Re: [PATCH v1 08/14] mm/mshare: Add basic page table sharing using mshare

Dave Hansen
2022-04-11T18:48:58Z

On 4/11/22 09:05, Khalid Aziz wrote:
> This patch adds basic page table sharing across tasks by making
> mshare syscall. It does this by creating a new mm_struct which
> hosts the shared vmas and page tables. This mm_struct is
> maintained as long as there is at least one task using the mshare'd
> range. It is cleaned up by the last mshare_unlink syscall.

This sounds like a really good idea because it (in theory) totally
separates the lifetime of the *source* of the page tables from the
lifetime of the process that creates the mshare.

> diff --git a/mm/internal.h b/mm/internal.h
> index cf50a471384e..68f82f0f8b66 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -718,6 +718,8 @@ void vunmap_range_noflush(unsigned long start, unsigned long end);
> int numa_migrate_prep(struct page *page, struct vm_area_struct *vma,
> unsigned long addr, int page_nid, int *flags);
>
> +extern vm_fault_t find_shared_vma(struct vm_area_struct **vma,
> + unsigned long *addrp);
> static inline bool vma_is_shared(const struct vm_area_struct *vma)
> {
> return vma->vm_flags & VM_SHARED_PT;
> diff --git a/mm/memory.c b/mm/memory.c
> index c125c4969913..c77c0d643ea8 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4776,6 +4776,7 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> unsigned int flags, struct pt_regs *regs)
> {
> vm_fault_t ret;
> + bool shared = false;
>
> __set_current_state(TASK_RUNNING);
>
> @@ -4785,6 +4786,15 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> /* do counter updates before entering really critical section. */
> check_sync_rss_stat(current);
>
> + if (unlikely(vma_is_shared(vma))) {
> + ret = find_shared_vma(&vma, &address);
> + if (ret)
> + return ret;
> + if (!vma)
> + return VM_FAULT_SIGSEGV;
> + shared = true;
> + }
> +
> if (!arch_vma_access_permitted(vma, flags & FAULT_FLAG_WRITE,
> flags & FAULT_FLAG_INSTRUCTION,
> flags & FAULT_FLAG_REMOTE))
> @@ -4802,6 +4812,31 @@ vm_fault_t handle_mm_fault(struct vm_area_struct *vma, unsigned long address,
> else
> ret = __handle_mm_fault(vma, address, flags);
>
> + /*
> + * Release the read lock on shared VMA's parent mm unless
> + * __handle_mm_fault released the lock already.
> + * __handle_mm_fault sets VM_FAULT_RETRY in return value if
> + * it released mmap lock. If lock was released, that implies
> + * the lock would have been released on task's original mm if
> + * this were not a shared PTE vma. To keep lock state consistent,
> + * make sure to release the lock on task's original mm
> + */
> + if (shared) {
> + int release_mmlock = 1;
> +
> + if (!(ret & VM_FAULT_RETRY)) {
> + mmap_read_unlock(vma->vm_mm);
> + release_mmlock = 0;
> + } else if ((flags & FAULT_FLAG_ALLOW_RETRY) &&
> + (flags & FAULT_FLAG_RETRY_NOWAIT)) {
> + mmap_read_unlock(vma->vm_mm);
> + release_mmlock = 0;
> + }
> +
> + if (release_mmlock)
> + mmap_read_unlock(current->mm);
> + }

Are we guaranteed that current->mm == the original vma->vm_mm? Just a
quick scan of handle_mm_fault() users shows a few suspect ones like
hmm_range_fault() or iommu_v2.c::do_fault().

> diff --git a/mm/mshare.c b/mm/mshare.c
> index cd2f7ad24d9d..d1896adcb00f 100644
> --- a/mm/mshare.c
> +++ b/mm/mshare.c
> @@ -17,18 +17,49 @@
> #include <linux/pseudo_fs.h>
> #include <linux/fileattr.h>
> #include <linux/refcount.h>
> +#include <linux/mman.h>
> #include <linux/sched/mm.h>
> #include <uapi/linux/magic.h>
> #include <uapi/linux/limits.h>
>
> struct mshare_data {
> - struct mm_struct *mm;
> + struct mm_struct *mm, *host_mm;
> mode_t mode;
> refcount_t refcnt;
> };
>
> static struct super_block *msharefs_sb;
>
> +/* Returns holding the host mm's lock for read. Caller must release. */
> +vm_fault_t
> +find_shared_vma(struct vm_area_struct **vmap, unsigned long *addrp)
> +{
> + struct vm_area_struct *vma, *guest = *vmap;
> + struct mshare_data *info = guest->vm_private_data;
> + struct mm_struct *host_mm = info->mm;
> + unsigned long host_addr;
> + pgd_t *pgd, *guest_pgd;
> +
> + host_addr = *addrp - guest->vm_start + host_mm->mmap_base;
> + pgd = pgd_offset(host_mm, host_addr);
> + guest_pgd = pgd_offset(current->mm, *addrp);
> + if (!pgd_same(*guest_pgd, *pgd)) {
> + set_pgd(guest_pgd, *pgd);
> + return VM_FAULT_NOPAGE;
> + }

Is digging around in the other process's page tables OK without holding
any locks?

> + *addrp = host_addr;
> + mmap_read_lock(host_mm);
> + vma = find_vma(host_mm, host_addr);
> +
> + /* XXX: expand stack? */
> + if (vma && vma->vm_start > host_addr)
> + vma = NULL;
> +
> + *vmap = vma;
> + return 0;
> +}
> +
> static void
> msharefs_evict_inode(struct inode *inode)
> {
> @@ -169,11 +200,13 @@ SYSCALL_DEFINE5(mshare, const char __user *, name, unsigned long, addr,
> unsigned long, len, int, oflag, mode_t, mode)
> {
> struct mshare_data *info;
> - struct mm_struct *mm;
> struct filename *fname = getname(name);
> struct dentry *dentry;
> struct inode *inode;
> struct qstr namestr;
> + struct vm_area_struct *vma, *next, *new_vma;
> + struct mm_struct *new_mm;
> + unsigned long end;
> int err = PTR_ERR(fname);
>
> /*
> @@ -193,6 +226,8 @@ SYSCALL_DEFINE5(mshare, const char __user *, name, unsigned long, addr,
> if (IS_ERR(fname))
> goto err_out;
>
> + end = addr + len;
> +
> /*
> * Does this mshare entry exist already? If it does, calling
> * mshare with O_EXCL|O_CREAT is an error
> @@ -205,49 +240,165 @@ SYSCALL_DEFINE5(mshare, const char __user *, name, unsigned long, addr,
> inode_lock(d_inode(msharefs_sb->s_root));
> dentry = d_lookup(msharefs_sb->s_root, &namestr);
> if (dentry && (oflag & (O_EXCL|O_CREAT))) {
> + inode = d_inode(dentry);
> err = -EEXIST;
> dput(dentry);
> goto err_unlock_inode;
> }
>
> if (dentry) {
> + unsigned long mapaddr, prot = PROT_NONE;
> +
> inode = d_inode(dentry);
> if (inode == NULL) {
> + mmap_write_unlock(current->mm);
> err = -EINVAL;
> goto err_out;
> }
> info = inode->i_private;
> - refcount_inc(&info->refcnt);
> dput(dentry);
> +
> + /*
> + * Map in the address range as anonymous mappings
> + */
> + oflag &= (O_RDONLY | O_WRONLY | O_RDWR);
> + if (oflag & O_RDONLY)
> + prot |= PROT_READ;
> + else if (oflag & O_WRONLY)
> + prot |= PROT_WRITE;
> + else if (oflag & O_RDWR)
> + prot |= (PROT_READ | PROT_WRITE);
> + mapaddr = vm_mmap(NULL, addr, len, prot,
> + MAP_FIXED | MAP_SHARED | MAP_ANONYMOUS, 0);
> + if (IS_ERR((void *)mapaddr)) {
> + err = -EINVAL;
> + goto err_out;
> + }
> +
> + refcount_inc(&info->refcnt);
> +
> + /*
> + * Now that we have mmap'd the mshare'd range, update vma
> + * flags and vm_mm pointer for this mshare'd range.
> + */
> + mmap_write_lock(current->mm);
> + vma = find_vma(current->mm, addr);
> + if (vma && vma->vm_start < addr) {
> + mmap_write_unlock(current->mm);
> + err = -EINVAL;
> + goto err_out;
> + }

How do you know that this is the same anonymous VMA that you set up
above? Couldn't it have been unmapped and remapped to be something
random before the mmap_write_lock() is reacquired?

> + while (vma && vma->vm_start < (addr + len)) {
> + vma->vm_private_data = info;
> + vma->vm_mm = info->mm;
> + vma->vm_flags |= VM_SHARED_PT;
> + next = vma->vm_next;
> + vma = next;
> + }

This vma is still in the mm->mm_rb tree, right? I'm kinda surprised
that it's OK to have a VMA in mm->vm_rb have vma->vm_mm!=mm.

> } else {
> - mm = mm_alloc();
> - if (!mm)
> + unsigned long myaddr;
> + struct mm_struct *old_mm;
> +
> + old_mm = current->mm;
> + new_mm = mm_alloc();
> + if (!new_mm)
> return -ENOMEM;
> info = kzalloc(sizeof(*info), GFP_KERNEL);
> if (!info) {
> err = -ENOMEM;
> goto err_relmm;
> }
> - mm->mmap_base = addr;
> - mm->task_size = addr + len;
> - if (!mm->task_size)
> - mm->task_size--;
> - info->mm = mm;
> + new_mm->mmap_base = addr;
> + new_mm->task_size = addr + len;
> + if (!new_mm->task_size)
> + new_mm->task_size--;
> + info->mm = new_mm;
> + info->host_mm = old_mm;
> info->mode = mode;
> refcount_set(&info->refcnt, 1);
> +
> + /*
> + * VMAs for this address range may or may not exist.
> + * If VMAs exist, they should be marked as shared at
> + * this point and page table info should be copied
> + * over to newly created mm_struct. TODO: If VMAs do not
> + * exist, create them and mark them as shared.
> + */

At this point, there are just too many TODO's in this series to look at
it seriously. I think what you have here is an entertaining
proof-of-concept, but it's looking to me to be obviously still RFC
quality. Do you seriously think anyone could even *think* about
applying this series at this point?