Re: [PATCH -next 1/2] mm: compaction: convert to use a folio in isolate_migratepages_block()

[Kefeng Wang]

On 2023/6/12 22:41, Matthew Wilcox wrote:
> On Mon, Jun 12, 2023 at 10:34:13PM +0800, Kefeng Wang wrote:
> > @@ -959,7 +960,8 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >   				 * Hugepage was successfully isolated and placed
> >   				 * on the cc->migratepages list.
> >   				 */
> > -				low_pfn += compound_nr(page) - 1;
> > +				folio = page_folio(page);
> > +				low_pfn += folio_nr_pages(folio) - 1;
> >   				goto isolate_success_no_list;
>
> Why is this safe?  That is, how do we know that the folio can't be
> dissolved under us at this point, then reallocated and hit the
> VM_BUG_ON_PGFLAGS(PageTail(page), page) in folio_flags() when we
> test folio_test_large()?

This is successfully isolated path, after isolate_hugetlb(),  the folio
reference is incremented, so I think the folio can't be dissolved here,
correct me if I am wrong.

> > @@ -1132,30 +1137,30 @@ isolate_migratepages_block(struct compact_control *cc, unsigned long low_pfn,
> >   			 * and it's on LRU. It can only be a THP so the order
> >   			 * is safe to read and it's 0 for tail pages.
> >   			 */
>
> ^^^ This comment needs to be updated too.
will update
> > -		mod_node_page_state(page_pgdat(page),
> > -				NR_ISOLATED_ANON + page_is_file_lru(page),
> > -				thp_nr_pages(page));
> > +		lruvec_del_folio(lruvec, folio);
> > +		mod_node_page_state(folio_pgdat(folio),
> > +				NR_ISOLATED_ANON + folio_is_file_lru(folio),
> > +				folio_nr_pages(folio));
>
> 		node_stat_mod_folio()
> ok

Thanks


>