When page table locked is held, the page can't be freed from under us.
So use get_page() to get the extra page reference to simplify the code.
No functional change intended.
Signed-off-by: Miaohe Lin <[email protected]>
---
mm/memcontrol.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 93e3cc581b51..4ca382efb1ca 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -5670,8 +5670,9 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
*/
if (is_device_private_entry(ent)) {
page = pfn_swap_entry_to_page(ent);
- if (!get_page_unless_zero(page))
- return NULL;
+ /* Get a page reference while we know the page can't be freed. */
+ get_page(page);
+
return page;
}
--
2.33.0
On Sat, Jul 15, 2023 at 11:28:02AM +0800, Miaohe Lin wrote:
> When page table locked is held, the page can't be freed from under us.
But the page isn't mapped into the page table ... there's a swap entry
in the page table, so I don't think your logic holds.
> So use get_page() to get the extra page reference to simplify the code.
> No functional change intended.
>
> Signed-off-by: Miaohe Lin <[email protected]>
> ---
> mm/memcontrol.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 93e3cc581b51..4ca382efb1ca 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -5670,8 +5670,9 @@ static struct page *mc_handle_swap_pte(struct vm_area_struct *vma,
> */
> if (is_device_private_entry(ent)) {
> page = pfn_swap_entry_to_page(ent);
> - if (!get_page_unless_zero(page))
> - return NULL;
> + /* Get a page reference while we know the page can't be freed. */
> + get_page(page);
> +
> return page;
> }
>
> --
> 2.33.0
>
>
On 2023/7/15 11:56, Matthew Wilcox wrote:
> On Sat, Jul 15, 2023 at 11:28:02AM +0800, Miaohe Lin wrote:
>> When page table locked is held, the page can't be freed from under us.
>
> But the page isn't mapped into the page table ... there's a swap entry
> in the page table, so I don't think your logic holds.
>
IIUC, device_private_entry will hold one page refcnt when it's set to page table.
And there's similar code in do_swap_page():
vm_fault_t do_swap_page(struct vm_fault *vmf)
if (unlikely(non_swap_entry(entry))) {
if (is_device_private_entry(entry))
/*
* Get a page reference while we know the page can't be
* freed.
*/
get_page(vmf->page);
pte_unmap_unlock(vmf->pte, vmf->ptl);
ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
put_page(vmf->page);
...
If my logic doesn't hold, do_swap_page() will need to fix the code. Or am I miss something?
Thanks Matthew.
On 2023/7/17 10:28, Miaohe Lin wrote:
> On 2023/7/15 11:56, Matthew Wilcox wrote:
>> On Sat, Jul 15, 2023 at 11:28:02AM +0800, Miaohe Lin wrote:
>>> When page table locked is held, the page can't be freed from under us.
>>
>> But the page isn't mapped into the page table ... there's a swap entry
>> in the page table, so I don't think your logic holds.
>>
>
> IIUC, device_private_entry will hold one page refcnt when it's set to page table.
Take remove_migration_pte() as example, it will hold extra one page refcnt when set device private entry:
remove_migration_pte()
...
folio_get(folio);
...
if (unlikely(is_device_private_page(new))) {
make_[writable|readable]_device_private_entry();
}
...
set_pte_at
> And there's similar code in do_swap_page():
>
> vm_fault_t do_swap_page(struct vm_fault *vmf)
> if (unlikely(non_swap_entry(entry))) {
> if (is_device_private_entry(entry))
> /*
> * Get a page reference while we know the page can't be
> * freed.
> */
> get_page(vmf->page);
> pte_unmap_unlock(vmf->pte, vmf->ptl);
> ret = vmf->page->pgmap->ops->migrate_to_ram(vmf);
> put_page(vmf->page);
> ...
>
> If my logic doesn't hold, do_swap_page() will need to fix the code. Or am I miss something?
Can I have your opinion?
Thanks.