@ -1842,6 +1843,15 @@ 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(flag) do { \
> + (flag) = current->flags & PF_MEMALLOC_NOIO; \
> + current->flags |= PF_MEMALLOC_NOIO; \
> +} while (0)
> +#define memalloc_noio_restore(flag) do { \
> + current->flags = (current->flags & ~PF_MEMALLOC_NOIO) | flag; \
> +} while (0)
> +

Again with the ghastly macros. Please, do this properly in regular old
C, as previously discussed. It really doesn't matter what daft things
local_irq_save() did 20 years ago. Just do it right!

Also, you can probably put the unlikely() inside memalloc_noio() and
avoid repeating it at all the callsites.

And it might be neater to do:

/*
* Nice comment goes here
*/
static inline gfp_t memalloc_noio_flags(gfp_t flags)
{
if (unlikely(current->flags & PF_MEMALLOC_NOIO))
flags &= ~GFP_IOFS;
return flags;
}

> * task->jobctl flags
> */
>
> ...
>
> @@ -2304,6 +2304,12 @@ unsigned long try_to_free_pages(struct zonelist *zonelist, int order,
> .gfp_mask = sc.gfp_mask,
> };
>
> + if (unlikely(memalloc_noio())) {
> + gfp_mask &= ~GFP_IOFS;
> + sc.gfp_mask = gfp_mask;
> + shrink.gfp_mask = sc.gfp_mask;
> + }

We can avoid writing to shrink.gfp_mask twice. And maybe sc.gfp_mask
as well. Unclear, I didn't think about it too hard ;)

> throttle_direct_reclaim(gfp_mask, zonelist, nodemask);
>
> /*
>
> ...
>


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