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

[John Hubbard <jhubbard@nvidia.com>]

On 12/8/22 14:21, Peter Xu wrote:
> 
> 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.
> 

It does! And now for the comment, I'll think you'll find that this suffices:

		/*
		 * All callers that get here will already hold the i_mmap_rwsem.
		 * Therefore, no additional locks need to be taken before
		 * calling hugetlb_walk().
		 */

...which, considering all the data above, is probably the mother of
all summaries. :)  But really, it's all that people need to know here, and
it's readily understandable without wondering what KSM has to do with this,
for example.


thanks,
-- 
John Hubbard
NVIDIA