This is about VFIO aka PCI passthrough used from QEMU.
KVM is irrelevant here.
QEMU is a machine emulator. It allocates guest RAM from anonymous memory
and these pages are movable which is ok. They may happen to be allocated
from the contiguous memory allocation zone (CMA). Which is also ok as
long they are movable.
However if the guest starts using VFIO (which can be hotplugged into
the guest), in most cases it involves DMA which requires guest RAM pages
to be pinned and not move once their addresses are programmed to
the hardware for DMA.
So we end up in a situation when quite many pages in CMA are not movable
anymore. And we get bunch of these:
[77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
[77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
[77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
This is a very rough patch to start the conversation about how to move
pages properly. mm/page_alloc.c does this and
arch/powerpc/mm/mmu_context_iommu.c exploits it.
Please do not comment on the style and code placement,
this is just to give some context :)
Obviously, this does not work well - it manages to migrate only few pages
and crashes as it is missing locks/disabling interrupts and I probably
should not just remove pages from LRU list (normally, I guess, only these
can migrate) and a million of other things.
The questions are:
- what is the correct way of telling if the page is in CMA?
is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
- how to tell MM to move page away? I am calling migrate_pages() with
an get_new_page callback which allocates a page with GFP_USER but without
GFP_MOVABLE which should allocate new page out of CMA which seems ok but
there is a little convern that we might want to add MOVABLE back when
VFIO device is unplugged from the guest.
- do I need to isolate pages by using isolate_migratepages_range,
reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
I dropped them for now and the patch uses only @migratepages from
the compact_control struct.
- are there any flags in madvise() to address this (could not
locate any relevant)?
- what else is missing? disabled interrupts? locks?
Thanks!
Signed-off-by: Alexey Kardashevskiy <[email protected]>
---
arch/powerpc/mm/mmu_context_iommu.c | 40 +++++++++++++++++++++++++++++++------
mm/page_alloc.c | 36 +++++++++++++++++++++++++++++++++
2 files changed, 70 insertions(+), 6 deletions(-)
diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
index da6a216..bf6850e 100644
--- a/arch/powerpc/mm/mmu_context_iommu.c
+++ b/arch/powerpc/mm/mmu_context_iommu.c
@@ -72,12 +72,15 @@ bool mm_iommu_preregistered(void)
}
EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
+extern int mm_iommu_move_page(unsigned long pfn);
+
long mm_iommu_get(unsigned long ua, unsigned long entries,
struct mm_iommu_table_group_mem_t **pmem)
{
struct mm_iommu_table_group_mem_t *mem;
long i, j, ret = 0, locked_entries = 0;
struct page *page = NULL;
+ unsigned long moved = 0, tried = 0;
if (!current || !current->mm)
return -ESRCH; /* process exited */
@@ -122,15 +125,29 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
}
for (i = 0; i < entries; ++i) {
+ unsigned long pfn;
+
if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
1/* pages */, 1/* iswrite */, &page)) {
- for (j = 0; j < i; ++j)
- put_page(pfn_to_page(
- mem->hpas[j] >> PAGE_SHIFT));
- vfree(mem->hpas);
- kfree(mem);
ret = -EFAULT;
- goto unlock_exit;
+ goto put_exit;
+ }
+
+ pfn = page_to_pfn(page);
+ if (get_pageblock_migratetype(page) == MIGRATE_CMA)
+ {
+ unsigned long pfnold = pfn;
+ put_page(page);
+ page = NULL;
+ mm_iommu_move_page(pfn);
+ if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
+ 1/* pages */, 1/* iswrite */, &page)) {
+ ret = -EFAULT;
+ goto put_exit;
+ }
+ pfn = page_to_pfn(page);
+ if (pfn != pfnold)
+ ++moved;
}
mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
@@ -144,6 +161,17 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
list_add_rcu(&mem->next, ¤t->mm->context.iommu_group_mem_list);
+ printk("***K*** %s %u: tried %lu, moved %lu of %lu\n", __func__, __LINE__,
+ tried, moved, entries);
+
+put_exit:
+ if (ret) {
+ for (j = 0; j < i; ++j)
+ put_page(pfn_to_page(mem->hpas[j] >> PAGE_SHIFT));
+ vfree(mem->hpas);
+ kfree(mem);
+ }
+
unlock_exit:
if (locked_entries && ret)
mm_iommu_adjust_locked_vm(current->mm, locked_entries, false);
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index ef19f22..0639cce 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -7008,3 +7008,39 @@ bool is_free_buddy_page(struct page *page)
return order < MAX_ORDER;
}
#endif
+
+static struct page *mm_iommu_new_page(struct page *page, unsigned long private,
+ int **reason)
+{
+ /*
+ * Anything but not GFP_MOVABLE!
+ */
+ return alloc_page(GFP_USER);
+}
+
+static void mm_iommu_free_page(struct page *page, unsigned long private)
+{
+ free_page(page_to_pfn(page) << PAGE_SHIFT);
+}
+
+int mm_iommu_move_page(unsigned long pfn)
+{
+ unsigned long ret, nr_reclaimed;
+ struct compact_control cc = {
+ .nr_migratepages = 0,
+ .order = -1,
+ .zone = page_zone(pfn_to_page(pfn)),
+ .mode = MIGRATE_SYNC,
+ .ignore_skip_hint = true,
+ };
+ struct page *page = pfn_to_page(pfn);
+ INIT_LIST_HEAD(&cc.migratepages);
+
+ if (PageLRU(page)) {
+ list_del(&page->lru);
+ }
+ ret = migrate_pages(&cc.migratepages, mm_iommu_new_page,
+ mm_iommu_free_page, 0, cc.mode, MR_CMA);
+
+ return ret;
+}
--
2.4.0.rc3.8.gfb3e7d5
On 08/05/2015 06:08 PM, Alexey Kardashevskiy wrote:
> This is about VFIO aka PCI passthrough used from QEMU.
> KVM is irrelevant here.
Anyone, any idea? Or the question is way too stupid? :)
>
> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
> and these pages are movable which is ok. They may happen to be allocated
> from the contiguous memory allocation zone (CMA). Which is also ok as
> long they are movable.
>
> However if the guest starts using VFIO (which can be hotplugged into
> the guest), in most cases it involves DMA which requires guest RAM pages
> to be pinned and not move once their addresses are programmed to
> the hardware for DMA.
>
> So we end up in a situation when quite many pages in CMA are not movable
> anymore. And we get bunch of these:
>
> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
>
> This is a very rough patch to start the conversation about how to move
> pages properly. mm/page_alloc.c does this and
> arch/powerpc/mm/mmu_context_iommu.c exploits it.
>
> Please do not comment on the style and code placement,
> this is just to give some context :)
>
> Obviously, this does not work well - it manages to migrate only few pages
> and crashes as it is missing locks/disabling interrupts and I probably
> should not just remove pages from LRU list (normally, I guess, only these
> can migrate) and a million of other things.
>
> The questions are:
>
> - what is the correct way of telling if the page is in CMA?
> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
>
> - how to tell MM to move page away? I am calling migrate_pages() with
> an get_new_page callback which allocates a page with GFP_USER but without
> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
> there is a little convern that we might want to add MOVABLE back when
> VFIO device is unplugged from the guest.
>
> - do I need to isolate pages by using isolate_migratepages_range,
> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
> I dropped them for now and the patch uses only @migratepages from
> the compact_control struct.
>
> - are there any flags in madvise() to address this (could not
> locate any relevant)?
>
> - what else is missing? disabled interrupts? locks?
>
> Thanks!
>
>
> Signed-off-by: Alexey Kardashevskiy <[email protected]>
> ---
> arch/powerpc/mm/mmu_context_iommu.c | 40 +++++++++++++++++++++++++++++++------
> mm/page_alloc.c | 36 +++++++++++++++++++++++++++++++++
> 2 files changed, 70 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> index da6a216..bf6850e 100644
> --- a/arch/powerpc/mm/mmu_context_iommu.c
> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> @@ -72,12 +72,15 @@ bool mm_iommu_preregistered(void)
> }
> EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
>
> +extern int mm_iommu_move_page(unsigned long pfn);
> +
> long mm_iommu_get(unsigned long ua, unsigned long entries,
> struct mm_iommu_table_group_mem_t **pmem)
> {
> struct mm_iommu_table_group_mem_t *mem;
> long i, j, ret = 0, locked_entries = 0;
> struct page *page = NULL;
> + unsigned long moved = 0, tried = 0;
>
> if (!current || !current->mm)
> return -ESRCH; /* process exited */
> @@ -122,15 +125,29 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
> }
>
> for (i = 0; i < entries; ++i) {
> + unsigned long pfn;
> +
> if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> 1/* pages */, 1/* iswrite */, &page)) {
> - for (j = 0; j < i; ++j)
> - put_page(pfn_to_page(
> - mem->hpas[j] >> PAGE_SHIFT));
> - vfree(mem->hpas);
> - kfree(mem);
> ret = -EFAULT;
> - goto unlock_exit;
> + goto put_exit;
> + }
> +
> + pfn = page_to_pfn(page);
> + if (get_pageblock_migratetype(page) == MIGRATE_CMA)
> + {
> + unsigned long pfnold = pfn;
> + put_page(page);
> + page = NULL;
> + mm_iommu_move_page(pfn);
> + if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> + 1/* pages */, 1/* iswrite */, &page)) {
> + ret = -EFAULT;
> + goto put_exit;
> + }
> + pfn = page_to_pfn(page);
> + if (pfn != pfnold)
> + ++moved;
> }
>
> mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
> @@ -144,6 +161,17 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
>
> list_add_rcu(&mem->next, ¤t->mm->context.iommu_group_mem_list);
>
> + printk("***K*** %s %u: tried %lu, moved %lu of %lu\n", __func__, __LINE__,
> + tried, moved, entries);
> +
> +put_exit:
> + if (ret) {
> + for (j = 0; j < i; ++j)
> + put_page(pfn_to_page(mem->hpas[j] >> PAGE_SHIFT));
> + vfree(mem->hpas);
> + kfree(mem);
> + }
> +
> unlock_exit:
> if (locked_entries && ret)
> mm_iommu_adjust_locked_vm(current->mm, locked_entries, false);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ef19f22..0639cce 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7008,3 +7008,39 @@ bool is_free_buddy_page(struct page *page)
> return order < MAX_ORDER;
> }
> #endif
> +
> +static struct page *mm_iommu_new_page(struct page *page, unsigned long private,
> + int **reason)
> +{
> + /*
> + * Anything but not GFP_MOVABLE!
> + */
> + return alloc_page(GFP_USER);
> +}
> +
> +static void mm_iommu_free_page(struct page *page, unsigned long private)
> +{
> + free_page(page_to_pfn(page) << PAGE_SHIFT);
> +}
> +
> +int mm_iommu_move_page(unsigned long pfn)
> +{
> + unsigned long ret, nr_reclaimed;
> + struct compact_control cc = {
> + .nr_migratepages = 0,
> + .order = -1,
> + .zone = page_zone(pfn_to_page(pfn)),
> + .mode = MIGRATE_SYNC,
> + .ignore_skip_hint = true,
> + };
> + struct page *page = pfn_to_page(pfn);
> + INIT_LIST_HEAD(&cc.migratepages);
> +
> + if (PageLRU(page)) {
> + list_del(&page->lru);
> + }
> + ret = migrate_pages(&cc.migratepages, mm_iommu_new_page,
> + mm_iommu_free_page, 0, cc.mode, MR_CMA);
> +
> + return ret;
> +}
>
--
Alexey
On 08/05/2015 10:08 AM, Alexey Kardashevskiy wrote:
> This is about VFIO aka PCI passthrough used from QEMU.
> KVM is irrelevant here.
>
> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
> and these pages are movable which is ok. They may happen to be allocated
> from the contiguous memory allocation zone (CMA). Which is also ok as
> long they are movable.
>
> However if the guest starts using VFIO (which can be hotplugged into
> the guest), in most cases it involves DMA which requires guest RAM pages
> to be pinned and not move once their addresses are programmed to
> the hardware for DMA.
>
> So we end up in a situation when quite many pages in CMA are not movable
> anymore. And we get bunch of these:
>
> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
IIRC CMA was for mobile devices and their camera/codec drivers and you
don't use QEMU on those? What do you need CMA for in your case?
> This is a very rough patch to start the conversation about how to move
> pages properly. mm/page_alloc.c does this and
> arch/powerpc/mm/mmu_context_iommu.c exploits it.
OK such conversation should probably start by mentioning the VM_PINNED
effort by Peter Zijlstra: https://lkml.org/lkml/2014/5/26/345
It's more general approach to dealing with pinned pages, and moving them
out of CMA area (and compacting them in general) prior pinning is one of
the things that should be done within that framework.
Then there's the effort to enable migrating pages other than LRU during
compaction (and thus CMA allocation): https://lwn.net/Articles/650864/
I don't know if that would be applicable in your use case, i.e. are the
pins for DMA short-lived and can the isolation/migration code wait a bit
for the transfer to finish so it can grab them, or something?
>
> Please do not comment on the style and code placement,
> this is just to give some context :)
>
> Obviously, this does not work well - it manages to migrate only few pages
> and crashes as it is missing locks/disabling interrupts and I probably
> should not just remove pages from LRU list (normally, I guess, only these
> can migrate) and a million of other things.
>
> The questions are:
>
> - what is the correct way of telling if the page is in CMA?
> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
Should be.
> - how to tell MM to move page away? I am calling migrate_pages() with
> an get_new_page callback which allocates a page with GFP_USER but without
> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
> there is a little convern that we might want to add MOVABLE back when
> VFIO device is unplugged from the guest.
Hmm, once the page is allocated, then the migratetype is not tracked
anywhere (except in page_owner debug data). But the unmovable
allocations might exhaust available unmovable pageblocks and lead to
fragmentation. So "add MOVABLE back" would be too late. Instead we would
need to tell the allocator somehow to give us movable page but outside
of CMA. CMA's own __alloc_contig_migrate_range() avoids this problem by
allocating movable pages, but the range has been already page-isolated
and thus the allocator won't see the pages there. You obviously can't
take this approach and isolate all CMA pageblocks like that. That smells
like a new __GFP_FLAG, meh.
> - do I need to isolate pages by using isolate_migratepages_range,
> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
> I dropped them for now and the patch uses only @migratepages from
> the compact_control struct.
You don't have to do reclaim_clean_pages_from_list(), but the isolation
has to be careful, yeah.
> - are there any flags in madvise() to address this (could not
> locate any relevant)?
AFAIK there's no madvise(I_WILL_BE_PINNING_THIS_RANGE).
> - what else is missing? disabled interrupts? locks?
See what isolate_migratepages_block() does.
> Thanks!
>
>
> Signed-off-by: Alexey Kardashevskiy <[email protected]>
> ---
> arch/powerpc/mm/mmu_context_iommu.c | 40 +++++++++++++++++++++++++++++++------
> mm/page_alloc.c | 36 +++++++++++++++++++++++++++++++++
> 2 files changed, 70 insertions(+), 6 deletions(-)
>
> diff --git a/arch/powerpc/mm/mmu_context_iommu.c b/arch/powerpc/mm/mmu_context_iommu.c
> index da6a216..bf6850e 100644
> --- a/arch/powerpc/mm/mmu_context_iommu.c
> +++ b/arch/powerpc/mm/mmu_context_iommu.c
> @@ -72,12 +72,15 @@ bool mm_iommu_preregistered(void)
> }
> EXPORT_SYMBOL_GPL(mm_iommu_preregistered);
>
> +extern int mm_iommu_move_page(unsigned long pfn);
> +
> long mm_iommu_get(unsigned long ua, unsigned long entries,
> struct mm_iommu_table_group_mem_t **pmem)
> {
> struct mm_iommu_table_group_mem_t *mem;
> long i, j, ret = 0, locked_entries = 0;
> struct page *page = NULL;
> + unsigned long moved = 0, tried = 0;
>
> if (!current || !current->mm)
> return -ESRCH; /* process exited */
> @@ -122,15 +125,29 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
> }
>
> for (i = 0; i < entries; ++i) {
> + unsigned long pfn;
> +
> if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> 1/* pages */, 1/* iswrite */, &page)) {
> - for (j = 0; j < i; ++j)
> - put_page(pfn_to_page(
> - mem->hpas[j] >> PAGE_SHIFT));
> - vfree(mem->hpas);
> - kfree(mem);
> ret = -EFAULT;
> - goto unlock_exit;
> + goto put_exit;
> + }
> +
> + pfn = page_to_pfn(page);
> + if (get_pageblock_migratetype(page) == MIGRATE_CMA)
> + {
> + unsigned long pfnold = pfn;
> + put_page(page);
> + page = NULL;
> + mm_iommu_move_page(pfn);
> + if (1 != get_user_pages_fast(ua + (i << PAGE_SHIFT),
> + 1/* pages */, 1/* iswrite */, &page)) {
> + ret = -EFAULT;
> + goto put_exit;
> + }
> + pfn = page_to_pfn(page);
> + if (pfn != pfnold)
> + ++moved;
> }
>
> mem->hpas[i] = page_to_pfn(page) << PAGE_SHIFT;
> @@ -144,6 +161,17 @@ long mm_iommu_get(unsigned long ua, unsigned long entries,
>
> list_add_rcu(&mem->next, ¤t->mm->context.iommu_group_mem_list);
>
> + printk("***K*** %s %u: tried %lu, moved %lu of %lu\n", __func__, __LINE__,
> + tried, moved, entries);
> +
> +put_exit:
> + if (ret) {
> + for (j = 0; j < i; ++j)
> + put_page(pfn_to_page(mem->hpas[j] >> PAGE_SHIFT));
> + vfree(mem->hpas);
> + kfree(mem);
> + }
> +
> unlock_exit:
> if (locked_entries && ret)
> mm_iommu_adjust_locked_vm(current->mm, locked_entries, false);
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index ef19f22..0639cce 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -7008,3 +7008,39 @@ bool is_free_buddy_page(struct page *page)
> return order < MAX_ORDER;
> }
> #endif
> +
> +static struct page *mm_iommu_new_page(struct page *page, unsigned long private,
> + int **reason)
> +{
> + /*
> + * Anything but not GFP_MOVABLE!
> + */
> + return alloc_page(GFP_USER);
> +}
> +
> +static void mm_iommu_free_page(struct page *page, unsigned long private)
> +{
> + free_page(page_to_pfn(page) << PAGE_SHIFT);
> +}
> +
> +int mm_iommu_move_page(unsigned long pfn)
> +{
> + unsigned long ret, nr_reclaimed;
> + struct compact_control cc = {
> + .nr_migratepages = 0,
> + .order = -1,
> + .zone = page_zone(pfn_to_page(pfn)),
> + .mode = MIGRATE_SYNC,
> + .ignore_skip_hint = true,
> + };
> + struct page *page = pfn_to_page(pfn);
> + INIT_LIST_HEAD(&cc.migratepages);
> +
> + if (PageLRU(page)) {
> + list_del(&page->lru);
> + }
> + ret = migrate_pages(&cc.migratepages, mm_iommu_new_page,
> + mm_iommu_free_page, 0, cc.mode, MR_CMA);
> +
> + return ret;
> +}
>
On 08/17/2015 05:45 PM, Vlastimil Babka wrote:
> On 08/05/2015 10:08 AM, Alexey Kardashevskiy wrote:
>> This is about VFIO aka PCI passthrough used from QEMU.
>> KVM is irrelevant here.
>>
>> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
>> and these pages are movable which is ok. They may happen to be allocated
>> from the contiguous memory allocation zone (CMA). Which is also ok as
>> long they are movable.
>>
>> However if the guest starts using VFIO (which can be hotplugged into
>> the guest), in most cases it involves DMA which requires guest RAM pages
>> to be pinned and not move once their addresses are programmed to
>> the hardware for DMA.
>>
>> So we end up in a situation when quite many pages in CMA are not movable
>> anymore. And we get bunch of these:
>>
>> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
>> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
>> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
>
> IIRC CMA was for mobile devices and their camera/codec drivers and you
> don't use QEMU on those? What do you need CMA for in your case?
I do not want QEMU to get memory from CMA, this is my point. It just
happens sometime that the kernel allocates movable pages from there.
>
>> This is a very rough patch to start the conversation about how to move
>> pages properly. mm/page_alloc.c does this and
>> arch/powerpc/mm/mmu_context_iommu.c exploits it.
>
> OK such conversation should probably start by mentioning the VM_PINNED
> effort by Peter Zijlstra: https://lkml.org/lkml/2014/5/26/345
>
> It's more general approach to dealing with pinned pages, and moving them
> out of CMA area (and compacting them in general) prior pinning is one of
> the things that should be done within that framework.
And I assume these patches did not go anywhere, right?...
> Then there's the effort to enable migrating pages other than LRU during
> compaction (and thus CMA allocation): https://lwn.net/Articles/650864/
> I don't know if that would be applicable in your use case, i.e. are the
> pins for DMA short-lived and can the isolation/migration code wait a bit
> for the transfer to finish so it can grab them, or something?
Pins for DMA are long-lived, pretty much as long as the guest is running.
So this "compaction" is too late.
>>
>> Please do not comment on the style and code placement,
>> this is just to give some context :)
>>
>> Obviously, this does not work well - it manages to migrate only few pages
>> and crashes as it is missing locks/disabling interrupts and I probably
>> should not just remove pages from LRU list (normally, I guess, only these
>> can migrate) and a million of other things.
>>
>> The questions are:
>>
>> - what is the correct way of telling if the page is in CMA?
>> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
>
> Should be.
>
>> - how to tell MM to move page away? I am calling migrate_pages() with
>> an get_new_page callback which allocates a page with GFP_USER but without
>> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
>> there is a little convern that we might want to add MOVABLE back when
>> VFIO device is unplugged from the guest.
>
> Hmm, once the page is allocated, then the migratetype is not tracked
> anywhere (except in page_owner debug data). But the unmovable allocations
> might exhaust available unmovable pageblocks and lead to fragmentation. So
> "add MOVABLE back" would be too late. Instead we would need to tell the
>allocator somehow to give us movable page but outside of CMA.
It is it movable, why do we care if it is in CMA or not?
> CMA's own
> __alloc_contig_migrate_range() avoids this problem by allocating movable
> pages, but the range has been already page-isolated and thus the allocator
> won't see the pages there.You obviously can't take this approach and
> isolate all CMA pageblocks like that. That smells like a new __GFP_FLAG, meh.
I understood (more or less) all of it except the __GFP_FLAG - when/what
would use it?
>> - do I need to isolate pages by using isolate_migratepages_range,
>> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
>> I dropped them for now and the patch uses only @migratepages from
>> the compact_control struct.
>
> You don't have to do reclaim_clean_pages_from_list(), but the isolation has
> to be careful, yeah.
The isolation here means the whole CMA zone isolation which I "obviously
can't take this approach"? :)
>> - are there any flags in madvise() to address this (could not
>> locate any relevant)?
>
> AFAIK there's no madvise(I_WILL_BE_PINNING_THIS_RANGE)
>
>> - what else is missing? disabled interrupts? locks?
>
> See what isolate_migratepages_block() does.
Thanks for the pointers! I'll have a closer look at Peter's patchset.
--
Alexey
On 08/17/2015 11:11 AM, Alexey Kardashevskiy wrote:
> On 08/17/2015 05:45 PM, Vlastimil Babka wrote:
>> On 08/05/2015 10:08 AM, Alexey Kardashevskiy wrote:
>>> This is about VFIO aka PCI passthrough used from QEMU.
>>> KVM is irrelevant here.
>>>
>>> QEMU is a machine emulator. It allocates guest RAM from anonymous memory
>>> and these pages are movable which is ok. They may happen to be allocated
>>> from the contiguous memory allocation zone (CMA). Which is also ok as
>>> long they are movable.
>>>
>>> However if the guest starts using VFIO (which can be hotplugged into
>>> the guest), in most cases it involves DMA which requires guest RAM pages
>>> to be pinned and not move once their addresses are programmed to
>>> the hardware for DMA.
>>>
>>> So we end up in a situation when quite many pages in CMA are not movable
>>> anymore. And we get bunch of these:
>>>
>>> [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
>>> [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
>>> [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
>>
>> IIRC CMA was for mobile devices and their camera/codec drivers and you
>> don't use QEMU on those? What do you need CMA for in your case?
>
>
> I do not want QEMU to get memory from CMA, this is my point. It just
> happens sometime that the kernel allocates movable pages from there.
I meant why the kernel used for QEMU has also CMA enabled and used (for
something else)? CMA is mostly used on mobile devices and they don't run
QEMU?
>
>>
>>> This is a very rough patch to start the conversation about how to move
>>> pages properly. mm/page_alloc.c does this and
>>> arch/powerpc/mm/mmu_context_iommu.c exploits it.
>>
>> OK such conversation should probably start by mentioning the VM_PINNED
>> effort by Peter Zijlstra: https://lkml.org/lkml/2014/5/26/345
>>
>> It's more general approach to dealing with pinned pages, and moving them
>> out of CMA area (and compacting them in general) prior pinning is one of
>> the things that should be done within that framework.
>
>
> And I assume these patches did not go anywhere, right?...
Not yet :)
>> Then there's the effort to enable migrating pages other than LRU during
>> compaction (and thus CMA allocation): https://lwn.net/Articles/650864/
>> I don't know if that would be applicable in your use case, i.e. are the
>> pins for DMA short-lived and can the isolation/migration code wait a bit
>> for the transfer to finish so it can grab them, or something?
>
>
> Pins for DMA are long-lived, pretty much as long as the guest is running.
> So this "compaction" is too late.
Oh, OK.
>>>
>>> Please do not comment on the style and code placement,
>>> this is just to give some context :)
>>>
>>> Obviously, this does not work well - it manages to migrate only few pages
>>> and crashes as it is missing locks/disabling interrupts and I probably
>>> should not just remove pages from LRU list (normally, I guess, only these
>>> can migrate) and a million of other things.
>>>
>>> The questions are:
>>>
>>> - what is the correct way of telling if the page is in CMA?
>>> is (get_pageblock_migratetype(page) == MIGRATE_CMA) good enough?
>>
>> Should be.
>>
>>> - how to tell MM to move page away? I am calling migrate_pages() with
>>> an get_new_page callback which allocates a page with GFP_USER but without
>>> GFP_MOVABLE which should allocate new page out of CMA which seems ok but
>>> there is a little convern that we might want to add MOVABLE back when
>>> VFIO device is unplugged from the guest.
>>
>> Hmm, once the page is allocated, then the migratetype is not tracked
>> anywhere (except in page_owner debug data). But the unmovable allocations
>> might exhaust available unmovable pageblocks and lead to fragmentation. So
>> "add MOVABLE back" would be too late. Instead we would need to tell the
>> allocator somehow to give us movable page but outside of CMA.
>
> It is it movable, why do we care if it is in CMA or not?
I did assume your pages are mostly movable, but with some temporary pins
they might not be movable reliably at arbitrary time. But if you say the
pins are long lived then it's probably best allocated without MOVABLE.
If the device is later unplugged, sync compaction will eventually move
the pages out of unmovable pageblocks.
>> CMA's own
>> __alloc_contig_migrate_range() avoids this problem by allocating movable
>> pages, but the range has been already page-isolated and thus the allocator
>> won't see the pages there.You obviously can't take this approach and
>> isolate all CMA pageblocks like that. That smells like a new __GFP_FLAG, meh.
>
>
> I understood (more or less) all of it except the __GFP_FLAG - when/what
> would use it?
Nevermind, wrt above.
>>> - do I need to isolate pages by using isolate_migratepages_range,
>>> reclaim_clean_pages_from_list like __alloc_contig_migrate_range does?
>>> I dropped them for now and the patch uses only @migratepages from
>>> the compact_control struct.
>>
>> You don't have to do reclaim_clean_pages_from_list(), but the isolation has
>> to be careful, yeah.
>
>
> The isolation here means the whole CMA zone isolation which I "obviously
> can't take this approach"? :)
Ah no, that's isolation from lru lists in this context. Unfortunately
the same word is used for both.
On Mon, 2015-08-17 at 19:11 +1000, Alexey Kardashevskiy wrote:
> On 08/17/2015 05:45 PM, Vlastimil Babka wrote:
> > On 08/05/2015 10:08 AM, Alexey Kardashevskiy wrote:
> > > This is about VFIO aka PCI passthrough used from QEMU.
> > > KVM is irrelevant here.
> > >
> > > QEMU is a machine emulator. It allocates guest RAM from anonymous
> > > memory
> > > and these pages are movable which is ok. They may happen to be
> > > allocated
> > > from the contiguous memory allocation zone (CMA). Which is also
> > > ok as
> > > long they are movable.
> > >
> > > However if the guest starts using VFIO (which can be hotplugged
> > > into
> > > the guest), in most cases it involves DMA which requires guest
> > > RAM pages
> > > to be pinned and not move once their addresses are programmed to
> > > the hardware for DMA.
> > >
> > > So we end up in a situation when quite many pages in CMA are not
> > > movable
> > > anymore. And we get bunch of these:
> > >
> > > [77306.513966] alloc_contig_range: [1f3800, 1f78c4) PFNs busy
> > > [77306.514448] alloc_contig_range: [1f3800, 1f78c8) PFNs busy
> > > [77306.514927] alloc_contig_range: [1f3800, 1f78cc) PFNs busy
> >
> > IIRC CMA was for mobile devices and their camera/codec drivers and
> > you
> > don't use QEMU on those? What do you need CMA for in your case?
>
> I do not want QEMU to get memory from CMA, this is my point. It just
> happens sometime that the kernel allocates movable pages from there.
You may want to explain why we have a CMA in the first place.... our
KVM implementation needs to allocate large chunks of physically
contiguous memory for each guest in order to contain the MMU hash table
for those guests.
We use a CMA whose size can be specified at boot but is generally a
pecentile of the total system memory to allocate these from.
However we don't want normal allocations that we *know* are going to be
pinned to be in that CMA, otherwise they would defeat its purpose, so
this patch is about moving stuff that we are about to pin out of the
CMA first.
Cheers,
Ben.
On Mon, 2015-08-17 at 11:53 +0200, Vlastimil Babka wrote:
> I meant why the kernel used for QEMU has also CMA enabled and used
> (for
> something else)? CMA is mostly used on mobile devices and they don't
> run
> QEMU?
I explained in a separeate reply but yes, we do use a CMA for KVM for
our MMU hash tables.
Cheers,
Ben.
On Mon, Aug 17, 2015 at 07:11:01PM +1000, Alexey Kardashevskiy wrote:
> >OK such conversation should probably start by mentioning the VM_PINNED
> >effort by Peter Zijlstra: https://lkml.org/lkml/2014/5/26/345
> >
> >It's more general approach to dealing with pinned pages, and moving them
> >out of CMA area (and compacting them in general) prior pinning is one of
> >the things that should be done within that framework.
>
>
> And I assume these patches did not go anywhere, right?...
I got lost in the IB code :/
Its on the TODO pile somewhere