On 6/21/2022 7:25 AM, David Hildenbrand wrote:
On 21.06.22 13:55, Alistair Popple wrote:
David Hildenbranddavid@redhat.com 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 Sierraalex.sierra@amd.com >>>> Acked-by: Felix KuehlingFelix.Kuehling@amd.com >>>> Reviewed-by: Alistair Poppleapopple@nvidia.com >>>> [hch: rebased ontop of the refcount changes, >>>> removed is_dev_private_or_coherent_page] >>>> Signed-off-by: Christoph Hellwighch@lst.de >>>> --- >>>> 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.
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)
Regards, Alex Sierra