Re: [PATCH v2 10/10] mm/hugetlb: Document why page_vma_mapped_walk() is safe to walk

[Peter Xu <peterx@redhat.com>]

On Thu, Dec 08, 2022 at 01:54:27PM -0800, John Hubbard wrote:
> On 12/8/22 13:05, Peter Xu wrote:
> > > > +		/*
> > > > +		 * NOTE: we don't need explicit lock here to walk the
> > > > +		 * hugetlb pgtable because either (1) potential callers of
> > > > +		 * hugetlb pvmw currently holds i_mmap_rwsem, or (2) the
> > > > +		 * caller will not walk a hugetlb vma (e.g. ksm or uprobe).
> > > > +		 * When one day this rule breaks, one will get a warning
> > > > +		 * in hugetlb_walk(), and then we'll figure out what to do.
> > > > +		 */
> > > 
> > > Confused. Is this documentation actually intended to refer to hugetlb_walk()
> > > itself, or just this call site? If the former, then let's move it over
> > > to be right before hugetlb_walk().
> > 
> > It is for this specific code path not hugetlb_walk().
> > 
> > The "holds i_mmap_rwsem" here is a true statement (not requirement) because
> > PVMW rmap walkers always have that.  That satisfies with hugetlb_walk()
> > requirements already even without holding the vma lock.
> > 
> 
> It's really hard to understand. Do you have a few extra words to explain it?
> I can help with actual comment wording perhaps, but I am still a bit in
> the dark as to the actual meaning. :)

Firstly, this patch (to be squashed into previous) is trying to document
page_vma_mapped_walk() on why it's not needed to further take any lock to
call hugetlb_walk().

To call hugetlb_walk() we need either of the locks listed below (in either
read or write mode), according to the rules we setup for it in patch 3:

  (1) hugetlb vma lock
  (2) i_mmap_rwsem lock

page_vma_mapped_walk() is called in below sites across the kernel:

__replace_page[179]            if (!page_vma_mapped_walk(&pvmw))
__damon_pa_mkold[24]           while (page_vma_mapped_walk(&pvmw)) {
__damon_pa_young[97]           while (page_vma_mapped_walk(&pvmw)) {
write_protect_page[1065]       if (!page_vma_mapped_walk(&pvmw))
remove_migration_pte[179]      while (page_vma_mapped_walk(&pvmw)) {
page_idle_clear_pte_refs_one[56] while (page_vma_mapped_walk(&pvmw)) {
page_mapped_in_vma[318]        if (!page_vma_mapped_walk(&pvmw))
folio_referenced_one[813]      while (page_vma_mapped_walk(&pvmw)) {
page_vma_mkclean_one[958]      while (page_vma_mapped_walk(pvmw)) {
try_to_unmap_one[1506]         while (page_vma_mapped_walk(&pvmw)) {
try_to_migrate_one[1881]       while (page_vma_mapped_walk(&pvmw)) {
page_make_device_exclusive_one[2205] while (page_vma_mapped_walk(&pvmw)) {

If we group them, we can see that most of them are during a rmap walk
(i.e., comes from a higher rmap_walk() stack), they are:

__damon_pa_mkold[24]           while (page_vma_mapped_walk(&pvmw)) {
__damon_pa_young[97]           while (page_vma_mapped_walk(&pvmw)) {
remove_migration_pte[179]      while (page_vma_mapped_walk(&pvmw)) {
page_idle_clear_pte_refs_one[56] while (page_vma_mapped_walk(&pvmw)) {
page_mapped_in_vma[318]        if (!page_vma_mapped_walk(&pvmw))
folio_referenced_one[813]      while (page_vma_mapped_walk(&pvmw)) {
page_vma_mkclean_one[958]      while (page_vma_mapped_walk(pvmw)) {
try_to_unmap_one[1506]         while (page_vma_mapped_walk(&pvmw)) {
try_to_migrate_one[1881]       while (page_vma_mapped_walk(&pvmw)) {
page_make_device_exclusive_one[2205] while (page_vma_mapped_walk(&pvmw)) {

Let's call it case (A).

We have another two special cases that are not during a rmap walk, they
are:

write_protect_page[1065]       if (!page_vma_mapped_walk(&pvmw))
__replace_page[179]            if (!page_vma_mapped_walk(&pvmw))

Let's call it case (B).

Case (A) is always safe because it always take the i_mmap_rwsem lock in
read mode.  It's done in rmap_walk_file() where:

	if (!locked) {
		if (i_mmap_trylock_read(mapping))
			goto lookup;

		if (rwc->try_lock) {
			rwc->contended = true;
			return;
		}

		i_mmap_lock_read(mapping);
	}

If locked==true it means the caller already holds the lock, so no need to
take it.  It justifies that all callers from rmap_walk() upon a hugetlb vma
is safe to call hugetlb_walk() already according to the rule of hugetlb_walk().

Case (B) contains two cases either in KSM path or uprobe path, and none of
the paths (afaict) can get a hugetlb vma involved.  IOW, the whole path of 

	if (unlikely(is_vm_hugetlb_page(vma))) {

In page_vma_mapped_walk() just should never trigger.

To summarize above into a shorter paragraph, it'll become the comment.

Hope it explains.  Thanks.

-- 
Peter Xu