Re: [RFC v1 2/3] udmabuf: Replace pages when there is FALLOC_FL_PUNCH_HOLE in memfd

Daniel Vetter
2023-08-02T12:40:43Z

On Tue, Jul 18, 2023 at 01:28:57AM -0700, Vivek Kasireddy wrote:
> When a hole is punched in the memfd or when a page is replaced for
> any reason, the udmabuf driver needs to get notified in order to
> update its list of pages with the new page. To accomplish this, we
> first identify the vma ranges where pages associated with a given
> udmabuf are mapped to and then register a handler for update_mapping
> mmu notifier for receiving mapping updates.
>
> Once we get notified about a new page faulted in at a given offset
> in the mapping (backed by shmem or hugetlbfs), the list of pages
> is updated and we also zap the relevant PTEs associated with the
> vmas that have mmap'd the udmabuf fd.
>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Jason Gunthorpe <jgg@nvidia.com>
> Cc: Gerd Hoffmann <kraxel@redhat.com>
> Cc: Dongwon Kim <dongwon.kim@intel.com>
> Cc: Junxiao Chang <junxiao.chang@intel.com>
> Signed-off-by: Vivek Kasireddy <vivek.kasireddy@intel.com>

I think the long thread made it clear already, so just for the record:
This wont work. udmabuf is very intentionally about pin_user_page
semantics, if you change the underlying mapping, you get to keep all the
pieces.

The _only_ way to make this work by implementing the dma_buf move
notification infrastructure, and most importers can't cope with such
dynamic dma-buf. And so most likely will not solve your use-case.

Everything else races in a fundamental and unfixable way.
-Daniel

> ---
> drivers/dma-buf/udmabuf.c | 172 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 172 insertions(+)
>
> diff --git a/drivers/dma-buf/udmabuf.c b/drivers/dma-buf/udmabuf.c
> index 10c47bf77fb5..189a36c41906 100644
> --- a/drivers/dma-buf/udmabuf.c
> +++ b/drivers/dma-buf/udmabuf.c
> @@ -4,6 +4,8 @@
> #include <linux/dma-buf.h>
> #include <linux/dma-resv.h>
> #include <linux/highmem.h>
> +#include <linux/rmap.h>
> +#include <linux/mmu_notifier.h>
> #include <linux/init.h>
> #include <linux/kernel.h>
> #include <linux/memfd.h>
> @@ -30,6 +32,23 @@ struct udmabuf {
> struct sg_table *sg;
> struct miscdevice *device;
> pgoff_t *offsets;
> + struct udmabuf_vma_range *ranges;
> + unsigned int num_ranges;
> + struct mmu_notifier notifier;
> + struct mutex mn_lock;
> + struct list_head mmap_vmas;
> +};
> +
> +struct udmabuf_vma_range {
> + struct file *memfd;
> + pgoff_t ubufindex;
> + unsigned long start;
> + unsigned long end;
> +};
> +
> +struct udmabuf_mmap_vma {
> + struct list_head vma_link;
> + struct vm_area_struct *vma;
> };
>
> static vm_fault_t udmabuf_vm_fault(struct vm_fault *vmf)
> @@ -42,28 +61,54 @@ static vm_fault_t udmabuf_vm_fault(struct vm_fault *vmf)
> if (pgoff >= ubuf->pagecount)
> return VM_FAULT_SIGBUS;
>
> + mutex_lock(&ubuf->mn_lock);
> pfn = page_to_pfn(ubuf->pages[pgoff]);
> if (ubuf->offsets) {
> pfn += ubuf->offsets[pgoff] >> PAGE_SHIFT;
> }
> + mutex_unlock(&ubuf->mn_lock);
>
> return vmf_insert_pfn(vma, vmf->address, pfn);
> }
>
> +static void udmabuf_vm_close(struct vm_area_struct *vma)
> +{
> + struct udmabuf *ubuf = vma->vm_private_data;
> + struct udmabuf_mmap_vma *mmap_vma;
> +
> + list_for_each_entry(mmap_vma, &ubuf->mmap_vmas, vma_link) {
> + if (mmap_vma->vma == vma) {
> + list_del(&mmap_vma->vma_link);
> + kfree(mmap_vma);
> + break;
> + }
> + }
> +}
> +
> static const struct vm_operations_struct udmabuf_vm_ops = {
> .fault = udmabuf_vm_fault,
> + .close = udmabuf_vm_close,
> };
>
> static int mmap_udmabuf(struct dma_buf *buf, struct vm_area_struct *vma)
> {
> struct udmabuf *ubuf = buf->priv;
> + struct udmabuf_mmap_vma *mmap_vma;
>
> if ((vma->vm_flags & (VM_SHARED | VM_MAYSHARE)) == 0)
> return -EINVAL;
>
> + mmap_vma = kmalloc(sizeof(*mmap_vma), GFP_KERNEL);
> + if (!mmap_vma)
> + return -ENOMEM;
> +
> vma->vm_ops = &udmabuf_vm_ops;
> vma->vm_private_data = ubuf;
> vm_flags_set(vma, VM_PFNMAP | VM_DONTEXPAND | VM_DONTDUMP);
> +
> + mmap_vma->vma = vma;
> + list_add(&mmap_vma->vma_link, &ubuf->mmap_vmas);
> +
> return 0;
> }
>
> @@ -109,6 +154,7 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
> if (ret < 0)
> goto err_alloc;
>
> + mutex_lock(&ubuf->mn_lock);
> for_each_sg(sg->sgl, sgl, ubuf->pagecount, i) {
> offset = ubuf->offsets ? ubuf->offsets[i] : 0;
> sg_set_page(sgl, ubuf->pages[i], PAGE_SIZE, offset);
> @@ -116,9 +162,12 @@ static struct sg_table *get_sg_table(struct device *dev, struct dma_buf *buf,
> ret = dma_map_sgtable(dev, sg, direction, 0);
> if (ret < 0)
> goto err_map;
> +
> + mutex_unlock(&ubuf->mn_lock);
> return sg;
>
> err_map:
> + mutex_unlock(&ubuf->mn_lock);
> sg_free_table(sg);
> err_alloc:
> kfree(sg);
> @@ -157,6 +206,9 @@ static void release_udmabuf(struct dma_buf *buf)
>
> for (pg = 0; pg < ubuf->pagecount; pg++)
> put_page(ubuf->pages[pg]);
> +
> + mmu_notifier_unregister(&ubuf->notifier, ubuf->notifier.mm);
> + kfree(ubuf->ranges);
> kfree(ubuf->offsets);
> kfree(ubuf->pages);
> kfree(ubuf);
> @@ -208,6 +260,93 @@ static const struct dma_buf_ops udmabuf_ops = {
> .end_cpu_access = end_cpu_udmabuf,
> };
>
> +static void invalidate_mmap_vmas(struct udmabuf *ubuf,
> + struct udmabuf_vma_range *range,
> + unsigned long address, unsigned long size)
> +{
> + struct udmabuf_mmap_vma *vma;
> + unsigned long start = range->ubufindex << PAGE_SHIFT;
> +
> + start += address - range->start;
> + list_for_each_entry(vma, &ubuf->mmap_vmas, vma_link) {
> + zap_vma_ptes(vma->vma, vma->vma->vm_start + start, size);
> + }
> +}
> +
> +static struct udmabuf_vma_range *find_udmabuf_range(struct udmabuf *ubuf,
> + unsigned long address)
> +{
> + struct udmabuf_vma_range *range;
> + int i;
> +
> + for (i = 0; i < ubuf->num_ranges; i++) {
> + range = &ubuf->ranges[i];
> + if (address >= range->start && address < range->end)
> + return range;
> + }
> +
> + return NULL;
> +}
> +
> +static void update_udmabuf(struct mmu_notifier *mn, struct mm_struct *mm,
> + unsigned long address, unsigned long pfn)
> +{
> + struct udmabuf *ubuf = container_of(mn, struct udmabuf, notifier);
> + struct udmabuf_vma_range *range = find_udmabuf_range(ubuf, address);
> + struct page *old_page, *new_page;
> + pgoff_t pgoff, pgshift = PAGE_SHIFT;
> + unsigned long size = 0;
> +
> + if (!range || !pfn_valid(pfn))
> + return;
> +
> + if (is_file_hugepages(range->memfd))
> + pgshift = huge_page_shift(hstate_file(range->memfd));
> +
> + mutex_lock(&ubuf->mn_lock);
> + pgoff = range->ubufindex + ((address - range->start) >> pgshift);
> + old_page = ubuf->pages[pgoff];
> + new_page = pfn_to_page(pfn);
> +
> + do {
> + ubuf->pages[pgoff] = new_page;
> + get_page(new_page);
> + put_page(old_page);
> + size += PAGE_SIZE;
> + } while (ubuf->pages[++pgoff] == old_page);
> +
> + mutex_unlock(&ubuf->mn_lock);
> + invalidate_mmap_vmas(ubuf, range, address, size);
> +}
> +
> +static const struct mmu_notifier_ops udmabuf_update_ops = {
> + .update_mapping = update_udmabuf,
> +};
> +
> +static struct vm_area_struct *find_guest_ram_vma(struct udmabuf *ubuf,
> + struct mm_struct *vmm_mm)
> +{
> + struct vm_area_struct *vma = NULL;
> + MA_STATE(mas, &vmm_mm->mm_mt, 0, 0);
> + unsigned long addr;
> + pgoff_t pg;
> +
> + mas_set(&mas, 0);
> + mmap_read_lock(vmm_mm);
> + mas_for_each(&mas, vma, ULONG_MAX) {
> + for (pg = 0; pg < ubuf->pagecount; pg++) {
> + addr = page_address_in_vma(ubuf->pages[pg], vma);
> + if (addr == -EFAULT)
> + break;
> + }
> + if (addr != -EFAULT)
> + break;
> + }
> + mmap_read_unlock(vmm_mm);
> +
> + return vma;
> +}
> +
> #define SEALS_WANTED (F_SEAL_SHRINK)
> #define SEALS_DENIED (F_SEAL_WRITE)
>
> @@ -218,6 +357,7 @@ static long udmabuf_create(struct miscdevice *device,
> DEFINE_DMA_BUF_EXPORT_INFO(exp_info);
> struct file *memfd = NULL;
> struct address_space *mapping = NULL;
> + struct vm_area_struct *guest_ram;
> struct udmabuf *ubuf;
> struct dma_buf *buf;
> pgoff_t pgoff, pgcnt, pgidx, pgbuf = 0, pglimit;
> @@ -252,6 +392,13 @@ static long udmabuf_create(struct miscdevice *device,
> goto err;
> }
>
> + ubuf->ranges = kmalloc_array(head->count, sizeof(*ubuf->ranges),
> + GFP_KERNEL);
> + if (!ubuf->ranges) {
> + ret = -ENOMEM;
> + goto err;
> + }
> +
> pgbuf = 0;
> for (i = 0; i < head->count; i++) {
> ret = -EBADFD;
> @@ -270,6 +417,8 @@ static long udmabuf_create(struct miscdevice *device,
> goto err;
> pgoff = list[i].offset >> PAGE_SHIFT;
> pgcnt = list[i].size >> PAGE_SHIFT;
> + ubuf->ranges[i].ubufindex = pgbuf;
> + ubuf->ranges[i].memfd = memfd;
> if (is_file_hugepages(memfd)) {
> if (!ubuf->offsets) {
> ubuf->offsets = kmalloc_array(ubuf->pagecount,
> @@ -299,6 +448,7 @@ static long udmabuf_create(struct miscdevice *device,
> get_page(hpage);
> ubuf->pages[pgbuf] = hpage;
> ubuf->offsets[pgbuf++] = chunkoff << PAGE_SHIFT;
> +
> if (++chunkoff == maxchunks) {
> put_page(hpage);
> hpage = NULL;
> @@ -334,6 +484,25 @@ static long udmabuf_create(struct miscdevice *device,
> goto err;
> }
>
> + guest_ram = find_guest_ram_vma(ubuf, current->mm);
> + if (!guest_ram)
> + goto err;
> +
> + ubuf->notifier.ops = &udmabuf_update_ops;
> + ret = mmu_notifier_register(&ubuf->notifier, current->mm);
> + if (ret)
> + goto err;
> +
> + ubuf->num_ranges = head->count;
> + for (i = 0; i < ubuf->num_ranges; i++) {
> + page = ubuf->pages[ubuf->ranges[i].ubufindex];
> + ubuf->ranges[i].start = page_address_in_vma(page, guest_ram);
> + ubuf->ranges[i].end = ubuf->ranges[i].start + list[i].size;
> + }
> +
> + INIT_LIST_HEAD(&ubuf->mmap_vmas);
> + mutex_init(&ubuf->mn_lock);
> +
> flags = 0;
> if (head->flags & UDMABUF_FLAGS_CLOEXEC)
> flags |= O_CLOEXEC;
> @@ -344,6 +513,9 @@ static long udmabuf_create(struct miscdevice *device,
> put_page(ubuf->pages[--pgbuf]);
> if (memfd)
> fput(memfd);
> + if (ubuf->notifier.mm)
> + mmu_notifier_unregister(&ubuf->notifier, ubuf->notifier.mm);
> + kfree(ubuf->ranges);
> kfree(ubuf->offsets);
> kfree(ubuf->pages);
> kfree(ubuf);
> --
> 2.39.2
>

--
Daniel Vetter
Software Engineer, Intel Corporation