[v2,1/4] mm: Set folio swapbacked iff folios are dirty in try_to_unmap_one

Commit Message

Barry Song Jan. 13, 2025, 3:38 a.m. UTC

From: Barry Song <[e-mail]>

The refcount may be temporarily or long-term increased, but this does
not change the fundamental nature of the folio already being lazy-
freed. Therefore, we only reset 'swapbacked' when we are certain the
folio is dirty and not droppable.

Suggested-by: David Hildenbrand <[e-mail]>
Signed-off-by: Barry Song <[e-mail]>
---
 mm/rmap.c | 49 ++++++++++++++++++++++---------------------------
 1 file changed, 22 insertions(+), 27 deletions(-)

Comments

David Hildenbrand Jan. 13, 2025, 1:19 p.m. UTC | #1

On 13.01.25 04:38, Barry Song wrote:
> From: Barry Song <[e-mail]>
> 
> The refcount may be temporarily or long-term increased, but this does
> not change the fundamental nature of the folio already being lazy-
> freed. Therefore, we only reset 'swapbacked' when we are certain the
> folio is dirty and not droppable.
> 
> Suggested-by: David Hildenbrand <[e-mail]>
> Signed-off-by: Barry Song <[e-mail]>
> ---

Acked-by: David Hildenbrand <[e-mail]>

David Hildenbrand Jan. 13, 2025, 1:20 p.m. UTC | #2

On 13.01.25 14:19, David Hildenbrand wrote:
> On 13.01.25 04:38, Barry Song wrote:
>> From: Barry Song <[e-mail]>
>>
>> The refcount may be temporarily or long-term increased, but this does
>> not change the fundamental nature of the folio already being lazy-
>> freed. Therefore, we only reset 'swapbacked' when we are certain the
>> folio is dirty and not droppable.
>>
>> Suggested-by: David Hildenbrand <[e-mail]>
>> Signed-off-by: Barry Song <[e-mail]>
>> ---
> 
> Acked-by: David Hildenbrand <[e-mail]>
> 

Ah, should this have a Fixes: ?

Because of a speculative reference, we might not reclaim MADV_FREE 
folios as we silently mark them as swapbacked again, which sounds fairly 
wrong.

Barry Song Jan. 13, 2025, 9:56 p.m. UTC | #3

On Tue, Jan 14, 2025 at 2:21 AM David Hildenbrand <[e-mail]> wrote:
>
> On 13.01.25 14:19, David Hildenbrand wrote:
> > On 13.01.25 04:38, Barry Song wrote:
> >> From: Barry Song <[e-mail]>
> >>
> >> The refcount may be temporarily or long-term increased, but this does
> >> not change the fundamental nature of the folio already being lazy-
> >> freed. Therefore, we only reset 'swapbacked' when we are certain the
> >> folio is dirty and not droppable.
> >>
> >> Suggested-by: David Hildenbrand <[e-mail]>
> >> Signed-off-by: Barry Song <[e-mail]>
> >> ---
> >
> > Acked-by: David Hildenbrand <[e-mail]>
> >

Thanks!

>
> Ah, should this have a Fixes: ?

That makes sense to me.

>
> Because of a speculative reference, we might not reclaim MADV_FREE
> folios as we silently mark them as swapbacked again, which sounds fairly
> wrong.
>

I assume the tag should be applied to Mauricio's commit 6c8e2a256915a2 ("mm:
fix race between MADV_FREE reclaim and blkdev direct IO read") and  also
Mauricio is CC'ed.

> --
> Cheers,
>
> David / dhildenb
>

Thanks
Barry

Baolin Wang Jan. 14, 2025, 2:55 a.m. UTC | #4

On 2025/1/13 11:38, Barry Song wrote:
> From: Barry Song <[e-mail]>
> 
> The refcount may be temporarily or long-term increased, but this does
> not change the fundamental nature of the folio already being lazy-
> freed. Therefore, we only reset 'swapbacked' when we are certain the
> folio is dirty and not droppable.
> 
> Suggested-by: David Hildenbrand <[e-mail]>
> Signed-off-by: Barry Song <[e-mail]>

LGTM. Feel free to add:
Reviewed-by: Baolin Wang <[e-mail]>

Lance Yang Jan. 14, 2025, 6:05 a.m. UTC | #5

On Tue, Jan 14, 2025 at 10:56 AM Baolin Wang
<[e-mail]> wrote:
>
>
>
> On 2025/1/13 11:38, Barry Song wrote:
> > From: Barry Song <[e-mail]>
> >
> > The refcount may be temporarily or long-term increased, but this does
> > not change the fundamental nature of the folio already being lazy-
> > freed. Therefore, we only reset 'swapbacked' when we are certain the
> > folio is dirty and not droppable.
> >
> > Suggested-by: David Hildenbrand <[e-mail]>
> > Signed-off-by: Barry Song <[e-mail]>
>
> LGTM. Feel free to add:
> Reviewed-by: Baolin Wang <[e-mail]>

LGTM!
Reviewed-by: Lance Yang <[e-mail]>

Thanks,
Lance

Patch

diff --git a/mm/rmap.c b/mm/rmap.c
index c6c4d4ea29a7..de6b8c34e98c 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -1868,34 +1868,29 @@  static bool try_to_unmap_one(struct folio *folio, struct vm_area_struct *vma,
 				 */
 				smp_rmb();
 
-				/*
-				 * The only page refs must be one from isolation
-				 * plus the rmap(s) (dropped by discard:).
-				 */
-				if (ref_count == 1 + map_count &&
-				    (!folio_test_dirty(folio) ||
-				     /*
-				      * Unlike MADV_FREE mappings, VM_DROPPABLE
-				      * ones can be dropped even if they've
-				      * been dirtied.
-				      */
-				     (vma->vm_flags & VM_DROPPABLE))) {
-					dec_mm_counter(mm, MM_ANONPAGES);
-					goto discard;
-				}
-
-				/*
-				 * If the folio was redirtied, it cannot be
-				 * discarded. Remap the page to page table.
-				 */
-				set_pte_at(mm, address, pvmw.pte, pteval);
-				/*
-				 * Unlike MADV_FREE mappings, VM_DROPPABLE ones
-				 * never get swap backed on failure to drop.
-				 */
-				if (!(vma->vm_flags & VM_DROPPABLE))
+				if (folio_test_dirty(folio) && !(vma->vm_flags & VM_DROPPABLE)) {
+					/*
+					 * redirtied either using the page table or a previously
+					 * obtained GUP reference.
+					 */
+					set_pte_at(mm, address, pvmw.pte, pteval);
 					folio_set_swapbacked(folio);
-				goto walk_abort;
+					goto walk_abort;
+				} else if (ref_count != 1 + map_count) {
+					/*
+					 * Additional reference. Could be a GUP reference or any
+					 * speculative reference. GUP users must mark the folio
+					 * dirty if there was a modification. This folio cannot be
+					 * reclaimed right now either way, so act just like nothing
+					 * happened.
+					 * We'll come back here later and detect if the folio was
+					 * dirtied when the additional reference is gone.
+					 */
+					set_pte_at(mm, address, pvmw.pte, pteval);
+					goto walk_abort;
+				}
+				dec_mm_counter(mm, MM_ANONPAGES);
+				goto discard;
 			}
 
 			if (swap_duplicate(entry) < 0) {