On Mon, Jan 12, 2026 at 4:22 PM Baolin Wang baolin.wang@linux.alibaba.com wrote:
On 1/12/26 1:56 PM, Kairui Song wrote:
On Mon, Jan 12, 2026 at 12:00 PM Baolin Wang baolin.wang@linux.alibaba.com wrote:
On 1/12/26 1:53 AM, Kairui Song wrote:
From: Kairui Song kasong@tencent.com
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. As a result the order could be a stalled value if the entry is split after the xa_get_order and before the xa_cmpxchg_irq. In fact that are more way for other races to occur during the time window.
To fix that, open code the Xarray cmpxchg and put the order retrivial and value checking in the same critical section. Also ensure the order won't exceed the truncate border.
I observed random swapoff hangs and swap entry leaks when stress testing ZSWAP with shmem. After applying this patch, the problem is resolved.
Fixes: 809bc86517cc ("mm: shmem: support large folio swap out") Cc: stable@vger.kernel.org Signed-off-by: Kairui Song kasong@tencent.com
mm/shmem.c | 35 +++++++++++++++++++++++------------ 1 file changed, 23 insertions(+), 12 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c index 0b4c8c70d017..e160da0cd30f 100644 --- a/mm/shmem.c +++ b/mm/shmem.c @@ -961,18 +961,28 @@ static void shmem_delete_from_page_cache(struct folio *folio, void *radswap) * the number of pages being freed. 0 means entry not found in XArray (0 pages * being freed). */ -static long shmem_free_swap(struct address_space *mapping,
pgoff_t index, void *radswap)+static long shmem_free_swap(struct address_space *mapping, pgoff_t index,
unsigned int max_nr, 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;void *entry;
old = xa_cmpxchg_irq(&mapping->i_pages, index, radswap, NULL, 0);if (old != radswap)return 0;swap_put_entries_direct(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);if (index == round_down(xas.xa_index, nr_pages) && nr_pages < max_nr)xas_store(&xas, NULL);elsenr_pages = 0;}xas_unlock_irq(&xas);if (nr_pages)swap_put_entries_direct(radix_to_swp_entry(radswap), nr_pages);
return 1 << order;
return nr_pages;Thanks for the analysis, and it makes sense to me. Would the following implementation be simpler and also address your issue (we will not release the lock in __xa_cmpxchg() since gfp = 0)?
Hi Baolin,
static long shmem_free_swap(struct address_space *mapping, pgoff_t index, void *radswap) { XA_STATE(xas, &mapping->i_pages, index); int order; void *old;
xas_lock_irq(&xas); order = xas_get_order(&xas);Thanks for the suggestion. I did consider implementing it this way, but I was worried that the order could grow upwards. For example shmem_undo_range is trying to free 0-95 and there is an entry at 64 with order 5 (64 - 95). Before shmem_free_swap is called, the entry was swapped in, then the folio was freed, then an order 6 folio was allocated there and swapped out again using the same entry.
Then here it will free the whole order 6 entry (64 - 127), while shmem_undo_range is only supposed to erase (0-96).
Good point. However, this cannot happen during swapoff, because the 'end' is set to -1 in shmem_evict_inode().
That's not only for swapff, shmem_truncate_range / falloc can also use it right?
Actually, the real question is how to handle the case where a large swap entry happens to cross the 'end' when calling shmem_truncate_range(). If the shmem mapping stores a folio, we would split that large folio by truncate_inode_partial_folio(). If the shmem mapping stores a large swap entry, then as you noted, the truncation range can indeed exceed the 'end'.
But with your change, that large swap entry would not be truncated, and I’m not sure whether that might cause other issues. Perhaps the best approach is to first split the large swap entry and only truncate the swap entries within the 'end' boundary like the truncate_inode_partial_folio() does.
Right... I was thinking that the shmem_undo_range iterates the undo range twice IIUC, in the second try it will retry if shmem_free_swap returns 0:
swaps_freed = shmem_free_swap(mapping, indices[i], end - indices[i], folio); if (!swaps_freed) { /* Swap was replaced by page: retry */ index = indices[i]; break; }
So I thought shmem_free_swap returning 0 is good enough. Which is not, it may cause the second loop to retry forever.
Alternatively, this patch could only focus on the race on the order, which seems uncontested. As for handling large swap entries that go beyond the 'end', should we address that in a follow-up, for example by splitting? What do you think?
I think a partial fix is still wrong, How about we just handle the split here, like this?
static int shmem_free_swap(struct address_space *mapping, pgoff_t index, unsigned int max_nr, void *radswap) { XA_STATE(xas, &mapping->i_pages, index); int nr_pages = 0, ret; void *entry; bool split;
retry: xas_lock_irq(&xas); entry = xas_load(&xas); if (entry == radswap) { nr_pages = 1 << xas_get_order(&xas); /* * Check if the order growed upwards and a larger entry is * now covering the target entry. In this case caller may need to * restart the iteration. */ if (index != round_down(xas.xa_index, nr_pages)) { xas_unlock_irq(&xas); return 0; }
/* Check if we are freeing part of a large entry. */ if (nr_pages > max_nr) { xas_unlock_irq(&xas); /* Let the caller decide what to do by returning 0 if split failed. */ if (shmem_split_large_entry(mapping, index + max_nr, radswap, mapping_gfp(mapping))) return 0; goto retry; }
xas_store(&xas, NULL); xas_unlock_irq(&xas);
swap_put_entries_direct(radix_to_swp_entry(radswap), nr_pages); return nr_pages; }
xas_unlock_irq(&xas); return 0; }