On Fri 23-07-21 22:58:34, Andreas Gruenbacher wrote:
> Introduce a new fault_in_iov_iter helper for manually faulting in an iterator.
> Other than fault_in_pages_writeable(), this function is non-destructive.
>
> We'll use fault_in_iov_iter in gfs2 once we've determined that the iterator
> passed to .read_iter or .write_iter isn't in memory.
>
> Signed-off-by: Andreas Gruenbacher <agruenba@xxxxxxxxxx>
...
> +unsigned long fault_in_user_pages(unsigned long start, unsigned long len,
> + bool write)
> +{
> + struct mm_struct *mm = current->mm;
> + struct vm_area_struct *vma = NULL;
> + unsigned long end, nstart, nend;
> + int locked = 0;
> + int gup_flags;
> +
> + /*
> + * FIXME: Make sure this function doesn't succeed for pages that cannot
> + * be accessed; otherwise we could end up in a loop trying to fault in
> + * and then access the pages. (It's okay if a page gets evicted and we
> + * need more than one retry.)
> + */
> +
> + /*
> + * FIXME: Are these the right FOLL_* flags?
> + */
How about the FIXMEs here? I guess we should answer these questions before
merging and remove the comments. Regarding the first FIXME I tend to agree
that if we cannot fault in the first page, we should return the error
rather than returning 0 as you do now. OTOH the caller can check for 0 and
understand there's something wrong going on as well. But the error would be
probably a bit clearer.
> +
> + gup_flags = FOLL_TOUCH | FOLL_POPULATE;
I don't think FOLL_POPULATE makes sense here. It makes sense only with
FOLL_MLOCK and determines whether mlock(2) should fault in missing pages or
not.
Honza
> + if (write)
> + gup_flags |= FOLL_WRITE;
> +
> + end = PAGE_ALIGN(start + len);
> + for (nstart = start & PAGE_MASK; nstart < end; nstart = nend) {
> + unsigned long nr_pages;
> + long ret;
> +
> + if (!locked) {
> + locked = 1;
> + mmap_read_lock(mm);
> + vma = find_vma(mm, nstart);
> + } else if (nstart >= vma->vm_end)
> + vma = vma->vm_next;
> + if (!vma || vma->vm_start >= end)
> + break;
> + nend = min(end, vma->vm_end);
> + if (vma->vm_flags & (VM_IO | VM_PFNMAP))
> + continue;
> + if (nstart < vma->vm_start)
> + nstart = vma->vm_start;
> + nr_pages = (nend - nstart) / PAGE_SIZE;
> + ret = __get_user_pages_locked(mm, nstart, nr_pages,
> + NULL, NULL, &locked, gup_flags);
> + if (ret <= 0)
> + break;
> + nend = nstart + ret * PAGE_SIZE;
> + }
> + if (locked)
> + mmap_read_unlock(mm);
> + if (nstart > start)
> + return min(nstart - start, len);
> + return 0;
> +}
> +
> /**
> * get_dump_page() - pin user page in memory while writing it to core dump
> * @addr: user address
> --
> 2.26.3
>
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
