mm: zswap: don't fail a large-folio swapin whose range is not in zswap

thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample zswap_never_enabled() to decide whether a swapin may use a large folio. zswap_load() samples the same one-way static key again once the read reaches it. Nothing serialises the two reads, and in between the task allocates and pins a high-order folio, which can sleep.

If zswap is enabled for the first time in that window, a large folio that was correctly permitted reaches zswap_load(), which rejects every large folio with -EINVAL. swap_read_folio() treats anything other than -ENOENT as "zswap handled it" and skips the backing-device read, so the folio comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO for shmem. The data is intact on the swap device - it was written there before zswap was ever enabled - and the not-uptodate folio stays in the swap cache, so every retry of the fault fails the same way. With panic_on_warn the WARN takes the machine down rather than the task.

Scan the range instead of rejecting the folio. The caller has pinned every slot before issuing the read, so zswap cannot start a store or a writeback into the range and the scan is stable. If nothing in the range is in zswap it is all on the backing device: return -ENOENT and let swap_read_folio() read it.

A range that does have a slot in zswap is still refused, because zswap stores large folios as order-0 entries and cannot reconstruct one. That stays reachable - a slot shared with another task can be stored inside the same window - and refusing is correct, since the alternative is returning the stale device copy. Report it as -EIO rather than -EINVAL:
the request is valid, zswap just cannot serve it. The only caller distinguishes -ENOENT from everything else, so that part is a documentation fix.

Fixes 242d12c98174 ("mm: support large folios swap-in for sync io devices")

Cc [email protected]

Co-developed-by Alexandre Ghiti <[email protected]>

Signed-off-by Alexandre Ghiti <[email protected]>

Signed-off-by Usama Arif <[email protected]>

---
 mm/zswap.c | 58 ++++++++++++++++++++++++++++++++++++++++--------------
 1 file changed, 43 insertions(+), 15 deletions(-)

mm/zswap.c +43 -16

index 37f34e406c8e3..fd36ac38e9a1e 100644
@@ -1571,6 +1571,32 @@ bool zswap_store(struct folio *folio)
        return ret;
 }
 
+/**
+ * zswap_is_present() - is any slot in [entry, entry + nr) in zswap?
+ * @entry: base swap entry of the range
+ * @nr: number of contiguous slots to check
+ *
+ * Context: The caller must keep the range pinned, otherwise the answer can
+ * change under it.
+ * Return: true if at least one slot in the range is in zswap.
+ */
+static bool zswap_is_present(swp_entry_t entry, unsigned int nr)
+{
+       pgoff_t offset = swp_offset(entry);
+       struct xarray *tree = swap_zswap_tree(entry);
+       unsigned long index = offset;
+
+       /*
+        * A pinned range is at most SWAPFILE_CLUSTER slots and is aligned to
+        * its own size, so one tree covers all of it and a single lookup is
+        * enough. Scanning only part of the range would report a false
+        * "absent" and let the caller read a stale copy from the device.
+        */
+       BUILD_BUG_ON(SWAPFILE_CLUSTER > ZSWAP_ADDRESS_SPACE_PAGES);
+
+       return xa_find(tree, &index, offset + nr - 1, XA_PRESENT);
+}
+
 /**
  * zswap_load() - load a folio from zswap
  * @folio: folio to load

@@ -1578,15 +1604,12 @@ bool zswap_store(struct folio *folio)
  * Return: 0 on success, with the folio unlocked and marked up-to-date, or one
  * of the following error codes:
  *
- *  -EIO: if the swapped out content was in zswap, but could not be loaded
- *  into the page due to a decompression failure. The folio is unlocked, but
- *  NOT marked up-to-date, so that an IO error is emitted (e.g. do_swap_page()
- *  will SIGBUS).
- *
- *  -EINVAL: if the swapped out content was in zswap, but the page belongs
- *  to a large folio, which is not supported by zswap. The folio is unlocked,
- *  but NOT marked up-to-date, so that an IO error is emitted (e.g.
- *  do_swap_page() will SIGBUS).
+ *  -EIO: if the swapped out content was in zswap but could not be handed
+ *  back, either because decompression failed or because a slot in a
+ *  large-folio range is still in zswap and zswap cannot reconstruct a large
+ *  folio from per-page entries. The folio is unlocked, but NOT marked
+ *  up-to-date, so that an IO error is emitted (e.g. do_swap_page() will
+ *  SIGBUS).
  *
  *  -ENOENT: if the swapped out content was not in zswap. The folio remains
  *  locked on return.

@@ -1605,13 +1628,18 @@ int zswap_load(struct folio *folio)
                return -ENOENT;
 
        /*
-        * Large folios should not be swapped in while zswap is being used, as
-        * they are not properly handled. Zswap does not properly load large
-        * folios, and a large folio may only be partially in zswap.
+        * A large folio can legitimately reach zswap_load() with its whole
+        * range on the backing device, so scan the range rather than rejecting
+        * it outright. The caller has pinned every slot, so zswap cannot start
+        * a store or a writeback into the range while we look.
         */
-       if (WARN_ON_ONCE(folio_test_large(folio))) {
-               folio_unlock(folio);
-               return -EINVAL;
+       if (folio_test_large(folio)) {
+               if (WARN_ON_ONCE(zswap_is_present(swp,
+                                                 folio_nr_pages(folio)))) {
+                       folio_unlock(folio);
+                       return -EIO;
+               }
+               return -ENOENT;
        }
 
        entry = xa_load(tree, offset);
--

2.53.0-Meta

thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample zswap_never_enabled() to decide whether a swapin may use a large folio. zswap_load() samples the same one-way static key again once the read reaches it. Nothing serialises the two reads, and in between the task allocates and pins a high-order folio, which can sleep.

If zswap is enabled for the first time in that window, a large folio that was correctly permitted reaches zswap_load(), which rejects every large folio with -EINVAL. swap_read_folio() treats anything other than -ENOENT as "zswap handled it" and skips the backing-device read, so the folio comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO for shmem. The data is intact on the swap device - it was written there before zswap was ever enabled - and the not-uptodate folio stays in the swap cache, so every retry of the fault fails the same way. With panic_on_warn the WARN takes the machine down rather than the task.

Scan the range instead of rejecting the folio. The caller has pinned every slot before issuing the read, so zswap cannot start a store or a writeback into the range and the scan is stable. If nothing in the range is in zswap it is all on the backing device: return -ENOENT and let swap_read_folio() read it.

A range that does have a slot in zswap is still refused, because zswap stores large folios as order-0 entries and cannot reconstruct one. That stays reachable - a slot shared with another task can be stored inside the same window - and refusing is correct, since the alternative is returning the stale device copy. Report it as -EIO rather than -EINVAL:
the request is valid, zswap just cannot serve it. The only caller distinguishes -ENOENT from everything else, so that part is a documentation fix.

Fixes 242d12c98174 ("mm: support large folios swap-in for sync io devices")

Cc [email protected]

Co-developed-by Alexandre Ghiti <[email protected]>

Signed-off-by Alexandre Ghiti <[email protected]>

Signed-off-by Usama Arif <[email protected]>

thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample zswap_never_enabled() to decide whether a swapin may use a large folio. zswap_load() samples the same one-way static key again once the read reaches it. Nothing serialises the two reads, and in between the task allocates and pins a high-order folio, which can sleep.

If zswap is enabled for the first time in that window, a large folio that was correctly permitted reaches zswap_load(), which rejects every large folio with -EINVAL. swap_read_folio() treats anything other than -ENOENT as "zswap handled it" and skips the backing-device read, so the folio comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO for shmem. The data is intact on the swap device - it was written there before zswap was ever enabled - and the not-uptodate folio stays in the swap cache, so every retry of the fault fails the same way. With panic_on_warn the WARN takes the machine down rather than the task.

Scan the range instead of rejecting the folio. The caller has pinned every slot before issuing the read, so zswap cannot start a store or a writeback into the range and the scan is stable. If nothing in the range is in zswap it is all on the backing device: return -ENOENT and let swap_read_folio() read it.

A range that does have a slot in zswap is still refused, because zswap stores large folios as order-0 entries and cannot reconstruct one. That stays reachable - a slot shared with another task can be stored inside the same window - and refusing is correct, since the alternative is returning the stale device copy. Report it as -EIO rather than -EINVAL:
the request is valid, zswap just cannot serve it. The only caller distinguishes -ENOENT from everything else, so that part is a documentation fix.

Fixes 242d12c98174 ("mm: support large folios swap-in for sync io devices")

Cc [email protected]

Quoted 22 lines · On Mon, 7 Sep 2026 09:19:38 -0700 Usama Arif <[email protected]> wrote:

thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample zswap_never_enabled() to decide whether a swapin may use a large folio. zswap_load() samples the same one-way static key again once the read reaches it. Nothing serialises the two reads, and in between the task allocates and pins a high-order folio, which can sleep.

If zswap is enabled for the first time in that window, a large folio that was correctly permitted reaches zswap_load(), which rejects every large folio with -EINVAL. swap_read_folio() treats anything other than -ENOENT as "zswap handled it" and skips the backing-device read, so the folio comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO for shmem. The data is intact on the swap device - it was written there before zswap was ever enabled - and the not-uptodate folio stays in the swap cache, so every retry of the fault fails the same way. With panic_on_warn the WARN takes the machine down rather than the task.

Scan the range instead of rejecting the folio. The caller has pinned every slot before issuing the read, so zswap cannot start a store or a writeback into the range and the scan is stable. If nothing in the range is in zswap it is all on the backing device: return -ENOENT and let swap_read_folio() read it.

Quoted 8 lines

A range that does have a slot in zswap is still refused, because zswap stores large folios as order-0 entries and cannot reconstruct one. That stays reachable - a slot shared with another task can be stored inside the same window - and refusing is correct, since the alternative is returning the stale device copy. Report it as -EIO rather than -EINVAL:
the request is valid, zswap just cannot serve it. The only caller distinguishes -ENOENT from everything else, so that part is a documentation fix.

So to hit this bug the user needs to enable zswap system-wide during a teeny race window in the swapin code?

I suspect nobody has ever hit this and couldn't do so if they tried?

Quoted 3 lines

Fixes 242d12c98174 ("mm: support large folios swap-in for sync io devices")

Cc [email protected]

Documentation/process/stable-kernel-rules.rst, with which I agree:

Rules on what kind of patches are accepted, and which ones are not, into the "-stable" tree:

- It or an equivalent fix must already exist in Linux mainline (upstream).
- It must be obviously correct and tested.
- It cannot be bigger than 100 lines, with context.
- It must follow the :ref:Documentation/process/submitting-patches.rst <submittingpatches> rules.
- It must either fix a real bug that bothers people or just add a device ID. To elaborate on the former:

- It fixes a problem like an oops, a hang, data corruption, a real security issue, a hardware quirk, a build error (but not for things marked CONFIG_BROKEN), or some "oh, that's not good" issue.
- Serious issues as reported by a user of a distribution kernel may also be considered if they fix a notable performance or interactivity issue. As these fixes are not as obvious and have a higher risk of a subtle regression they should only be submitted by a distribution kernel maintainer and include an addendum linking to a bugzilla entry if it exists and additional information on the user-visible impact.
- No "This could be a problem..." type of things like a "theoretical race condition", unless an explanation of how the bug can be exploited is also provided.
- No "trivial" fixes without benefit for users (spelling changes, whitespace cleanups, etc).

If this patch meets the above then its changelog needs an update!

Quoted 36 lines · Andrew Morton

Quoted 22 lines · On Mon, 7 Sep 2026 09:19:38 -0700 Usama Arif <[email protected]> wrote:

thp_swapin_suitable_orders() and shmem_swap_alloc_folio() sample zswap_never_enabled() to decide whether a swapin may use a large folio. zswap_load() samples the same one-way static key again once the read reaches it. Nothing serialises the two reads, and in between the task allocates and pins a high-order folio, which can sleep.

If zswap is enabled for the first time in that window, a large folio that was correctly permitted reaches zswap_load(), which rejects every large folio with -EINVAL. swap_read_folio() treats anything other than -ENOENT as "zswap handled it" and skips the backing-device read, so the folio comes back unlocked and not uptodate: SIGBUS for an anonymous fault, -EIO for shmem. The data is intact on the swap device - it was written there before zswap was ever enabled - and the not-uptodate folio stays in the swap cache, so every retry of the fault fails the same way. With panic_on_warn the WARN takes the machine down rather than the task.

Scan the range instead of rejecting the folio. The caller has pinned every slot before issuing the read, so zswap cannot start a store or a writeback into the range and the scan is stable. If nothing in the range is in zswap it is all on the backing device: return -ENOENT and let swap_read_folio() read it.

Quoted 8 lines

A range that does have a slot in zswap is still refused, because zswap stores large folios as order-0 entries and cannot reconstruct one. That stays reachable - a slot shared with another task can be stored inside the same window - and refusing is correct, since the alternative is returning the stale device copy. Report it as -EIO rather than -EINVAL:
the request is valid, zswap just cannot serve it. The only caller distinguishes -ENOENT from everything else, so that part is a documentation fix.

So to hit this bug the user needs to enable zswap system-wide during a teeny race window in the swapin code?

Quoted 1 line

I suspect nobody has ever hit this and couldn't do so if they tried?

Yes, I think it would be very very difficult to hit this.

It was part of my PMD swap series, where its actually needed for the feature.

I think we can drop cc:stable, unless you think its needed Yosry?