Re: [PATCH 4/5] drm/xe: Helper to populate a userptr or hmmptr

["Hellstrom, Thomas" <thomas.hellstrom@intel.com>]

Hi,

On Mon, 2024-03-18 at 14:49 +0000, Zeng, Oak wrote:
> 
> 
> > -----Original Message-----
> > From: Hellstrom, Thomas <thomas.hellstrom@intel.com>
> > Sent: Monday, March 18, 2024 9:13 AM
> > To: intel-xe@lists.freedesktop.org; Zeng, Oak <oak.zeng@intel.com>
> > Cc: Brost, Matthew <matthew.brost@intel.com>; Welty, Brian
> > <brian.welty@intel.com>; airlied@gmail.com; Ghimiray, Himal Prasad
> > <himal.prasad.ghimiray@intel.com>
> > Subject: Re: [PATCH 4/5] drm/xe: Helper to populate a userptr or
> > hmmptr
> > 
> > Hi, Oak,
> > 
> > Found another thing, see below:
> > 
> > On Wed, 2024-03-13 at 23:35 -0400, Oak Zeng wrote:
> > > Add a helper function xe_hmm_populate_range to populate
> > > a a userptr or hmmptr range. This functions calls hmm_range_fault
> > > to read CPU page tables and populate all pfns/pages of this
> > > virtual address range.
> > > 
> > > If the populated page is system memory page, dma-mapping is
> > > performed
> > > to get a dma-address which can be used later for GPU to access
> > > pages.
> > > 
> > > If the populated page is device private page, we calculate the
> > > dpa (
> > > device physical address) of the page.
> > > 
> > > The dma-address or dpa is then saved in userptr's sg table. This
> > > is
> > > prepare work to replace the get_user_pages_fast code in userptr
> > > code
> > > path. The helper function will also be used to populate hmmptr
> > > later.
> > > 
> > > Signed-off-by: Oak Zeng <oak.zeng@intel.com>
> > > Co-developed-by: Niranjana Vishwanathapura
> > > <niranjana.vishwanathapura@intel.com>
> > > Signed-off-by: Niranjana Vishwanathapura
> > > <niranjana.vishwanathapura@intel.com>
> > > Cc: Matthew Brost <matthew.brost@intel.com>
> > > Cc: Thomas Hellström <thomas.hellstrom@intel.com>
> > > Cc: Brian Welty <brian.welty@intel.com>
> > > ---
> > >  drivers/gpu/drm/xe/Makefile |   3 +-
> > >  drivers/gpu/drm/xe/xe_hmm.c | 213
> > > ++++++++++++++++++++++++++++++++++++
> > >  drivers/gpu/drm/xe/xe_hmm.h |  12 ++
> > >  3 files changed, 227 insertions(+), 1 deletion(-)
> > >  create mode 100644 drivers/gpu/drm/xe/xe_hmm.c
> > >  create mode 100644 drivers/gpu/drm/xe/xe_hmm.h
> > > 
> > > diff --git a/drivers/gpu/drm/xe/Makefile
> > > b/drivers/gpu/drm/xe/Makefile
> > > index 840467080e59..29dcbc938b01 100644
> > > --- a/drivers/gpu/drm/xe/Makefile
> > > +++ b/drivers/gpu/drm/xe/Makefile
> > > @@ -143,7 +143,8 @@ xe-y += xe_bb.o \
> > >  	xe_wait_user_fence.o \
> > >  	xe_wa.o \
> > >  	xe_wopcm.o \
> > > -	xe_svm_devmem.o
> > > +	xe_svm_devmem.o \
> > > +	xe_hmm.o
> > > 
> > >  # graphics hardware monitoring (HWMON) support
> > >  xe-$(CONFIG_HWMON) += xe_hwmon.o
> > > diff --git a/drivers/gpu/drm/xe/xe_hmm.c
> > > b/drivers/gpu/drm/xe/xe_hmm.c
> > > new file mode 100644
> > > index 000000000000..c45c2447d386
> > > --- /dev/null
> > > +++ b/drivers/gpu/drm/xe/xe_hmm.c
> > > @@ -0,0 +1,213 @@
> > > +// SPDX-License-Identifier: MIT
> > > +/*
> > > + * Copyright © 2024 Intel Corporation
> > > + */
> > > +
> > > +#include <linux/mmu_notifier.h>
> > > +#include <linux/dma-mapping.h>
> > > +#include <linux/memremap.h>
> > > +#include <linux/swap.h>
> > > +#include <linux/mm.h>
> > > +#include "xe_hmm.h"
> > > +#include "xe_vm.h"
> > > +
> > > +/**
> > > + * mark_range_accessed() - mark a range is accessed, so core mm
> > > + * have such information for memory eviction or write back to
> > > + * hard disk
> > > + *
> > > + * @range: the range to mark
> > > + * @write: if write to this range, we mark pages in this range
> > > + * as dirty
> > > + */
> > > +static void mark_range_accessed(struct hmm_range *range, bool
> > > write)
> > > +{
> > > +	struct page *page;
> > > +	u64 i, npages;
> > > +
> > > +	npages = ((range->end - 1) >> PAGE_SHIFT) - (range-
> > > >start >>
> > > PAGE_SHIFT) + 1;
> > > +	for (i = 0; i < npages; i++) {
> > > +		page = hmm_pfn_to_page(range->hmm_pfns[i]);
> > > +		if (write) {
> > > +			lock_page(page);
> > > +			set_page_dirty(page);
> > > +			unlock_page(page);
> > > +		}
> > > +		mark_page_accessed(page);
> > > +	}
> > > +}
> > > +
> > > +/**
> > > + * build_sg() - build a scatter gather table for all the
> > > physical
> > > pages/pfn
> > > + * in a hmm_range. dma-address is save in sg table and will be
> > > used
> > > to program
> > > + * GPU page table later.
> > > + *
> > > + * @xe: the xe device who will access the dma-address in sg
> > > table
> > > + * @range: the hmm range that we build the sg table from. range-
> > > > hmm_pfns[]
> > > + * has the pfn numbers of pages that back up this hmm address
> > > range.
> > > + * @st: pointer to the sg table.
> > > + * @write: whether we write to this range. This decides dma map
> > > direction
> > > + * for system pages. If write we map it bi-diretional; otherwise
> > > + * DMA_TO_DEVICE
> > > + *
> > > + * All the contiguous pfns will be collapsed into one entry in
> > > + * the scatter gather table. This is for the convenience of
> > > + * later on operations to bind address range to GPU page table.
> > > + *
> > > + * The dma_address in the sg table will later be used by GPU to
> > > + * access memory. So if the memory is system memory, we need to
> > > + * do a dma-mapping so it can be accessed by GPU/DMA. If the
> > > memory
> > > + * is GPU local memory (of the GPU who is going to access
> > > memory),
> > > + * we need gpu dpa (device physical address), and there is no
> > > need
> > > + * of dma-mapping.
> > > + *
> > > + * FIXME: dma-mapping for peer gpu device to access remote gpu's
> > > + * memory. Add this when you support p2p
> > > + *
> > > + * This function allocates the storage of the sg table. It is
> > > + * caller's responsibility to free it calling sg_free_table.
> > > + *
> > > + * Returns 0 if successful; -ENOMEM if fails to allocate memory
> > > + */
> > > +static int build_sg(struct xe_device *xe, struct hmm_range
> > > *range,
> > > +			     struct sg_table *st, bool write)
> > > +{
> > > +	struct device *dev = xe->drm.dev;
> > > +	struct scatterlist *sg;
> > > +	u64 i, npages;
> > > +
> > > +	sg = NULL;
> > > +	st->nents = 0;
> > > +	npages = ((range->end - 1) >> PAGE_SHIFT) - (range-
> > > >start >>
> > > PAGE_SHIFT) + 1;
> > > +
> > > +	if (unlikely(sg_alloc_table(st, npages, GFP_KERNEL)))
> > > +		return -ENOMEM;
> > > +
> > > +	for (i = 0; i < npages; i++) {
> > > +		struct page *page;
> > > +		unsigned long addr;
> > > +		struct xe_mem_region *mr;
> > > +
> > > +		page = hmm_pfn_to_page(range->hmm_pfns[i]);
> > > +		if (is_device_private_page(page)) {
> > > +			mr = page_to_mem_region(page);
> > > +			addr = vram_pfn_to_dpa(mr, range-
> > > > hmm_pfns[i]);
> > > +		} else {
> > > +			addr = dma_map_page(dev, page, 0,
> > > PAGE_SIZE,
> > > +					write ?
> > > DMA_BIDIRECTIONAL :
> > > DMA_TO_DEVICE);
> > > +		}
> > > +
> > > +		if (sg && (addr == (sg_dma_address(sg) + sg-
> > > > length))) {
> > > +			sg->length += PAGE_SIZE;
> > > +			sg_dma_len(sg) += PAGE_SIZE;
> > > +			continue;
> > > +		}
> > > +
> > > +		sg =  sg ? sg_next(sg) : st->sgl;
> > > +		sg_dma_address(sg) = addr;
> > > +		sg_dma_len(sg) = PAGE_SIZE;
> > > +		sg->length = PAGE_SIZE;
> > > +		st->nents++;
> > > +	}
> > > +
> > > +	sg_mark_end(sg);
> > > +	return 0;
> > > +}
> > > +
> > > +/**
> > > + * xe_hmm_populate_range() - Populate physical pages of a
> > > virtual
> > > + * address range
> > > + *
> > > + * @vma: vma has information of the range to populate. only vma
> > > + * of userptr and hmmptr type can be populated.
> > > + * @hmm_range: pointer to hmm_range struct. hmm_rang->hmm_pfns
> > > + * will hold the populated pfns.
> > > + * @write: populate pages with write permission
> > > + *
> > > + * This function populate the physical pages of a virtual
> > > + * address range. The populated physical pages is saved in
> > > + * userptr's sg table. It is similar to get_user_pages but call
> > > + * hmm_range_fault.
> > > + *
> > > + * This function also read mmu notifier sequence # (
> > > + * mmu_interval_read_begin), for the purpose of later
> > > + * comparison (through mmu_interval_read_retry).
> > > + *
> > > + * This must be called with mmap read or write lock held.
> > > + *
> > > + * This function allocates the storage of the userptr sg table.
> > > + * It is caller's responsibility to free it calling
> > > sg_free_table.
> > > + *
> > > + * returns: 0 for succuss; negative error no on failure
> > > + */
> > > +int xe_hmm_populate_range(struct xe_vma *vma, struct hmm_range
> > > *hmm_range,
> > > +						bool write)
> > > +{
> > > +	unsigned long timeout =
> > > +		jiffies +
> > > msecs_to_jiffies(HMM_RANGE_DEFAULT_TIMEOUT);
> > > +	unsigned long *pfns, flags = HMM_PFN_REQ_FAULT;
> > > +	struct xe_userptr_vma *userptr_vma;
> > > +	struct xe_userptr *userptr;
> > > +	u64 start = vma->gpuva.va.addr;
> > > +	u64 end = start + vma->gpuva.va.range;
> > > +	struct xe_vm *vm = xe_vma_vm(vma);
> > > +	u64 npages;
> > > +	int ret;
> > > +
> > > +	userptr_vma = to_userptr_vma(vma);
> > > +	userptr = &userptr_vma->userptr;
> > > +	mmap_assert_locked(userptr->notifier.mm);
> > > +
> > > +	npages = ((end - 1) >> PAGE_SHIFT) - (start >>
> > > PAGE_SHIFT) +
> > > 1;
> > > +	pfns = kvmalloc_array(npages, sizeof(*pfns),
> > > GFP_KERNEL);
> > > +	if (unlikely(!pfns))
> > > +		return -ENOMEM;
> > > +
> > > +	if (write)
> > > +		flags |= HMM_PFN_REQ_WRITE;
> > > +
> > > +	memset64((u64 *)pfns, (u64)flags, npages);
> > > +	hmm_range->hmm_pfns = pfns;
> > > +	hmm_range->notifier_seq =
> > > mmu_interval_read_begin(&userptr-
> > > > notifier);
> > > +	hmm_range->notifier = &userptr->notifier;
> > > +	hmm_range->start = start;
> > > +	hmm_range->end = end;
> > > +	hmm_range->pfn_flags_mask = HMM_PFN_REQ_FAULT |
> > > HMM_PFN_REQ_WRITE;
> > > +	/**
> > > +	 * FIXME:
> > > +	 * Set the the dev_private_owner can prevent
> > > hmm_range_fault
> > > to fault
> > > +	 * in the device private pages owned by caller. See
> > > function
> > > +	 * hmm_vma_handle_pte. In multiple GPU case, this should
> > > be
> > > set to the
> > > +	 * device owner of the best migration destination. e.g.,
> > > device0/vm0
> > > +	 * has a page fault, but we have determined the best
> > > placement of
> > > +	 * the fault address should be on device1, we should set
> > > below to
> > > +	 * device1 instead of device0.
> > > +	 */
> > > +	hmm_range->dev_private_owner = vm->xe->drm.dev;
> > > +
> > > +	while (true) {
> > > +		ret = hmm_range_fault(hmm_range);
> > > +		if (time_after(jiffies, timeout))
> > > +			break;
> > > +
> > > +		if (ret == -EBUSY)
> > > +			continue;
> > 
> > If (ret == -EBUSY) it looks from the hmm_range_fault()
> > implementation
> > like hmm_range->notifier_seq has become invalid and without calling
> > mmu_interval_read_begin() again, we will end up in an infinite
> > loop?
> > 
> 
> I noticed this thing before and had a read_begin in the while loop.
> But after a second thought, function xe_hmm_populate_range is called
> inside a mmap lock, so after the read_begin is called above, there
> can't be a invalidation before mmap unlock. So theoretically EBUSY
> can't happen?
> 
> Oak

Invalidations can happen due to many different things that don't need
the mmap lock. File truncation and I think page reclaim are typical
examples.

/Thomas


> 
> > /Thomas
> > 
> > 
> > 
> > > +		break;
> > > +	}
> > > +
> > > +	if (ret)
> > > +		goto free_pfns;
> > > +
> > > +	ret = build_sg(vm->xe, hmm_range, &userptr->sgt, write);
> > > +	if (ret)
> > > +		goto free_pfns;
> > > +
> > > +	mark_range_accessed(hmm_range, write);
> > > +	userptr->sg = &userptr->sgt;
> > > +	userptr->notifier_seq = hmm_range->notifier_seq;
> > > +
> > > +free_pfns:
> > > +	kvfree(pfns);
> > > +	return ret;
> > > +}
> > > +
> > > diff --git a/drivers/gpu/drm/xe/xe_hmm.h
> > > b/drivers/gpu/drm/xe/xe_hmm.h
> > > new file mode 100644
> > > index 000000000000..960f3f6d36ae
> > > --- /dev/null
> > > +++ b/drivers/gpu/drm/xe/xe_hmm.h
> > > @@ -0,0 +1,12 @@
> > > +// SPDX-License-Identifier: MIT
> > > +/*
> > > + * Copyright © 2024 Intel Corporation
> > > + */
> > > +
> > > +#include <linux/types.h>
> > > +#include <linux/hmm.h>
> > > +#include "xe_vm_types.h"
> > > +#include "xe_svm.h"
> > > +
> > > +int xe_hmm_populate_range(struct xe_vma *vma, struct hmm_range
> > > *hmm_range,
> > > +						bool write);
>