On 16.12.24 17:51, Dev Jain wrote:
In contrast to PMD-collapse, we do not need to operate on two levels of pagetable
simultaneously. Therefore, downgrade the mmap lock from write to read mode. Still
take the anon_vma lock in exclusive mode so as to not waste time in the rmap path,
which is anyways going to fail since the PTEs are going to be changed. Under the PTL,
copy page contents, clear the PTEs, remove folio pins, and (try to) unmap the
old folios. Set the PTEs to the new folio using the set_ptes() API.
Signed-off-by: Dev Jain <dev.jain@xxxxxxx>
---
Note: I have been trying hard to get rid of the locks in here: we still are
taking the PTL around the page copying; dropping the PTL and taking it after
the copying should lead to a deadlock, for example:
khugepaged madvise(MADV_COLD)
folio_lock() lock(ptl)
lock(ptl) folio_lock()
We can create a locked folio list, altogether drop both the locks, take the PTL,
do everything which __collapse_huge_page_isolate() does *except* the isolation and
again try locking folios, but then it will reduce efficiency of khugepaged
and almost looks like a forced solution :)
Please note the following discussion if anyone is interested:
https://lore.kernel.org/all/66bb7496-a445-4ad7-8e56-4f2863465c54@xxxxxxx/
(Apologies for not CCing the mailing list from the start)
mm/khugepaged.c | 108 ++++++++++++++++++++++++++++++++++++++----------
1 file changed, 87 insertions(+), 21 deletions(-)
diff --git a/mm/khugepaged.c b/mm/khugepaged.c
index 88beebef773e..8040b130e677 100644
--- a/mm/khugepaged.c
+++ b/mm/khugepaged.c
@@ -714,24 +714,28 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
struct vm_area_struct *vma,
unsigned long address,
spinlock_t *ptl,
- struct list_head *compound_pagelist)
+ struct list_head *compound_pagelist, int order)
{
struct folio *src, *tmp;
pte_t *_pte;
pte_t pteval;
- for (_pte = pte; _pte < pte + HPAGE_PMD_NR;
+ for (_pte = pte; _pte < pte + (1UL << order);
_pte++, address += PAGE_SIZE) {
pteval = ptep_get(_pte);
if (pte_none(pteval) || is_zero_pfn(pte_pfn(pteval))) {
add_mm_counter(vma->vm_mm, MM_ANONPAGES, 1);
if (is_zero_pfn(pte_pfn(pteval))) {
- /*
- * ptl mostly unnecessary.
- */
- spin_lock(ptl);
- ptep_clear(vma->vm_mm, address, _pte);
- spin_unlock(ptl);
+ if (order == HPAGE_PMD_ORDER) {
+ /*
+ * ptl mostly unnecessary.
+ */
+ spin_lock(ptl);
+ ptep_clear(vma->vm_mm, address, _pte);
+ spin_unlock(ptl);
+ } else {
+ ptep_clear(vma->vm_mm, address, _pte);
+ }
ksm_might_unmap_zero_page(vma->vm_mm, pteval);
}
} else {
@@ -740,15 +744,20 @@ static void __collapse_huge_page_copy_succeeded(pte_t *pte,
src = page_folio(src_page);
if (!folio_test_large(src))
release_pte_folio(src);
- /*
- * ptl mostly unnecessary, but preempt has to
- * be disabled to update the per-cpu stats
- * inside folio_remove_rmap_pte().
- */
- spin_lock(ptl);
- ptep_clear(vma->vm_mm, address, _pte);
- folio_remove_rmap_pte(src, src_page, vma);
- spin_unlock(ptl);
+ if (order == HPAGE_PMD_ORDER) {
+ /*
+ * ptl mostly unnecessary, but preempt has to
+ * be disabled to update the per-cpu stats
+ * inside folio_remove_rmap_pte().
+ */
+ spin_lock(ptl);
+ ptep_clear(vma->vm_mm, address, _pte);
+ folio_remove_rmap_pte(src, src_page, vma);
+ spin_unlock(ptl);
+ } else {
+ ptep_clear(vma->vm_mm, address, _pte);
+ folio_remove_rmap_pte(src, src_page, vma);
+ }
As I've talked to Nico about this code recently ... :)
Are you clearing the PTE after the copy succeeded? If so, where is the
TLB flush?
How do you sync against concurrent write acess + GUP-fast?
The sequence really must be: (1) clear PTE/PMD + flush TLB (2) check if
there are unexpected page references (e.g., GUP) if so back off (3)
copy page content (4) set updated PTE/PMD.
To Nico, I suggested doing it simple initially, and still clear the
high-level PMD entry + flush under mmap write lock, then re-map the PTE
table after modifying the page table. It's not as efficient, but "harder
to get wrong".
Maybe that's already happening, but I stumbled over this clearing logic
in __collapse_huge_page_copy_succeeded(), so I'm curious.
--
Cheers,
David / dhildenb
