Messages in this thread | | | Subject | Re: [PATCH v2 2/5] mm/migrate: add a direction parameter to migrate_vma | From | Ralph Campbell <> | Date | Mon, 20 Jul 2020 13:49:09 -0700 |
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On 7/20/20 12:59 PM, Jason Gunthorpe wrote: > On Mon, Jul 20, 2020 at 12:54:53PM -0700, Ralph Campbell wrote: >>>> diff --git a/include/linux/migrate.h b/include/linux/migrate.h >>>> index 3e546cbf03dd..620f2235d7d4 100644 >>>> +++ b/include/linux/migrate.h >>>> @@ -180,6 +180,11 @@ static inline unsigned long migrate_pfn(unsigned long pfn) >>>> return (pfn << MIGRATE_PFN_SHIFT) | MIGRATE_PFN_VALID; >>>> } >>>> +enum migrate_vma_direction { >>>> + MIGRATE_VMA_FROM_SYSTEM, >>>> + MIGRATE_VMA_FROM_DEVICE_PRIVATE, >>>> +}; >>> >>> I would have guessed this is more natural as _FROM_DEVICE_ and >>> TO_DEVICE_ ? >> >> The caller controls where the destination memory is allocated so it isn't >> necessarily device private memory, it could be from system to system. >> The use case for system to system memory migration is for hardware >> like ARM SMMU or PCIe ATS where a single set of page tables is shared by >> the device and a CPU process over a coherent system memory bus. >> Also many integrated GPUs in SOCs fall into this category too. > > Maybe just TO/FROM_DEIVCE then? Even though the memory is not > DEVICE_PRIVATE it is still device owned pages right? > >> So to me, it makes more sense to specify the direction based on the >> source location. > > It feels strange because the driver doesn't always know or control the > source? > > Jason >
The driver can't really know where the source is currently located because the API is designed to not initially hold the page locks, migrate_vma_setup() only knows the source once it holds the page table locks and isolates/locks the pages being migrated. The direction and pgmap_owner are supposed to filter which pages the caller is interested in migrating. Perhaps the direction should instead be a flags field with separate bits for system memory and device private memory selecting source candidates for migration. I can imagine use cases for all 4 combinations of d->d, d->s, s->d, and s->s being valid.
I didn't really think a direction was needed, this was something that Christoph Hellwig seemed to think made the API safer.
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