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SubjectRe: [PATCH v5 1/6] mm/page_idle: Add per-pid idle page tracking using virtual index
On Mon, Aug 12, 2019 at 08:14:38PM +0200, Jann Horn wrote:
[snip]
> > +/* Helper to get the start and end frame given a pos and count */
> > +static int page_idle_get_frames(loff_t pos, size_t count, struct mm_struct *mm,
> > + unsigned long *start, unsigned long *end)
> > +{
> > + unsigned long max_frame;
> > +
> > + /* If an mm is not given, assume we want physical frames */
> > + max_frame = mm ? (mm->task_size >> PAGE_SHIFT) : max_pfn;
> > +
> > + if (pos % BITMAP_CHUNK_SIZE || count % BITMAP_CHUNK_SIZE)
> > + return -EINVAL;
> > +
> > + *start = pos * BITS_PER_BYTE;
> > + if (*start >= max_frame)
> > + return -ENXIO;
> > +
> > + *end = *start + count * BITS_PER_BYTE;
> > + if (*end > max_frame)
> > + *end = max_frame;
> > + return 0;
> > +}
>
> You could add some overflow checks for the multiplications. I haven't
> seen any place where it actually matters, but it seems unclean; and in
> particular, on a 32-bit architecture where the maximum user address is
> very high (like with a 4G:4G split), it looks like this function might
> theoretically return with `*start > *end`, which could be confusing to
> callers.

I could store the multiplication result in unsigned long long (since we are
bounds checking with max_frame, start > end would not occur). Something like
the following (with extraneous casts). But I'll think some more about the
point you are raising.

diff --git a/mm/page_idle.c b/mm/page_idle.c
index 1ea21bbb36cb..8fd7a5559986 100644
--- a/mm/page_idle.c
+++ b/mm/page_idle.c
@@ -135,6 +135,7 @@ static int page_idle_get_frames(loff_t pos, size_t count, struct mm_struct *mm,
unsigned long *start, unsigned long *end)
{
unsigned long max_frame;
+ unsigned long long tmp;

/* If an mm is not given, assume we want physical frames */
max_frame = mm ? (mm->task_size >> PAGE_SHIFT) : max_pfn;
@@ -142,13 +143,16 @@ static int page_idle_get_frames(loff_t pos, size_t count, struct mm_struct *mm,
if (pos % BITMAP_CHUNK_SIZE || count % BITMAP_CHUNK_SIZE)
return -EINVAL;

- *start = pos * BITS_PER_BYTE;
- if (*start >= max_frame)
+ tmp = pos * BITS_PER_BYTE;
+ if (tmp >= (unsigned long long)max_frame)
return -ENXIO;
+ *start = (unsigned long)tmp;

- *end = *start + count * BITS_PER_BYTE;
- if (*end > max_frame)
+ tmp = *start + count * BITS_PER_BYTE;
+ if (tmp > (unsigned long long)max_frame)
*end = max_frame;
+ else
+ *end = (unsigned long)tmp;
return 0;
}

> [...]
> > for (; pfn < end_pfn; pfn++) {
> > bit = pfn % BITMAP_CHUNK_BITS;
> > if (!bit)
> > *out = 0ULL;
> > - page = page_idle_get_page(pfn);
> > - if (page) {
> > - if (page_is_idle(page)) {
> > - /*
> > - * The page might have been referenced via a
> > - * pte, in which case it is not idle. Clear
> > - * refs and recheck.
> > - */
> > - page_idle_clear_pte_refs(page);
> > - if (page_is_idle(page))
> > - *out |= 1ULL << bit;
> > - }
> > + page = page_idle_get_page_pfn(pfn);
> > + if (page && page_idle_pte_check(page)) {
> > + *out |= 1ULL << bit;
> > put_page(page);
> > }
> The `page && !page_idle_pte_check(page)` case looks like it's missing
> a put_page(); you probably intended to write something like this?
>
> page = page_idle_get_page_pfn(pfn);
> if (page) {
> if (page_idle_pte_check(page))
> *out |= 1ULL << bit;
> put_page(page);
> }

Ah, you're right. Will fix, good eyes and thank you!

> [...]
> > +/* page_idle tracking for /proc/<pid>/page_idle */
> > +
> > +struct page_node {
> > + struct page *page;
> > + unsigned long addr;
> > + struct list_head list;
> > +};
> > +
> > +struct page_idle_proc_priv {
> > + unsigned long start_addr;
> > + char *buffer;
> > + int write;
> > +
> > + /* Pre-allocate and provide nodes to pte_page_idle_proc_add() */
> > + struct page_node *page_nodes;
> > + int cur_page_node;
> > + struct list_head *idle_page_list;
> > +};
>
> A linked list is a weird data structure to use if the list elements
> are just consecutive array elements.

Not all of the pages will be considered in the later parts of the code, some
pages are skipped.

However, in v4 I added an array to allocate all the page_node structures,
since Andrew did not want GFP_ATOMIC allocations of individual list nodes.

So I think I could get rid of the linked list and leave the unused
page_node(s) as NULL, but I don't know I have to check if that is possible. I
could be missing a corner case, regardless I'll make a note of this and try!

> > +/*
> > + * Add page to list to be set as idle later.
> > + */
> > +static void pte_page_idle_proc_add(struct page *page,
> > + unsigned long addr, struct mm_walk *walk)
> > +{
> > + struct page *page_get = NULL;
> > + struct page_node *pn;
> > + int bit;
> > + unsigned long frames;
> > + struct page_idle_proc_priv *priv = walk->private;
> > + u64 *chunk = (u64 *)priv->buffer;
> > +
> > + if (priv->write) {
> > + VM_BUG_ON(!page);
> > +
> > + /* Find whether this page was asked to be marked */
> > + frames = (addr - priv->start_addr) >> PAGE_SHIFT;
> > + bit = frames % BITMAP_CHUNK_BITS;
> > + chunk = &chunk[frames / BITMAP_CHUNK_BITS];
> > + if (((*chunk >> bit) & 1) == 0)
> > + return;
>
> This means that BITMAP_CHUNK_SIZE is UAPI on big-endian systems,
> right? My opinion is that it would be slightly nicer to design the
> UAPI such that incrementing virtual addresses are mapped to
> incrementing offsets in the buffer (iow, either use bytewise access or
> use little-endian), but I'm not going to ask you to redesign the UAPI
> this late.

That would also be slow and consume more memory in userspace buffers.
Currently, a 64-bit (8 byte) chunk accounts for 64 pages worth or 256KB.

Also I wanted to keep the interface consistent with the global
/sys/kernel/mm/page_idle interface.

> [...]
> > +ssize_t page_idle_proc_generic(struct file *file, char __user *ubuff,
> > + size_t count, loff_t *pos, int write)
> > +{
> [...]
> > + down_read(&mm->mmap_sem);
> [...]
> > +
> > + if (!write && !walk_error)
> > + ret = copy_to_user(ubuff, buffer, count);
> > +
> > + up_read(&mm->mmap_sem);
>
> I'd move the up_read() above the copy_to_user(); copy_to_user() can
> block, and there's no reason to hold the mmap_sem across
> copy_to_user().

Will do.

> Sorry about only chiming in at v5 with all this.

No problem, I'm glad you did!

thanks,

- Joel

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