mm/shmem, swap: fix race of truncate and swap entry split

From: Kairui Song <[email protected]>

The helper for shmem swap freeing is not handling the order of swap entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but it gets the entry order before that using xa_get_order without lock protection, and it may get an outdated order value if the entry is split or changed in other ways after the xa_get_order and before the xa_cmpxchg_irq.

And besides, the order could grow and be larger than expected, and cause truncation to erase data beyond the end border. For example, if the target entry and following entries are swapped in or freed, then a large folio was added in place and swapped out, using the same entry, the xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.

To fix that, open code the Xarray cmpxchg and put the order retrieval and value checking in the same critical section. Also, ensure the order won't exceed the end border, skip it if the entry goes across the border.

Skipping large swap entries crosses the end border is safe here. Shmem truncate iterates the range twice, in the first iteration, find_lock_entries already filtered such entries, and shmem will swapin the entries that cross the end border and partially truncate the folio (split the folio or at least zero part of it). So in the second loop here, if we see a swap entry that crosses the end order, it must at least have its content erased already.

I observed random swapoff hangs and kernel panics when stress testing ZSWAP with shmem. After applying this patch, all problems are gone.

Fixes 809bc86517cc ("mm: shmem: support large folio swap out")

Cc [email protected]

Signed-off-by Kairui Song <[email protected]>

From: Kairui Song <[email protected]>

The helper for shmem swap freeing is not handling the order of swap entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but it gets the entry order before that using xa_get_order without lock protection, and it may get an outdated order value if the entry is split or changed in other ways after the xa_get_order and before the xa_cmpxchg_irq.

And besides, the order could grow and be larger than expected, and cause truncation to erase data beyond the end border. For example, if the target entry and following entries are swapped in or freed, then a large folio was added in place and swapped out, using the same entry, the xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.

To fix that, open code the Xarray cmpxchg and put the order retrieval and value checking in the same critical section. Also, ensure the order won't exceed the end border, skip it if the entry goes across the border.

Skipping large swap entries crosses the end border is safe here. Shmem truncate iterates the range twice, in the first iteration, find_lock_entries already filtered such entries, and shmem will swapin the entries that cross the end border and partially truncate the folio (split the folio or at least zero part of it). So in the second loop here, if we see a swap entry that crosses the end order, it must at least have its content erased already.

I observed random swapoff hangs and kernel panics when stress testing ZSWAP with shmem. After applying this patch, all problems are gone.

Fixes 809bc86517cc ("mm: shmem: support large folio swap out")

Cc [email protected]

Signed-off-by Kairui Song <[email protected]>

---
Changes in v3:
- Rebased on top of mainline.
- Fix nr_pages calculation [ Baolin Wang ]
- Link to v2: https://lore.kernel.org/r/[email protected]

Changes in v2:
- Fix a potential retry loop issue and improvement to code style thanks
to Baoling Wang. I didn't split the change into two patches because a separate patch doesn't stand well as a fix.
- Link to v1: https://lore.kernel.org/r/[email protected]

---
  mm/shmem.c | 45 ++++++++++++++++++++++++++++++++++-----------
  1 file changed, 34 insertions(+), 11 deletions(-)

mm/shmem.c +34 -12

index ec6c01378e9d..6c3485d24d66 100644
@@ -962,17 +962,29 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
   * being freed).
   */
  static long shmem_free_swap(struct address_space *mapping,
-			    pgoff_t index, void *radswap)
+			    pgoff_t index, pgoff_t end, void *radswap)
  {
-	int order = xa_get_order(&mapping->i_pages, index);
-	void *old;
+	XA_STATE(xas, &mapping->i_pages, index);
+	unsigned int nr_pages = 0;
+	pgoff_t base;
+	void *entry;
 
-	old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
-	if (old != radswap)
-		return 0;
-	free_swap_and_cache_nr(radix_to_swp_entry(radswap), 1 << order);
+	xas_lock_irq(&xas);
+	entry = xas_load(&xas);
+	if (entry == radswap) {
+		nr_pages = 1 << xas_get_order(&xas);
+		base = round_down(xas.xa_index, nr_pages);
+		if (base < index || base + nr_pages - 1 > end)
+			nr_pages = 0;
+		else
+			xas_store(&xas, NULL);
+	}
+	xas_unlock_irq(&xas);
+
+	if (nr_pages)
+		free_swap_and_cache_nr(radix_to_swp_entry(radswap), nr_pages);
 
-	return 1 << order;
+	return nr_pages;
  }
 
  /*

@@ -1124,8 +1136,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
  			if (xa_is_value(folio)) {
  				if (unfalloc)
  					continue;
-				nr_swaps_freed += shmem_free_swap(mapping,
-							indices[i], folio);
+				nr_swaps_freed += shmem_free_swap(mapping, indices[i],
+								  end - 1, folio);
  				continue;
  			}
 

@@ -1191,12 +1203,23 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
  			folio = fbatch.folios[i];
 
  			if (xa_is_value(folio)) {
+				int order;
  				long swaps_freed;
 
  				if (unfalloc)
  					continue;
-				swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+				swaps_freed = shmem_free_swap(mapping, indices[i],
+							      end - 1, folio);
  				if (!swaps_freed) {
+					/*
+					 * If found a large swap entry cross the end border,
+					 * skip it as the truncate_inode_partial_folio above
+					 * should have at least zerod its content once.
+					 */
+					order = shmem_confirm_swap(mapping, indices[i],
+								   radix_to_swp_entry(folio));
+					if (order > 0 && indices[i] + (1 << order) > end)
+						continue;
  					/* Swap was replaced by page: retry */
  					index = indices[i];
  					break;
 
---

base-commit: 24d479d26b25bce5faea3ddd9fa8f3a6c3129ea7
change-id: 20260111-shmem-swap-fix-8d0e20a14b5d

Best regards,

From: Kairui Song <[email protected]>

The helper for shmem swap freeing is not handling the order of swap entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but it gets the entry order before that using xa_get_order without lock protection, and it may get an outdated order value if the entry is split or changed in other ways after the xa_get_order and before the xa_cmpxchg_irq.

And besides, the order could grow and be larger than expected, and cause truncation to erase data beyond the end border. For example, if the target entry and following entries are swapped in or freed, then a large folio was added in place and swapped out, using the same entry, the xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.

To fix that, open code the Xarray cmpxchg and put the order retrieval and value checking in the same critical section. Also, ensure the order won't exceed the end border, skip it if the entry goes across the border.

Skipping large swap entries crosses the end border is safe here. Shmem truncate iterates the range twice, in the first iteration, find_lock_entries already filtered such entries, and shmem will swapin the entries that cross the end border and partially truncate the folio (split the folio or at least zero part of it). So in the second loop here, if we see a swap entry that crosses the end order, it must at least have its content erased already.

I observed random swapoff hangs and kernel panics when stress testing ZSWAP with shmem. After applying this patch, all problems are gone.

Fixes 809bc86517cc ("mm: shmem: support large folio swap out")

Cc [email protected]

Signed-off-by Kairui Song <[email protected]>

---
Changes in v3:
- Rebased on top of mainline.
- Fix nr_pages calculation [ Baolin Wang ]
- Link to v2: https://lore.kernel.org/r/[email protected]

Changes in v2:
- Fix a potential retry loop issue and improvement to code style thanks
to Baoling Wang. I didn't split the change into two patches because a separate patch doesn't stand well as a fix.
- Link to v1: https://lore.kernel.org/r/[email protected]

---
 mm/shmem.c | 45 ++++++++++++++++++++++++++++++++++-----------
 1 file changed, 34 insertions(+), 11 deletions(-)

mm/shmem.c +34 -12

index ec6c01378e9d..6c3485d24d66 100644
@@ -962,17 +962,29 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap)
  * being freed).
  */
 static long shmem_free_swap(struct address_space *mapping,
-                           pgoff_t index, void *radswap)
+                           pgoff_t index, pgoff_t end, void *radswap)
 {
-       int order = xa_get_order(&mapping->i_pages, index);
-       void *old;
+       XA_STATE(xas, &mapping->i_pages, index);
+       unsigned int nr_pages = 0;
+       pgoff_t base;
+       void *entry;
 
-       old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);
-       if (old != radswap)
-               return 0;
-       free_swap_and_cache_nr(radix_to_swp_entry(radswap), 1 << order);
+       xas_lock_irq(&xas);
+       entry = xas_load(&xas);
+       if (entry == radswap) {
+               nr_pages = 1 << xas_get_order(&xas);
+               base = round_down(xas.xa_index, nr_pages);
+               if (base < index || base + nr_pages - 1 > end)
+                       nr_pages = 0;
+               else
+                       xas_store(&xas, NULL);
+       }
+       xas_unlock_irq(&xas);
+
+       if (nr_pages)
+               free_swap_and_cache_nr(radix_to_swp_entry(radswap), nr_pages);
 
-       return 1 << order;
+       return nr_pages;
 }
 
 /*

@@ -1124,8 +1136,8 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
                        if (xa_is_value(folio)) {
                                if (unfalloc)
                                        continue;
-                               nr_swaps_freed += shmem_free_swap(mapping,
-                                                       indices[i], folio);
+                               nr_swaps_freed += shmem_free_swap(mapping, indices[i],
+                                                                 end - 1, folio);
                                continue;
                        }
 

@@ -1191,12 +1203,23 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
                        folio = fbatch.folios[i];
 
                        if (xa_is_value(folio)) {
+                               int order;
                                long swaps_freed;
 
                                if (unfalloc)
                                        continue;
-                               swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+                               swaps_freed = shmem_free_swap(mapping, indices[i],
+                                                             end - 1, folio);
                                if (!swaps_freed) {
+                                       /*
+                                        * If found a large swap entry cross the end border,
+                                        * skip it as the truncate_inode_partial_folio above
+                                        * should have at least zerod its content once.
+                                        */
+                                       order = shmem_confirm_swap(mapping, indices[i],
+                                                                  radix_to_swp_entry(folio));
+                                       if (order > 0 && indices[i] + (1 << order) > end)
+                                               continue;
                                        /* Swap was replaced by page: retry */
                                        index = indices[i];
                                        break;
 
---

base-commit: 24d479d26b25bce5faea3ddd9fa8f3a6c3129ea7
change-id: 20260111-shmem-swap-fix-8d0e20a14b5d

Best regards,
--
Kairui Song <[email protected]>

From: Kairui Song <[email protected]>

The helper for shmem swap freeing is not handling the order of swap entries correctly. It uses xa_cmpxchg_irq to erase the swap entry, but it gets the entry order before that using xa_get_order without lock protection, and it may get an outdated order value if the entry is split or changed in other ways after the xa_get_order and before the xa_cmpxchg_irq.

And besides, the order could grow and be larger than expected, and cause truncation to erase data beyond the end border. For example, if the target entry and following entries are swapped in or freed, then a large folio was added in place and swapped out, using the same entry, the xa_cmpxchg_irq will still succeed, it's very unlikely to happen though.

To fix that, open code the Xarray cmpxchg and put the order retrieval and value checking in the same critical section. Also, ensure the order won't exceed the end border, skip it if the entry goes across the border.

Skipping large swap entries crosses the end border is safe here. Shmem truncate iterates the range twice, in the first iteration, find_lock_entries already filtered such entries, and shmem will swapin the entries that cross the end border and partially truncate the folio (split the folio or at least zero part of it). So in the second loop here, if we see a swap entry that crosses the end order, it must at least have its content erased already.

I observed random swapoff hangs and kernel panics when stress testing ZSWAP with shmem. After applying this patch, all problems are gone.

mm/shmem.c +0 -0

mm/shmem.c +5 -1

@@ -1191,12 +1203,23 @@
 		folio = fbatch.folios[i];
 
 		if (xa_is_value(folio)) {
+			int order;
 			long swaps_freed;
 
 			if (unfalloc)
 				continue;
-			swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+			swaps_freed = shmem_free_swap(mapping, indices[i],
+						      end - 1, folio);
 			if (!swaps_freed) {
+				/*
+				 * If found a large swap entry cross the end border,

Quoted 4 lines

mm/shmem.c +0 -0

[ ... ]

Quoted 20 lines

mm/shmem.c +10 -1

@@ -1191,12 +1203,23 @@
 		folio = fbatch.folios[i];
 
 		if (xa_is_value(folio)) {
+			int order;
 			long swaps_freed;
 
 			if (unfalloc)
 				continue;
-			swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+			swaps_freed = shmem_free_swap(mapping, indices[i],
+						      end - 1, folio);
 			if (!swaps_freed) {
+				/*
+				 * If found a large swap entry cross the end border,
+				 * skip it as the truncate_inode_partial_folio above
+				 * should have at least zerod its content once.
+				 */
+				order = shmem_confirm_swap(mapping, indices[i],
+							   radix_to_swp_entry(folio));

mm/shmem.c line 1225

Quoted 5 lines

mm/shmem.c +2 -0

+				if (order > 0 && indices[i] + (1 << order) > end)
+					continue;
 				/* Swap was replaced by page: retry */
 				index = indices[i];
 				break;

Could this code infinite loop if a large swap entry crosses the START boundary instead of the END boundary?

When shmem_free_swap() returns 0, it could be either because:
(a) The entry was replaced by a page, or
(b) The entry failed the boundary check (base < index || base + nr_pages - 1 > end)

The boundary check here only handles case (b) for entries crossing the END boundary:

    if (order > 0 && indices[i] + (1 << order) > end)
        continue;

But what happens if the entry crosses the START boundary? If find_get_entries() returns a large swap entry at indices[i] where the entry's base (calculated as indices[i] & ~((1 << order) - 1)) is less than the truncation start point, then shmem_free_swap() will return 0 due to the "base < index" check. The code will then call shmem_confirm_swap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index:

    index = indices[i];
    break;

The next iteration will find the same entry again at the same index, leading to an infinite loop. For example:

- Truncating range [18, 30]
- Large swap entry at [16, 23] (order 3, 8 pages)
- indices[i] = 18
- shmem_free_swap() sees base=16 < index=18, returns 0
- Check: 18 + 8 > 30 is false (26 <= 30)
- Retries with index=18
- Loop repeats indefinitely

Should the boundary check also handle the START case, perhaps:

    if (order > 0) {
        pgoff_t base = indices[i] & ~((1UL << order) - 1);
        if (base + (1 << order) - 1 > end || base < start)
            continue;
    }

where 'start' is preserved from before the loop?

Quoted 27 lines · Chris Mason

Quoted 4 lines

mm/shmem.c +0 -0

[ ... ]

Quoted 20 lines

mm/shmem.c +10 -1

@@ -1191,12 +1203,23 @@
 		folio = fbatch.folios[i];
 
 		if (xa_is_value(folio)) {
+			int order;
 			long swaps_freed;
 
 			if (unfalloc)
 				continue;
-			swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+			swaps_freed = shmem_free_swap(mapping, indices[i],
+						      end - 1, folio);
 			if (!swaps_freed) {
+				/*
+				 * If found a large swap entry cross the end border,
+				 * skip it as the truncate_inode_partial_folio above
+				 * should have at least zerod its content once.
+				 */
+				order = shmem_confirm_swap(mapping, indices[i],
+							   radix_to_swp_entry(folio));

Quoted 8 lines

mm/shmem.c line 1225

Quoted 5 lines

mm/shmem.c +2 -0

+				if (order > 0 && indices[i] + (1 << order) > end)
+					continue;
 				/* Swap was replaced by page: retry */
 				index = indices[i];
 				break;

Could this code infinite loop if a large swap entry crosses the START boundary instead of the END boundary?

Thanks for the review! It's really helpful :)

Quoted 25 lines

When shmem_free_swap() returns 0, it could be either because:
(a) The entry was replaced by a page, or
(b) The entry failed the boundary check (base < index || base + nr_pages - 1 > end)

The boundary check here only handles case (b) for entries crossing the END boundary:

    if (order > 0 && indices[i] + (1 << order) > end)
        continue;

But what happens if the entry crosses the START boundary? If find_get_entries() returns a large swap entry at indices[i] where the entry's base (calculated as indices[i] & ~((1 << order) - 1)) is less than the truncation start point, then shmem_free_swap() will return 0 due to the "base < index" check. The code will then call shmem_confirm_swap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index:

    index = indices[i];
    break;

The next iteration will find the same entry again at the same index, leading to an infinite loop. For example:

Quoted 7 lines

- Truncating range [18, 30]
- Large swap entry at [16, 23] (order 3, 8 pages)
- indices[i] = 18
- shmem_free_swap() sees base=16 < index=18, returns 0
- Check: 18 + 8 > 30 is false (26 <= 30)
- Retries with index=18
- Loop repeats indefinitely

I think this is a valid issue. And it's worse than that, during the while (index < end) loop a new large entry can land anywhere in the range, if one interaction's starting index points to the middle of any large entry, an infinite loop will occur: indices[0] are always equal to the index iteration value of that moments, shmem_free_swap will fail because the swap entry's index doesn't match indices[0], and so the index = indices[i]; break; keep it loop forever.

The chance seems very low though.

Quoted 2 lines

Should the boundary check also handle the START case, perhaps:

Quoted 5 lines

    if (order > 0) {
        pgoff_t base = indices[i] & ~((1UL << order) - 1);
        if (base + (1 << order) - 1 > end || base < start)
            continue;
    }

This still doesn't cover the case when a new large entry somehow lands in the range during the loop.

Quoted 2 lines

where 'start' is preserved from before the loop?

How about following patch:

From 863f38c757ee0898b6b7f0f8c695f551a1380ce8 Mon Sep 17 00:00:00 2001
From: Kairui Song <[email protected]>
Date: Thu, 29 Jan 2026 00:19:23 +0800
Subject: [PATCH] mm, shmem: prevent infinite loop on truncate race

Quoted 27 lines · Chris Mason

Quoted 4 lines

mm/shmem.c +0 -0

[ ... ]

Quoted 20 lines

mm/shmem.c +10 -1

@@ -1191,12 +1203,23 @@
  		folio = fbatch.folios[i];
 
  		if (xa_is_value(folio)) {
+			int order;
  			long swaps_freed;
 
  			if (unfalloc)
  				continue;
-			swaps_freed = shmem_free_swap(mapping, indices[i], folio);
+			swaps_freed = shmem_free_swap(mapping, indices[i],
+						      end - 1, folio);
  			if (!swaps_freed) {
+				/*
+				 * If found a large swap entry cross the end border,
+				 * skip it as the truncate_inode_partial_folio above
+				 * should have at least zerod its content once.
+				 */
+				order = shmem_confirm_swap(mapping, indices[i],
+							   radix_to_swp_entry(folio));

Quoted 8 lines

mm/shmem.c line 1225

Quoted 5 lines

mm/shmem.c +2 -0

+				if (order > 0 && indices[i] + (1 << order) > end)
+					continue;
  				/* Swap was replaced by page: retry */
  				index = indices[i];
  				break;

Could this code infinite loop if a large swap entry crosses the START boundary instead of the END boundary?

Thanks for the review! It's really helpful :)

Quoted 25 lines

When shmem_free_swap() returns 0, it could be either because:
(a) The entry was replaced by a page, or
(b) The entry failed the boundary check (base < index || base + nr_pages - 1 > end)

The boundary check here only handles case (b) for entries crossing the END boundary:

     if (order > 0 && indices[i] + (1 << order) > end)
         continue;

But what happens if the entry crosses the START boundary? If find_get_entries() returns a large swap entry at indices[i] where the entry's base (calculated as indices[i] & ~((1 << order) - 1)) is less than the truncation start point, then shmem_free_swap() will return 0 due to the "base < index" check. The code will then call shmem_confirm_swap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index:

     index = indices[i];
     break;

The next iteration will find the same entry again at the same index, leading to an infinite loop. For example:

Quoted 7 lines

- Truncating range [18, 30]
- Large swap entry at [16, 23] (order 3, 8 pages)
- indices[i] = 18
- shmem_free_swap() sees base=16 < index=18, returns 0
- Check: 18 + 8 > 30 is false (26 <= 30)
- Retries with index=18
- Loop repeats indefinitely

I think this is a valid issue. And it's worse than that, during the while (index < end) loop a new large entry can land anywhere in the range, if one interaction's starting index points to the middle of any large entry, an infinite loop will occur: indices[0] are always equal to the index iteration value of that moments, shmem_free_swap will fail because the swap entry's index doesn't match indices[0], and so the index = indices[i]; break; keep it loop forever.

The chance seems very low though.

Quoted 7 lines

Should the boundary check also handle the START case, perhaps:

     if (order > 0) {
         pgoff_t base = indices[i] & ~((1UL << order) - 1);
         if (base + (1 << order) - 1 > end || base < start)
             continue;
     }

This still doesn't cover the case when a new large entry somehow lands in the range during the loop.

Quoted 1 line

where 'start' is preserved from before the loop?

How about following patch:

From 863f38c757ee0898b6b7f0f8c695f551a1380ce8 Mon Sep 17 00:00:00 2001
From: Kairui Song <[email protected]>
Date: Thu, 29 Jan 2026 00:19:23 +0800
Subject: [PATCH] mm, shmem: prevent infinite loop on truncate race

When truncating a large swap entry, shmem_free_swap() returns 0 when the entry's index doesn't match the given index due to lookup alignment. The failure fallback path checks if the entry crosses the end border and aborts when it happens, so truncate won't erase an unexpected entry or range. But one scenario was ignored.

When index points to the middle of a large swap entry, and the large swap entry doesn't go across the end border, find_get_entries() will return that large swap entry as the first item in the batch with indices[0] equal to index. The entry's base index will be smaller than indices[0], so shmem_free_swap() will fail and return 0 due to the "base < index" check. The code will then call shmem_confirm_swap(), get the order, check if it crosses the END boundary (which it doesn't), and retry with the same index.

The next iteration will find the same entry again at the same index with same indices, leading to an infinite loop.

Fix this by retrying with a round-down index, and abort if the index is smaller than the truncate range.

Reported-by Chris Mason <[email protected]>

Closes https://lore.kernel.org/linux-mm/[email protected]/ open here

Fixes 809bc86517cc ("mm: shmem: support large folio swap out")

Fixes 8a1968bd997f ("mm/shmem, swap: fix race of truncate and swap entry split")

Signed-off-by Kairui Song <[email protected]>

---

  mm/shmem.c | 23 ++++++++++++++---------
  1 file changed, 14 insertions(+), 9 deletions(-)

mm/shmem.c +14 -9

index b9ddd38621a0..fe3719eb5a3c 100644
@@ -1211,17 +1211,22 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, uoff_t lend,
  				swaps_freed = shmem_free_swap(mapping, indices[i],
  							      end - 1, folio);
  				if (!swaps_freed) {
-					/*
-					 * If found a large swap entry cross the end border,
-					 * skip it as the truncate_inode_partial_folio above
-					 * should have at least zerod its content once.
-					 */
+					pgoff_t base = indices[i];
+
  					order = shmem_confirm_swap(mapping, indices[i],
  								   radix_to_swp_entry(folio));
-					if (order > 0 && indices[i] + (1 << order) > end)
-						continue;
-					/* Swap was replaced by page: retry */
-					index = indices[i];
+					/*
+					 * If found a large swap entry cross the end or start
+					 * border, skip it as the truncate_inode_partial_folio
+					 * above should have at least zerod its content once.
+					 */
+					if (order > 0) {
+						base = round_down(base, 1 << order);
+						if (base < start || base + (1 << order) > end)
+							continue;
+					}
+					/* Swap was replaced by page or extended, retry */
+					index = base;
  					break;
  				}
  				nr_swaps_freed += swaps_freed;