David Hildenbrand david@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 Sierra alex.sierra@amd.com >> Acked-by: Felix Kuehling Felix.Kuehling@amd.com >> Reviewed-by: Alistair Popple apopple@nvidia.com >> [hch: rebased ontop of the refcount changes, >> removed is_dev_private_or_coherent_page] >> Signed-off-by: Christoph Hellwig hch@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:
if (unlikely((flags & FOLL_LONGTERM) && (!is_pinnable_page(page) || is_device_coherent_page(page))))
- Alistair