[RFC] mm: a question about high-order check in __zone_watermark_ok()

[RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
(mm, page_alloc: only enforce watermarks for order-0 allocations)
rewrite the high-order check in __zone_watermark_ok(), but I think it
quietly fix a bug. Please see the following.

Before this patch, the high-order check is this:
__zone_watermark_ok()
	...
	for (o = 0; o < order; o++) {
		/* At the next order, this order's pages become unavailable */
		free_pages -= z->free_area[o].nr_free << o;

		/* Require fewer higher order pages to be free */
		min >>= 1;

		if (free_pages <= min)
			return false;
	}
	...

If we have cma memory, and we alloc a high-order movable page, then it's right.

But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
and there are a lot of high-order cma pages, but little high-order unmovable
pages, the it is still return *true*, but we will alloc *failed* finally, because
we cannot fallback from migrate_unmovable to migrate_cma, right?

Also if we doing __alloc_pages_slowpath(), the compact will not work, because
__zone_watermark_ok() always return true, and it lead to alloc a high-order
unmovable page failed, then do direct reclaim.

Thanks,
Xishi Qiu

[RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
(mm, page_alloc: only enforce watermarks for order-0 allocations)
rewrite the high-order check in __zone_watermark_ok(), but I think it
quietly fix a bug. Please see the following.

Before this patch, the high-order check is this:
__zone_watermark_ok()
	...
	for (o = 0; o < order; o++) {
		/* At the next order, this order's pages become unavailable */
		free_pages -= z->free_area[o].nr_free << o;

		/* Require fewer higher order pages to be free */
		min >>= 1;

		if (free_pages <= min)
			return false;
	}
	...

If we have cma memory, and we alloc a high-order movable page, then it's right.

But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
and there are a lot of high-order cma pages, but little high-order unmovable
pages, the it is still return *true*, but we will alloc *failed* finally, because
we cannot fallback from migrate_unmovable to migrate_cma, right?

Also if we doing __alloc_pages_slowpath(), the compact will not work, because
__zone_watermark_ok() always return true, and it lead to alloc a high-order
unmovable page failed, then do direct reclaim.

Thanks,
Xishi Qiu

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Vlastimil Babka]

[+CC Joonsoo Kim]

On 09/26/2016 10:47 AM, Xishi Qiu wrote:
> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> rewrite the high-order check in __zone_watermark_ok(), but I think it
> quietly fix a bug. Please see the following.
>
> Before this patch, the high-order check is this:
> __zone_watermark_ok()
> 	...
> 	for (o = 0; o < order; o++) {
> 		/* At the next order, this order's pages become unavailable */
> 		free_pages -= z->free_area[o].nr_free << o;
>
> 		/* Require fewer higher order pages to be free */
> 		min >>= 1;
>
> 		if (free_pages <= min)
> 			return false;
> 	}
> 	...
>
> If we have cma memory, and we alloc a high-order movable page, then it's right.
>
> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> and there are a lot of high-order cma pages, but little high-order unmovable
> pages, the it is still return *true*, but we will alloc *failed* finally, because
> we cannot fallback from migrate_unmovable to migrate_cma, right?

Yeah I think this limitation was known to CMA people.

> Also if we doing __alloc_pages_slowpath(), the compact will not work, because
> __zone_watermark_ok() always return true, and it lead to alloc a high-order
> unmovable page failed, then do direct reclaim.

I guess that can happen as well.

> Thanks,
> Xishi Qiu
>

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Vlastimil Babka]

[+CC Joonsoo Kim]

On 09/26/2016 10:47 AM, Xishi Qiu wrote:
> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> rewrite the high-order check in __zone_watermark_ok(), but I think it
> quietly fix a bug. Please see the following.
>
> Before this patch, the high-order check is this:
> __zone_watermark_ok()
> 	...
> 	for (o = 0; o < order; o++) {
> 		/* At the next order, this order's pages become unavailable */
> 		free_pages -= z->free_area[o].nr_free << o;
>
> 		/* Require fewer higher order pages to be free */
> 		min >>= 1;
>
> 		if (free_pages <= min)
> 			return false;
> 	}
> 	...
>
> If we have cma memory, and we alloc a high-order movable page, then it's right.
>
> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> and there are a lot of high-order cma pages, but little high-order unmovable
> pages, the it is still return *true*, but we will alloc *failed* finally, because
> we cannot fallback from migrate_unmovable to migrate_cma, right?

Yeah I think this limitation was known to CMA people.

> Also if we doing __alloc_pages_slowpath(), the compact will not work, because
> __zone_watermark_ok() always return true, and it lead to alloc a high-order
> unmovable page failed, then do direct reclaim.

I guess that can happen as well.

> Thanks,
> Xishi Qiu
>

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> rewrite the high-order check in __zone_watermark_ok(), but I think it
> quietly fix a bug. Please see the following.
> 
> Before this patch, the high-order check is this:
> __zone_watermark_ok()
> 	...
> 	for (o = 0; o < order; o++) {
> 		/* At the next order, this order's pages become unavailable */
> 		free_pages -= z->free_area[o].nr_free << o;
> 
> 		/* Require fewer higher order pages to be free */
> 		min >>= 1;
> 
> 		if (free_pages <= min)
> 			return false;
> 	}
> 	...
> 
> If we have cma memory, and we alloc a high-order movable page, then it's right.
> 
> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> and there are a lot of high-order cma pages, but little high-order unmovable
> pages, the it is still return *true*, but we will alloc *failed* finally, because
> we cannot fallback from migrate_unmovable to migrate_cma, right?

AFAIR CMA wmark check was always tricky and the above commit has made
the situation at least a bit more clear. Anyway IIRC 

#ifdef CONFIG_CMA
	/* If allocation can't use CMA areas don't use free CMA pages */
	if (!(alloc_flags & ALLOC_CMA))
		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif

	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
		return false;

should reduce the prioblem because a lot of CMA pages should just get us
below the wmark + reserve boundary.
-- 
Michal Hocko
SUSE Labs

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> rewrite the high-order check in __zone_watermark_ok(), but I think it
> quietly fix a bug. Please see the following.
> 
> Before this patch, the high-order check is this:
> __zone_watermark_ok()
> 	...
> 	for (o = 0; o < order; o++) {
> 		/* At the next order, this order's pages become unavailable */
> 		free_pages -= z->free_area[o].nr_free << o;
> 
> 		/* Require fewer higher order pages to be free */
> 		min >>= 1;
> 
> 		if (free_pages <= min)
> 			return false;
> 	}
> 	...
> 
> If we have cma memory, and we alloc a high-order movable page, then it's right.
> 
> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> and there are a lot of high-order cma pages, but little high-order unmovable
> pages, the it is still return *true*, but we will alloc *failed* finally, because
> we cannot fallback from migrate_unmovable to migrate_cma, right?

AFAIR CMA wmark check was always tricky and the above commit has made
the situation at least a bit more clear. Anyway IIRC 

#ifdef CONFIG_CMA
	/* If allocation can't use CMA areas don't use free CMA pages */
	if (!(alloc_flags & ALLOC_CMA))
		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
#endif

	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
		return false;

should reduce the prioblem because a lot of CMA pages should just get us
below the wmark + reserve boundary.
-- 
Michal Hocko
SUSE Labs

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/26 16:58, Michal Hocko wrote:

> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>> quietly fix a bug. Please see the following.
>>
>> Before this patch, the high-order check is this:
>> __zone_watermark_ok()
>> 	...
>> 	for (o = 0; o < order; o++) {
>> 		/* At the next order, this order's pages become unavailable */
>> 		free_pages -= z->free_area[o].nr_free << o;
>>
>> 		/* Require fewer higher order pages to be free */
>> 		min >>= 1;
>>
>> 		if (free_pages <= min)
>> 			return false;
>> 	}
>> 	...
>>
>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>
>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>> and there are a lot of high-order cma pages, but little high-order unmovable
>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> 
> AFAIR CMA wmark check was always tricky and the above commit has made
> the situation at least a bit more clear. Anyway IIRC 
> 
> #ifdef CONFIG_CMA
> 	/* If allocation can't use CMA areas don't use free CMA pages */
> 	if (!(alloc_flags & ALLOC_CMA))
> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> #endif
> 
> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> 		return false;
> 
> should reduce the prioblem because a lot of CMA pages should just get us
> below the wmark + reserve boundary.

Hi Michal,

If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
are also enough, but they are fragment, then it will triger the problem.
If we alloc a high-order unmovable page, water mark check return *true*, but we
will alloc *failed*, right?

Thanks,
Xishi Qiu

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/26 16:58, Michal Hocko wrote:

> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>> quietly fix a bug. Please see the following.
>>
>> Before this patch, the high-order check is this:
>> __zone_watermark_ok()
>> 	...
>> 	for (o = 0; o < order; o++) {
>> 		/* At the next order, this order's pages become unavailable */
>> 		free_pages -= z->free_area[o].nr_free << o;
>>
>> 		/* Require fewer higher order pages to be free */
>> 		min >>= 1;
>>
>> 		if (free_pages <= min)
>> 			return false;
>> 	}
>> 	...
>>
>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>
>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>> and there are a lot of high-order cma pages, but little high-order unmovable
>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> 
> AFAIR CMA wmark check was always tricky and the above commit has made
> the situation at least a bit more clear. Anyway IIRC 
> 
> #ifdef CONFIG_CMA
> 	/* If allocation can't use CMA areas don't use free CMA pages */
> 	if (!(alloc_flags & ALLOC_CMA))
> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> #endif
> 
> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> 		return false;
> 
> should reduce the prioblem because a lot of CMA pages should just get us
> below the wmark + reserve boundary.

Hi Michal,

If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
are also enough, but they are fragment, then it will triger the problem.
If we alloc a high-order unmovable page, water mark check return *true*, but we
will alloc *failed*, right?

Thanks,
Xishi Qiu

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> On 2016/9/26 16:58, Michal Hocko wrote:
> 
> > On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> >> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> >> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> >> rewrite the high-order check in __zone_watermark_ok(), but I think it
> >> quietly fix a bug. Please see the following.
> >>
> >> Before this patch, the high-order check is this:
> >> __zone_watermark_ok()
> >> 	...
> >> 	for (o = 0; o < order; o++) {
> >> 		/* At the next order, this order's pages become unavailable */
> >> 		free_pages -= z->free_area[o].nr_free << o;
> >>
> >> 		/* Require fewer higher order pages to be free */
> >> 		min >>= 1;
> >>
> >> 		if (free_pages <= min)
> >> 			return false;
> >> 	}
> >> 	...
> >>
> >> If we have cma memory, and we alloc a high-order movable page, then it's right.
> >>
> >> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> >> and there are a lot of high-order cma pages, but little high-order unmovable
> >> pages, the it is still return *true*, but we will alloc *failed* finally, because
> >> we cannot fallback from migrate_unmovable to migrate_cma, right?
> > 
> > AFAIR CMA wmark check was always tricky and the above commit has made
> > the situation at least a bit more clear. Anyway IIRC 
> > 
> > #ifdef CONFIG_CMA
> > 	/* If allocation can't use CMA areas don't use free CMA pages */
> > 	if (!(alloc_flags & ALLOC_CMA))
> > 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> > #endif
> > 
> > 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> > 		return false;
> > 
> > should reduce the prioblem because a lot of CMA pages should just get us
> > below the wmark + reserve boundary.
> 
> Hi Michal,
> 
> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> are also enough, but they are fragment, then it will triger the problem.
> If we alloc a high-order unmovable page, water mark check return *true*, but we
> will alloc *failed*, right?

As Vlastimil has written. There were known issues with the wmark checks
and high order requests.
-- 
Michal Hocko
SUSE Labs

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> On 2016/9/26 16:58, Michal Hocko wrote:
> 
> > On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> >> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> >> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> >> rewrite the high-order check in __zone_watermark_ok(), but I think it
> >> quietly fix a bug. Please see the following.
> >>
> >> Before this patch, the high-order check is this:
> >> __zone_watermark_ok()
> >> 	...
> >> 	for (o = 0; o < order; o++) {
> >> 		/* At the next order, this order's pages become unavailable */
> >> 		free_pages -= z->free_area[o].nr_free << o;
> >>
> >> 		/* Require fewer higher order pages to be free */
> >> 		min >>= 1;
> >>
> >> 		if (free_pages <= min)
> >> 			return false;
> >> 	}
> >> 	...
> >>
> >> If we have cma memory, and we alloc a high-order movable page, then it's right.
> >>
> >> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> >> and there are a lot of high-order cma pages, but little high-order unmovable
> >> pages, the it is still return *true*, but we will alloc *failed* finally, because
> >> we cannot fallback from migrate_unmovable to migrate_cma, right?
> > 
> > AFAIR CMA wmark check was always tricky and the above commit has made
> > the situation at least a bit more clear. Anyway IIRC 
> > 
> > #ifdef CONFIG_CMA
> > 	/* If allocation can't use CMA areas don't use free CMA pages */
> > 	if (!(alloc_flags & ALLOC_CMA))
> > 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> > #endif
> > 
> > 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> > 		return false;
> > 
> > should reduce the prioblem because a lot of CMA pages should just get us
> > below the wmark + reserve boundary.
> 
> Hi Michal,
> 
> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> are also enough, but they are fragment, then it will triger the problem.
> If we alloc a high-order unmovable page, water mark check return *true*, but we
> will alloc *failed*, right?

As Vlastimil has written. There were known issues with the wmark checks
and high order requests.
-- 
Michal Hocko
SUSE Labs

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/26 17:43, Michal Hocko wrote:

> On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
>> On 2016/9/26 16:58, Michal Hocko wrote:
>>
>>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>>>> quietly fix a bug. Please see the following.
>>>>
>>>> Before this patch, the high-order check is this:
>>>> __zone_watermark_ok()
>>>> 	...
>>>> 	for (o = 0; o < order; o++) {
>>>> 		/* At the next order, this order's pages become unavailable */
>>>> 		free_pages -= z->free_area[o].nr_free << o;
>>>>
>>>> 		/* Require fewer higher order pages to be free */
>>>> 		min >>= 1;
>>>>
>>>> 		if (free_pages <= min)
>>>> 			return false;
>>>> 	}
>>>> 	...
>>>>
>>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>>>
>>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>>>> and there are a lot of high-order cma pages, but little high-order unmovable
>>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
>>>
>>> AFAIR CMA wmark check was always tricky and the above commit has made
>>> the situation at least a bit more clear. Anyway IIRC 
>>>
>>> #ifdef CONFIG_CMA
>>> 	/* If allocation can't use CMA areas don't use free CMA pages */
>>> 	if (!(alloc_flags & ALLOC_CMA))
>>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>>> #endif
>>>
>>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
>>> 		return false;
>>>
>>> should reduce the prioblem because a lot of CMA pages should just get us
>>> below the wmark + reserve boundary.
>>
>> Hi Michal,
>>
>> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
>> are also enough, but they are fragment, then it will triger the problem.
>> If we alloc a high-order unmovable page, water mark check return *true*, but we
>> will alloc *failed*, right?
> 
> As Vlastimil has written. There were known issues with the wmark checks
> and high order requests.

Shall we backport to stable?

Thanks,
Xishi Qiu

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/26 17:43, Michal Hocko wrote:

> On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
>> On 2016/9/26 16:58, Michal Hocko wrote:
>>
>>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>>>> quietly fix a bug. Please see the following.
>>>>
>>>> Before this patch, the high-order check is this:
>>>> __zone_watermark_ok()
>>>> 	...
>>>> 	for (o = 0; o < order; o++) {
>>>> 		/* At the next order, this order's pages become unavailable */
>>>> 		free_pages -= z->free_area[o].nr_free << o;
>>>>
>>>> 		/* Require fewer higher order pages to be free */
>>>> 		min >>= 1;
>>>>
>>>> 		if (free_pages <= min)
>>>> 			return false;
>>>> 	}
>>>> 	...
>>>>
>>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>>>
>>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>>>> and there are a lot of high-order cma pages, but little high-order unmovable
>>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
>>>
>>> AFAIR CMA wmark check was always tricky and the above commit has made
>>> the situation at least a bit more clear. Anyway IIRC 
>>>
>>> #ifdef CONFIG_CMA
>>> 	/* If allocation can't use CMA areas don't use free CMA pages */
>>> 	if (!(alloc_flags & ALLOC_CMA))
>>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>>> #endif
>>>
>>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
>>> 		return false;
>>>
>>> should reduce the prioblem because a lot of CMA pages should just get us
>>> below the wmark + reserve boundary.
>>
>> Hi Michal,
>>
>> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
>> are also enough, but they are fragment, then it will triger the problem.
>> If we alloc a high-order unmovable page, water mark check return *true*, but we
>> will alloc *failed*, right?
> 
> As Vlastimil has written. There were known issues with the wmark checks
> and high order requests.

Shall we backport to stable?

Thanks,
Xishi Qiu

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
> On 2016/9/26 17:43, Michal Hocko wrote:
> 
> > On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> >> On 2016/9/26 16:58, Michal Hocko wrote:
> >>
> >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
> >>>> quietly fix a bug. Please see the following.
> >>>>
> >>>> Before this patch, the high-order check is this:
> >>>> __zone_watermark_ok()
> >>>> 	...
> >>>> 	for (o = 0; o < order; o++) {
> >>>> 		/* At the next order, this order's pages become unavailable */
> >>>> 		free_pages -= z->free_area[o].nr_free << o;
> >>>>
> >>>> 		/* Require fewer higher order pages to be free */
> >>>> 		min >>= 1;
> >>>>
> >>>> 		if (free_pages <= min)
> >>>> 			return false;
> >>>> 	}
> >>>> 	...
> >>>>
> >>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
> >>>>
> >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> >>>> and there are a lot of high-order cma pages, but little high-order unmovable
> >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
> >>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> >>>
> >>> AFAIR CMA wmark check was always tricky and the above commit has made
> >>> the situation at least a bit more clear. Anyway IIRC 
> >>>
> >>> #ifdef CONFIG_CMA
> >>> 	/* If allocation can't use CMA areas don't use free CMA pages */
> >>> 	if (!(alloc_flags & ALLOC_CMA))
> >>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> >>> #endif
> >>>
> >>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> >>> 		return false;
> >>>
> >>> should reduce the prioblem because a lot of CMA pages should just get us
> >>> below the wmark + reserve boundary.
> >>
> >> Hi Michal,
> >>
> >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> >> are also enough, but they are fragment, then it will triger the problem.
> >> If we alloc a high-order unmovable page, water mark check return *true*, but we
> >> will alloc *failed*, right?
> > 
> > As Vlastimil has written. There were known issues with the wmark checks
> > and high order requests.
> 
> Shall we backport to stable?

I dunno, it was a part of a larger series with high atomic reserves and
changes which sound a bit intrusive for the stable kernel. Considering
that CMA was known to be problematic and there are still some issues
left I do not think this is worth the trouble/risk.
-- 
Michal Hocko
SUSE Labs

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Michal Hocko]

On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
> On 2016/9/26 17:43, Michal Hocko wrote:
> 
> > On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> >> On 2016/9/26 16:58, Michal Hocko wrote:
> >>
> >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
> >>>> quietly fix a bug. Please see the following.
> >>>>
> >>>> Before this patch, the high-order check is this:
> >>>> __zone_watermark_ok()
> >>>> 	...
> >>>> 	for (o = 0; o < order; o++) {
> >>>> 		/* At the next order, this order's pages become unavailable */
> >>>> 		free_pages -= z->free_area[o].nr_free << o;
> >>>>
> >>>> 		/* Require fewer higher order pages to be free */
> >>>> 		min >>= 1;
> >>>>
> >>>> 		if (free_pages <= min)
> >>>> 			return false;
> >>>> 	}
> >>>> 	...
> >>>>
> >>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
> >>>>
> >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> >>>> and there are a lot of high-order cma pages, but little high-order unmovable
> >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
> >>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> >>>
> >>> AFAIR CMA wmark check was always tricky and the above commit has made
> >>> the situation at least a bit more clear. Anyway IIRC 
> >>>
> >>> #ifdef CONFIG_CMA
> >>> 	/* If allocation can't use CMA areas don't use free CMA pages */
> >>> 	if (!(alloc_flags & ALLOC_CMA))
> >>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> >>> #endif
> >>>
> >>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> >>> 		return false;
> >>>
> >>> should reduce the prioblem because a lot of CMA pages should just get us
> >>> below the wmark + reserve boundary.
> >>
> >> Hi Michal,
> >>
> >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> >> are also enough, but they are fragment, then it will triger the problem.
> >> If we alloc a high-order unmovable page, water mark check return *true*, but we
> >> will alloc *failed*, right?
> > 
> > As Vlastimil has written. There were known issues with the wmark checks
> > and high order requests.
> 
> Shall we backport to stable?

I dunno, it was a part of a larger series with high atomic reserves and
changes which sound a bit intrusive for the stable kernel. Considering
that CMA was known to be problematic and there are still some issues
left I do not think this is worth the trouble/risk.
-- 
Michal Hocko
SUSE Labs

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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Joonsoo Kim]

On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
> On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
> > On 2016/9/26 17:43, Michal Hocko wrote:
> > 
> > > On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> > >> On 2016/9/26 16:58, Michal Hocko wrote:
> > >>
> > >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> > >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> > >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> > >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
> > >>>> quietly fix a bug. Please see the following.
> > >>>>
> > >>>> Before this patch, the high-order check is this:
> > >>>> __zone_watermark_ok()
> > >>>> 	...
> > >>>> 	for (o = 0; o < order; o++) {
> > >>>> 		/* At the next order, this order's pages become unavailable */
> > >>>> 		free_pages -= z->free_area[o].nr_free << o;
> > >>>>
> > >>>> 		/* Require fewer higher order pages to be free */
> > >>>> 		min >>= 1;
> > >>>>
> > >>>> 		if (free_pages <= min)
> > >>>> 			return false;
> > >>>> 	}
> > >>>> 	...
> > >>>>
> > >>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
> > >>>>
> > >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> > >>>> and there are a lot of high-order cma pages, but little high-order unmovable
> > >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
> > >>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> > >>>
> > >>> AFAIR CMA wmark check was always tricky and the above commit has made
> > >>> the situation at least a bit more clear. Anyway IIRC 
> > >>>
> > >>> #ifdef CONFIG_CMA
> > >>> 	/* If allocation can't use CMA areas don't use free CMA pages */
> > >>> 	if (!(alloc_flags & ALLOC_CMA))
> > >>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> > >>> #endif
> > >>>
> > >>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> > >>> 		return false;
> > >>>
> > >>> should reduce the prioblem because a lot of CMA pages should just get us
> > >>> below the wmark + reserve boundary.
> > >>
> > >> Hi Michal,
> > >>
> > >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> > >> are also enough, but they are fragment, then it will triger the problem.
> > >> If we alloc a high-order unmovable page, water mark check return *true*, but we
> > >> will alloc *failed*, right?
> > > 
> > > As Vlastimil has written. There were known issues with the wmark checks
> > > and high order requests.
> > 
> > Shall we backport to stable?
> 
> I dunno, it was a part of a larger series with high atomic reserves and
> changes which sound a bit intrusive for the stable kernel. Considering
> that CMA was known to be problematic and there are still some issues
> left I do not think this is worth the trouble/risk.

CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
removed due to Mel's high atomic reserve series.

That series is rather large and has some problems so I think that it
is not suitable for stable tree.

Thanks.

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Joonsoo Kim]

On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
> On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
> > On 2016/9/26 17:43, Michal Hocko wrote:
> > 
> > > On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
> > >> On 2016/9/26 16:58, Michal Hocko wrote:
> > >>
> > >>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
> > >>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
> > >>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
> > >>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
> > >>>> quietly fix a bug. Please see the following.
> > >>>>
> > >>>> Before this patch, the high-order check is this:
> > >>>> __zone_watermark_ok()
> > >>>> 	...
> > >>>> 	for (o = 0; o < order; o++) {
> > >>>> 		/* At the next order, this order's pages become unavailable */
> > >>>> 		free_pages -= z->free_area[o].nr_free << o;
> > >>>>
> > >>>> 		/* Require fewer higher order pages to be free */
> > >>>> 		min >>= 1;
> > >>>>
> > >>>> 		if (free_pages <= min)
> > >>>> 			return false;
> > >>>> 	}
> > >>>> 	...
> > >>>>
> > >>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
> > >>>>
> > >>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
> > >>>> and there are a lot of high-order cma pages, but little high-order unmovable
> > >>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
> > >>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
> > >>>
> > >>> AFAIR CMA wmark check was always tricky and the above commit has made
> > >>> the situation at least a bit more clear. Anyway IIRC 
> > >>>
> > >>> #ifdef CONFIG_CMA
> > >>> 	/* If allocation can't use CMA areas don't use free CMA pages */
> > >>> 	if (!(alloc_flags & ALLOC_CMA))
> > >>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
> > >>> #endif
> > >>>
> > >>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
> > >>> 		return false;
> > >>>
> > >>> should reduce the prioblem because a lot of CMA pages should just get us
> > >>> below the wmark + reserve boundary.
> > >>
> > >> Hi Michal,
> > >>
> > >> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
> > >> are also enough, but they are fragment, then it will triger the problem.
> > >> If we alloc a high-order unmovable page, water mark check return *true*, but we
> > >> will alloc *failed*, right?
> > > 
> > > As Vlastimil has written. There were known issues with the wmark checks
> > > and high order requests.
> > 
> > Shall we backport to stable?
> 
> I dunno, it was a part of a larger series with high atomic reserves and
> changes which sound a bit intrusive for the stable kernel. Considering
> that CMA was known to be problematic and there are still some issues
> left I do not think this is worth the trouble/risk.

CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
removed due to Mel's high atomic reserve series.

That series is rather large and has some problems so I think that it
is not suitable for stable tree.

Thanks.

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
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Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/28 13:52, Joonsoo Kim wrote:

> On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
>> On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
>>> On 2016/9/26 17:43, Michal Hocko wrote:
>>>
>>>> On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
>>>>> On 2016/9/26 16:58, Michal Hocko wrote:
>>>>>
>>>>>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>>>>>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>>>>>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>>>>>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>>>>>>> quietly fix a bug. Please see the following.
>>>>>>>
>>>>>>> Before this patch, the high-order check is this:
>>>>>>> __zone_watermark_ok()
>>>>>>> 	...
>>>>>>> 	for (o = 0; o < order; o++) {
>>>>>>> 		/* At the next order, this order's pages become unavailable */
>>>>>>> 		free_pages -= z->free_area[o].nr_free << o;
>>>>>>>
>>>>>>> 		/* Require fewer higher order pages to be free */
>>>>>>> 		min >>= 1;
>>>>>>>
>>>>>>> 		if (free_pages <= min)
>>>>>>> 			return false;
>>>>>>> 	}
>>>>>>> 	...
>>>>>>>
>>>>>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>>>>>>
>>>>>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>>>>>>> and there are a lot of high-order cma pages, but little high-order unmovable
>>>>>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>>>>>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
>>>>>>
>>>>>> AFAIR CMA wmark check was always tricky and the above commit has made
>>>>>> the situation at least a bit more clear. Anyway IIRC 
>>>>>>
>>>>>> #ifdef CONFIG_CMA
>>>>>> 	/* If allocation can't use CMA areas don't use free CMA pages */
>>>>>> 	if (!(alloc_flags & ALLOC_CMA))
>>>>>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>>>>>> #endif
>>>>>>
>>>>>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
>>>>>> 		return false;
>>>>>>
>>>>>> should reduce the prioblem because a lot of CMA pages should just get us
>>>>>> below the wmark + reserve boundary.
>>>>>
>>>>> Hi Michal,
>>>>>
>>>>> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
>>>>> are also enough, but they are fragment, then it will triger the problem.
>>>>> If we alloc a high-order unmovable page, water mark check return *true*, but we
>>>>> will alloc *failed*, right?
>>>>
>>>> As Vlastimil has written. There were known issues with the wmark checks
>>>> and high order requests.
>>>
>>> Shall we backport to stable?
>>
>> I dunno, it was a part of a larger series with high atomic reserves and
>> changes which sound a bit intrusive for the stable kernel. Considering
>> that CMA was known to be problematic and there are still some issues
>> left I do not think this is worth the trouble/risk.
> 
> CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
> removed due to Mel's high atomic reserve series.
> 
> That series is rather large and has some problems so I think that it
> is not suitable for stable tree.
> 
> Thanks.
> 

OK, I know, thank you very much.

Thanks,
Xishi Qiu

> .
> 

Re: [RFC] mm: a question about high-order check in __zone_watermark_ok()

[Xishi Qiu]

On 2016/9/28 13:52, Joonsoo Kim wrote:

> On Mon, Sep 26, 2016 at 01:02:31PM +0200, Michal Hocko wrote:
>> On Mon 26-09-16 18:17:50, Xishi Qiu wrote:
>>> On 2016/9/26 17:43, Michal Hocko wrote:
>>>
>>>> On Mon 26-09-16 17:16:54, Xishi Qiu wrote:
>>>>> On 2016/9/26 16:58, Michal Hocko wrote:
>>>>>
>>>>>> On Mon 26-09-16 16:47:57, Xishi Qiu wrote:
>>>>>>> commit 97a16fc82a7c5b0cfce95c05dfb9561e306ca1b1
>>>>>>> (mm, page_alloc: only enforce watermarks for order-0 allocations)
>>>>>>> rewrite the high-order check in __zone_watermark_ok(), but I think it
>>>>>>> quietly fix a bug. Please see the following.
>>>>>>>
>>>>>>> Before this patch, the high-order check is this:
>>>>>>> __zone_watermark_ok()
>>>>>>> 	...
>>>>>>> 	for (o = 0; o < order; o++) {
>>>>>>> 		/* At the next order, this order's pages become unavailable */
>>>>>>> 		free_pages -= z->free_area[o].nr_free << o;
>>>>>>>
>>>>>>> 		/* Require fewer higher order pages to be free */
>>>>>>> 		min >>= 1;
>>>>>>>
>>>>>>> 		if (free_pages <= min)
>>>>>>> 			return false;
>>>>>>> 	}
>>>>>>> 	...
>>>>>>>
>>>>>>> If we have cma memory, and we alloc a high-order movable page, then it's right.
>>>>>>>
>>>>>>> But if we alloc a high-order unmovable page(e.g. alloc kernel stack in dup_task_struct()),
>>>>>>> and there are a lot of high-order cma pages, but little high-order unmovable
>>>>>>> pages, the it is still return *true*, but we will alloc *failed* finally, because
>>>>>>> we cannot fallback from migrate_unmovable to migrate_cma, right?
>>>>>>
>>>>>> AFAIR CMA wmark check was always tricky and the above commit has made
>>>>>> the situation at least a bit more clear. Anyway IIRC 
>>>>>>
>>>>>> #ifdef CONFIG_CMA
>>>>>> 	/* If allocation can't use CMA areas don't use free CMA pages */
>>>>>> 	if (!(alloc_flags & ALLOC_CMA))
>>>>>> 		free_cma = zone_page_state(z, NR_FREE_CMA_PAGES);
>>>>>> #endif
>>>>>>
>>>>>> 	if (free_pages - free_cma <= min + z->lowmem_reserve[classzone_idx])
>>>>>> 		return false;
>>>>>>
>>>>>> should reduce the prioblem because a lot of CMA pages should just get us
>>>>>> below the wmark + reserve boundary.
>>>>>
>>>>> Hi Michal,
>>>>>
>>>>> If we have many high-order cma pages, and the left pages (unmovable/movable/reclaimable)
>>>>> are also enough, but they are fragment, then it will triger the problem.
>>>>> If we alloc a high-order unmovable page, water mark check return *true*, but we
>>>>> will alloc *failed*, right?
>>>>
>>>> As Vlastimil has written. There were known issues with the wmark checks
>>>> and high order requests.
>>>
>>> Shall we backport to stable?
>>
>> I dunno, it was a part of a larger series with high atomic reserves and
>> changes which sound a bit intrusive for the stable kernel. Considering
>> that CMA was known to be problematic and there are still some issues
>> left I do not think this is worth the trouble/risk.
> 
> CMA problem is known one. I mentioned it on my ZONE_CMA series v1 but
> removed due to Mel's high atomic reserve series.
> 
> That series is rather large and has some problems so I think that it
> is not suitable for stable tree.
> 
> Thanks.
> 

OK, I know, thank you very much.

Thanks,
Xishi Qiu

> .
> 



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