Re: [PATCH v29 2/6] fs/proc/task_mmu: Implement IOCTL to get and optionally clear info about PTEs

[Michał Mirosław]

On Fri, Aug 11, 2023 at 11:08:38PM +0500, Muhammad Usama Anjum wrote:
> The PAGEMAP_SCAN IOCTL on the pagemap file can be used to get or optionally
> clear the info about page table entries. The following operations are supported
> in this IOCTL:
> - Scan the address range and get the memory ranges matching the provided criteria.
>   This is performed by default when the output buffer is specified.

Nit: This is actually performed always, but you can disable the output part
by passing {NULL, 0} for the buffer.

[...]
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -19,6 +19,8 @@
>  #include <linux/shmem_fs.h>
>  #include <linux/uaccess.h>
>  #include <linux/pkeys.h>
> +#include <linux/minmax.h>
> +#include <linux/overflow.h>
>  
>  #include <asm/elf.h>
>  #include <asm/tlb.h>
> @@ -1749,11 +1751,682 @@ static int pagemap_release(struct inode *inode, struct file *file)
>  	return 0;
>  }
>  
> +#define PM_SCAN_CATEGORIES	(PAGE_IS_WPALLOWED | PAGE_IS_WRITTEN |	\
> +				 PAGE_IS_FILE |	PAGE_IS_PRESENT |	\
> +				 PAGE_IS_SWAPPED | PAGE_IS_PFNZERO |	\
> +				 PAGE_IS_HUGE)
> +#define PM_SCAN_FLAGS		(PM_SCAN_WP_MATCHING | PM_SCAN_CHECK_WPASYNC)
> +
> +struct pagemap_scan_private {
> +	struct pm_scan_arg arg;
> +	unsigned long masks_of_interest, cur_vma_category;
> +	struct page_region *vec_buf;
> +	unsigned long vec_buf_len, vec_buf_index, found_pages, walk_end_addr;
> +	struct page_region __user *vec_out;
> +};
[...]
> +static unsigned long pagemap_thp_category(pmd_t pmd)
> +{
> +	unsigned long categories = PAGE_IS_HUGE;
> +
> +	/*
> +	 * THPs don't support file-backed memory. So PAGE_IS_FILE
> +	 * hasn't been checked here.

"hasn't been" -> "is not"
(same for HugeTLB comment)

> +static bool pagemap_scan_push_range(unsigned long categories,
> +				    struct pagemap_scan_private *p,
> +				    unsigned long addr, unsigned long end)
> +{
> +	struct page_region *cur_buf = &p->vec_buf[p->vec_buf_index];
> +
> +	/*
> +	 * When there is no output buffer provided at all, the sentinel values
> +	 * won't match here. There is no other way for `cur_buf->end` to be
> +	 * non-zero other than it being non-empty.
> +	 */
> +	if (addr == cur_buf->end && categories == cur_buf->categories) {
> +		cur_buf->end = end;
> +		return true;
> +	}
> +
> +	if (cur_buf->end) {
> +		if (p->vec_buf_index >= p->vec_buf_len - 1)
> +			return false;
> +
> +		cur_buf = &p->vec_buf[++p->vec_buf_index];
> +	}
> +
> +	cur_buf->start = addr;
> +	cur_buf->end = end;
> +	cur_buf->categories = categories;
> +
> +	return true;
> +}
> +
> +static void pagemap_scan_backout_range(struct pagemap_scan_private *p,
> +				       unsigned long addr, unsigned long end)
> +{
> +	struct page_region *cur_buf = &p->vec_buf[p->vec_buf_index];
> +
> +	if (cur_buf->start != addr) {
> +		cur_buf->end = addr;
> +	} else {
> +		cur_buf->start = cur_buf->end = 0;
> +		if (p->vec_buf_index > 0)
> +			p->vec_buf_index--;

There is no need to move to the previous index, as if the walk ends at
this moment, the flush_buffer() code will ignore the empty last range.

> +	}
> +
> +	p->found_pages -= (end - addr) / PAGE_SIZE;
> +}
> +
> +static int pagemap_scan_output(unsigned long categories,
> +			       struct pagemap_scan_private *p,
> +			       unsigned long addr, unsigned long *end)
> +{
> +	unsigned long n_pages, total_pages;
> +	int ret = 0;
> +
> +	if (!p->vec_buf)
> +		return 0;
> +
> +	categories &= p->arg.return_mask;
> +
> +	n_pages = (*end - addr) / PAGE_SIZE;
> +	if (check_add_overflow(p->found_pages, n_pages, &total_pages) ||
> +	    total_pages > p->arg.max_pages) {
> +		size_t n_too_much = total_pages - p->arg.max_pages;
> +		*end -= n_too_much * PAGE_SIZE;
> +		n_pages -= n_too_much;
> +		ret = -ENOSPC;
> +	}
> +
> +	if (!pagemap_scan_push_range(categories, p, addr, *end)) {
> +		*end = addr;
> +		n_pages = 0;
> +		ret = -ENOSPC;
> +	}
> +
> +	p->found_pages += n_pages;
> +	if (ret)
> +		p->walk_end_addr = *end;
> +
> +	return ret;
> +}
[...]
> +static int pagemap_scan_init_bounce_buffer(struct pagemap_scan_private *p)
> +{
> +	if (!p->arg.vec_len)
> +		return 0;

The removal of `cur_buf` lost the case of empty non-NULL output buffer
passed in args.  That was requesting the walk to stop at first matching
page (with the address returned in `walk_end`).  The push_range() call
is still checking that, but since neither the buffer nor the sentinel
values are set, the case is not possible to invoke.

> +	/*
> +	 * Allocate a smaller buffer to get output from inside the page
> +	 * walk functions and walk the range in PAGEMAP_WALK_SIZE chunks.
> +	 */

I think this is no longer true? We can now allocate arbitrary number of
entries, but should probably have at least 512 to cover one PMD of pages.
So it would be better to have a constant that holds the number of
entries in the bounce buffer.

> +	p->vec_buf_len = min_t(size_t, PAGEMAP_WALK_SIZE >> PAGE_SHIFT,
> +			       p->arg.vec_len);
> +	p->vec_buf = kmalloc_array(p->vec_buf_len, sizeof(*p->vec_buf),
> +				   GFP_KERNEL);
> +	if (!p->vec_buf)
> +		return -ENOMEM;
> +
> +	p->vec_buf[0].end = 0;

p->vec_buf->start = p->vec_buf->end = 0;

> +	p->vec_out = (struct page_region __user *)p->arg.vec;
> +
> +	return 0;
> +}
> +
> +static int pagemap_scan_flush_buffer(struct pagemap_scan_private *p)
> +{
> +	const struct page_region *buf = p->vec_buf;
> +	int n = (int)p->vec_buf_index;

Why do you need an `int` here (requiring a cast)?

> +	if (p->arg.vec_len == 0)
> +		return 0;

This should be actually `if (!buf)` as this notes that we don't have any
buffer allocated (due to no output requested).

> +	if (buf[n].end && buf[n].end != buf[n].start)
> +		n++;

Testing `buf[n].end` is redundant, as the range is nonempty if
`end != start`.

> +	if (!n)
> +		return 0;
> +
> +	if (copy_to_user(p->vec_out, buf, n * sizeof(*buf)))
> +		return -EFAULT;
> +
> +	p->arg.vec_len -= n;
> +	p->vec_out += n;
> +
> +	p->vec_buf_index = 0;
> +	p->vec_buf_len = min_t(size_t, p->vec_buf_len, p->arg.vec_len);
> +	p->vec_buf[0].end = 0;

buf->start = buf->end = 0;

> +	return n;
> +}
> +
> +static long do_pagemap_scan(struct mm_struct *mm, unsigned long uarg)
> +{
> +	struct mmu_notifier_range range;
> +	struct pagemap_scan_private p = {0};
> +	unsigned long walk_start;
> +	size_t n_ranges_out = 0;
> +	int ret;
> +
> +	ret = pagemap_scan_get_args(&p.arg, uarg);
> +	if (ret)
> +		return ret;
> +
> +	p.masks_of_interest = p.arg.category_inverted | p.arg.category_mask |
> +			      p.arg.category_anyof_mask | p.arg.return_mask;

`category_inverted` can be left out, because if a set bit it is not also in one
of the masks, then its value is going to be ignored.

[...]
> +	for (walk_start = p.arg.start; walk_start < p.arg.end;
> +			walk_start = p.arg.walk_end) {
> +		int n_out;
> +
> +		if (fatal_signal_pending(current)) {
> +			ret = -EINTR;
> +			break;
> +		}
> +
> +		ret = mmap_read_lock_killable(mm);
> +		if (ret)
> +			break;
> +		ret = walk_page_range(mm, walk_start, p.arg.end,
> +				      &pagemap_scan_ops, &p);
> +		mmap_read_unlock(mm);
> +
> +		n_out = pagemap_scan_flush_buffer(&p);
> +		if (n_out < 0)
> +			ret = n_out;
> +		else
> +			n_ranges_out += n_out;
> +
> +		p.walk_end_addr = p.walk_end_addr ? p.walk_end_addr : p.arg.end;

Why is `p.walk_end_addr` needed? It is not used in the loop code. Shoudn't
it be `p.arg.walk_end` as used in the `for` loop continuation statement?

> +		if (ret != -ENOSPC || p.arg.vec_len == 0 ||
> +		    p.found_pages == p.arg.max_pages)
> +			break;

Nit: I think you could split this into two or three separate `if (x)
break;` for easier reading. The `vec_len` and `found_pages` are
buffer-full tests, so could go along, but `ret != ENOSPC` is checking an
error condition aborting the scan before it ends.

> +	}
> +
> +	/* ENOSPC signifies early stop (buffer full) from the walk. */
> +	if (!ret || ret == -ENOSPC)
> +		ret = n_ranges_out;
> +
> +	p.arg.walk_end = p.walk_end_addr ? p.walk_end_addr : walk_start;
> +	if (pagemap_scan_writeback_args(&p.arg, uarg))
> +		ret = -EFAULT;
[...]
> --- a/include/uapi/linux/fs.h
> +++ b/include/uapi/linux/fs.h
[...]
> +/*
> + * struct pm_scan_arg - Pagemap ioctl argument
> + * @size:		Size of the structure
> + * @flags:		Flags for the IOCTL
> + * @start:		Starting address of the region
> + * @end:		Ending address of the region
> + * @walk_end		Address where the scan stopped (written by kernel).
> + *			walk_end == end (tag removed) informs that the scan completed on entire range.

I'm not sure `tag removed` is enough to know what tag was removed.
Maybe something like "with address tags cleared" would fit?

Best Regards
Michał Mirosław