Re: [PATCH v4 3/4] mm: Support batched unmap for lazyfree large folios during reclamation
From: Lance Yang
Date: Thu Jun 26 2025 - 09:53:51 EST
On 2025/6/26 21:16, David Hildenbrand wrote:
On 26.06.25 14:44, Lance Yang wrote:
On 2025/6/26 17:29, Lance Yang wrote:
Before I send out the real patch, I'd like to get some quick feedback to
ensure I've understood the discussion correctly ;)
Does this look like the right direction?
diff --git a/mm/rmap.c b/mm/rmap.c
index fb63d9256f09..5ebffe2137e4 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1845,23 +1845,37 @@ void folio_remove_rmap_pud(struct folio
*folio, struct page *page,
#endif
}
-/* We support batch unmapping of PTEs for lazyfree large folios */
-static inline bool can_batch_unmap_folio_ptes(unsigned long addr,
- struct folio *folio, pte_t *ptep)
+static inline unsigned int folio_unmap_pte_batch(struct folio *folio,
+ struct page_vma_mapped_walk *pvmw,
+ enum ttu_flags flags, pte_t pte)
{
const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
- int max_nr = folio_nr_pages(folio);
- pte_t pte = ptep_get(ptep);
+ unsigned long end_addr, addr = pvmw->address;
+ struct vm_area_struct *vma = pvmw->vma;
+ unsigned int max_nr;
+
+ if (flags & TTU_HWPOISON)
+ return 1;
+ if (!folio_test_large(folio))
+ return 1;
+ /* We may only batch within a single VMA and a single page
table. */
+ end_addr = pmd_addr_end(addr, vma->vm_end);
+ max_nr = (end_addr - addr) >> PAGE_SHIFT;
+
+ /* We only support lazyfree batching for now ... */
if (!folio_test_anon(folio) || folio_test_swapbacked(folio))
- return false;
+ return 1;
if (pte_unused(pte))
- return false;
- if (pte_pfn(pte) != folio_pfn(folio))
- return false;
+ return 1;
+
+ /* ... where we must be able to batch the whole folio. */
+ if (pte_pfn(pte) != folio_pfn(folio) || max_nr !=
folio_nr_pages(folio))
+ return 1;
+ max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr,
fpb_flags,
+ NULL, NULL, NULL);
- return folio_pte_batch(folio, addr, ptep, pte, max_nr,
fpb_flags, NULL,
- NULL, NULL) == max_nr;
+ return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
}
/*
@@ -2024,9 +2038,7 @@ static bool try_to_unmap_one(struct folio
*folio, struct vm_area_struct *vma,
if (pte_dirty(pteval))
folio_mark_dirty(folio);
} else if (likely(pte_present(pteval))) {
- if (folio_test_large(folio) && !(flags & TTU_HWPOISON) &&
- can_batch_unmap_folio_ptes(address, folio, pvmw.pte))
- nr_pages = folio_nr_pages(folio);
+ nr_pages = folio_unmap_pte_batch(folio, &pvmw, flags,
pteval);
end_addr = address + nr_pages * PAGE_SIZE;
flush_cache_range(vma, address, end_addr);
@@ -2206,13 +2218,16 @@ static bool try_to_unmap_one(struct folio
*folio, struct vm_area_struct *vma,
hugetlb_remove_rmap(folio);
} else {
folio_remove_rmap_ptes(folio, subpage, nr_pages, vma);
- folio_ref_sub(folio, nr_pages - 1);
}
if (vma->vm_flags & VM_LOCKED)
mlock_drain_local();
- folio_put(folio);
- /* We have already batched the entire folio */
- if (nr_pages > 1)
+ folio_put_refs(folio, nr_pages);
+
+ /*
+ * If we are sure that we batched the entire folio and cleared
+ * all PTEs, we can just optimize and stop right here.
+ */
+ if (nr_pages == folio_nr_pages(folio))
goto walk_done;
continue;
walk_abort:
--
Oops ... Through testing on my machine, I found that the logic doesn't
behave as expected because I messed up the meaning of max_nr (the
available
scan room in the page table) with folio_nr_pages(folio) :(
With the following change:
diff --git a/mm/rmap.c b/mm/rmap.c
index 5ebffe2137e4..b1407348e14e 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1850,9 +1850,9 @@ static inline unsigned int
folio_unmap_pte_batch(struct folio *folio,
enum ttu_flags flags, pte_t pte)
{
const fpb_t fpb_flags = FPB_IGNORE_DIRTY | FPB_IGNORE_SOFT_DIRTY;
+ unsigned int max_nr, nr_pages = folio_nr_pages(folio);
unsigned long end_addr, addr = pvmw->address;
struct vm_area_struct *vma = pvmw->vma;
- unsigned int max_nr;
if (flags & TTU_HWPOISON)
return 1;
@@ -1870,12 +1870,13 @@ static inline unsigned int
folio_unmap_pte_batch(struct folio *folio,
return 1;
/* ... where we must be able to batch the whole folio. */
Why is that still required? :)
Sorry ... I was still stuck in the "all-or-nothing" mindset ...
So, IIUC, you mean we should completely remove the "max_nr < nr_pages"
check and just let folio_pte_batch handle whatever partial batch it
safely can.
- if (pte_pfn(pte) != folio_pfn(folio) || max_nr !=
folio_nr_pages(folio))
+ if (pte_pfn(pte) != folio_pfn(folio) || max_nr < nr_pages)
return 1;
- max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, max_nr,
fpb_flags,
- NULL, NULL, NULL);
- return (max_nr != folio_nr_pages(folio)) ? 1 : max_nr;
+ max_nr = folio_pte_batch(folio, addr, pvmw->pte, pte, nr_pages,
+ fpb_flags, NULL, NULL, NULL);
+
+ return (max_nr != nr_pages) ? 1 : max_nr;
Why is that still required? :)
Then simply return the number of PTEs that consecutively map to the
large folio. Right?