On 6/21/2022 7:16 PM, Alistair Popple wrote:
David Hildenbrand <david@xxxxxxxxxx> writes:
On 21.06.22 18:08, Sierra Guiza, Alejandro (Alex) wrote:
On 6/21/2022 7:25 AM, David Hildenbrand wrote:
On 21.06.22 13:55, Alistair Popple wrote:
David Hildenbrand<david@xxxxxxxxxx> writes:
On 21.06.22 13:25, Felix Kuehling wrote:
Am 6/17/22 um 23:19 schrieb David Hildenbrand:
On 17.06.22 21:27, Sierra Guiza, Alejandro (Alex) wrote:
On 6/17/2022 12:33 PM, David Hildenbrand wrote:
On 17.06.22 19:20, Sierra Guiza, Alejandro (Alex) wrote:
On 6/17/2022 4:40 AM, David Hildenbrand wrote:
On 31.05.22 22:00, Alex Sierra wrote:
Device memory that is cache coherent from device and CPU point of view.
This is used on platforms that have an advanced system bus (like CAPI
or CXL). Any page of a process can be migrated to such memory. However,
no one should be allowed to pin such memory so that it can always be
evicted.
Signed-off-by: Alex Sierra<alex.sierra@xxxxxxx>
Acked-by: Felix Kuehling<Felix.Kuehling@xxxxxxx>
Reviewed-by: Alistair Popple<apopple@xxxxxxxxxx>
[hch: rebased ontop of the refcount changes,
removed is_dev_private_or_coherent_page]
Signed-off-by: Christoph Hellwig<hch@xxxxxx>
---
include/linux/memremap.h | 19 +++++++++++++++++++
mm/memcontrol.c | 7 ++++---
mm/memory-failure.c | 8 ++++++--
mm/memremap.c | 10 ++++++++++
mm/migrate_device.c | 16 +++++++---------
mm/rmap.c | 5 +++--
6 files changed, 49 insertions(+), 16 deletions(-)
diff --git a/include/linux/memremap.h b/include/linux/memremap.h
index 8af304f6b504..9f752ebed613 100644
--- a/include/linux/memremap.h
+++ b/include/linux/memremap.h
@@ -41,6 +41,13 @@ struct vmem_altmap {
* A more complete discussion of unaddressable memory may be found in
* include/linux/hmm.h and Documentation/vm/hmm.rst.
*
+ * MEMORY_DEVICE_COHERENT:
+ * Device memory that is cache coherent from device and CPU point of view. This
+ * is used on platforms that have an advanced system bus (like CAPI or CXL). A
+ * driver can hotplug the device memory using ZONE_DEVICE and with that memory
+ * type. Any page of a process can be migrated to such memory. However no one
Any page might not be right, I'm pretty sure. ... just thinking about special pages
like vdso, shared zeropage, ... pinned pages ...
Well, you cannot migrate long term pages, that's what I meant :)
+ * should be allowed to pin such memory so that it can always be evicted.
+ *
* MEMORY_DEVICE_FS_DAX:
* Host memory that has similar access semantics as System RAM i.e. DMA
* coherent and supports page pinning. In support of coordinating page
@@ -61,6 +68,7 @@ struct vmem_altmap {
enum memory_type {
/* 0 is reserved to catch uninitialized type fields */
MEMORY_DEVICE_PRIVATE = 1,
+ MEMORY_DEVICE_COHERENT,
MEMORY_DEVICE_FS_DAX,
MEMORY_DEVICE_GENERIC,
MEMORY_DEVICE_PCI_P2PDMA,
@@ -143,6 +151,17 @@ static inline bool folio_is_device_private(const struct folio *folio)
In general, this LGTM, and it should be correct with PageAnonExclusive I think.
However, where exactly is pinning forbidden?
Long-term pinning is forbidden since it would interfere with the device
memory manager owning the
device-coherent pages (e.g. evictions in TTM). However, normal pinning
is allowed on this device type.
I don't see updates to folio_is_pinnable() in this patch.
Device coherent type pages should return true here, as they are pinnable
pages.
That function is only called for long-term pinnings in try_grab_folio().
So wouldn't try_grab_folio() simply pin these pages? What am I missing?
As far as I understand this return NULL for long term pin pages.
Otherwise they get refcount incremented.
I don't follow.
You're saying
a) folio_is_pinnable() returns true for device coherent pages
and that
b) device coherent pages don't get long-term pinned
Yet, the code says
struct folio *try_grab_folio(struct page *page, int refs, unsigned int flags)
{
if (flags & FOLL_GET)
return try_get_folio(page, refs);
else if (flags & FOLL_PIN) {
struct folio *folio;
/*
* Can't do FOLL_LONGTERM + FOLL_PIN gup fast path if not in a
* right zone, so fail and let the caller fall back to the slow
* path.
*/
if (unlikely((flags & FOLL_LONGTERM) &&
!is_pinnable_page(page)))
return NULL;
...
return folio;
}
}
What prevents these pages from getting long-term pinned as stated in this patch?
Long-term pinning is handled by __gup_longterm_locked, which migrates
pages returned by __get_user_pages_locked that cannot be long-term
pinned. try_grab_folio is OK to grab the pages. Anything that can't be
long-term pinned will be migrated afterwards, and
__get_user_pages_locked will be retried. The migration of
DEVICE_COHERENT pages was implemented by Alistair in patch 5/13
("mm/gup: migrate device coherent pages when pinning instead of failing").
Thanks.
__gup_longterm_locked()->check_and_migrate_movable_pages()
Which checks folio_is_pinnable() and doesn't do anything if set.
Sorry to be dense here, but I don't see how what's stated in this patch
works without adjusting folio_is_pinnable().
Ugh, I think you might be right about try_grab_folio().
We didn't update folio_is_pinnable() to include device coherent pages
because device coherent pages are pinnable. It is really just
FOLL_LONGTERM that we want to prevent here.
For normal PUP that is done by my change in
check_and_migrate_movable_pages() which migrates pages being pinned with
FOLL_LONGTERM. But I think I incorrectly assumed we would take the
pte_devmap() path in gup_pte_range(), which we don't for coherent pages.
So I think the check in try_grab_folio() needs to be:
I think I said it already (and I might be wrong without reading the
code), but folio_is_pinnable() is *only* called for long-term pinnings.
It should actually be called folio_is_longterm_pinnable().
That's where that check should go, no?
David, I think you're right. We didn't catch this since the LONGTERM gup
test we added to hmm-test only calls to pin_user_pages. Apparently
try_grab_folio is called only from fast callers (ex.
pin_user_pages_fast/get_user_pages_fast). I have added a conditional
similar to what Alistair has proposed to return null on LONGTERM &&
(coherent_pages || folio_is_pinnable) at try_grab_folio. Also a new gup
test was added with LONGTERM set that calls pin_user_pages_fast.
Returning null under this condition it does causes the migration from
dev to system memory.
Why can't coherent memory simply put its checks into
folio_is_pinnable()? I don't get it why we have to do things differently
here.
I'd made the reasonable assumption that
folio_is_pinnable()/is_pinnable_page() were used to check if the
folio/page is pinnable or not regardless of FOLL_LONGTERM. Looking at
the code more closely though I see both are actually only used on paths
checking for FOLL_LONGTERM pinning.
So I agree - we should rename these
folio_is_longterm_pinnable()/is_longterm_pinnable_page() and add the
check for coherent pages there. Thanks for pointing that out.
- Alistair
Will do in the next patch series.
Regards,
Alex Sierra
Actually, Im having different problems with a call to PageAnonExclusive
from try_to_migrate_one during page fault from a HMM test that first
migrate pages to device private and forks to mark as COW these pages.
Apparently is catching the first BUG VM_BUG_ON_PGFLAGS(!PageAnon(page),
page)
With or without this series? A backtrace would be great.
