[PATCH v3 1/7] mm/shmem, swap: improve cached mTHP handling and fix potential hung

[PATCH v3 1/7] mm/shmem, swap: improve cached mTHP handling and fix potential hung

[Kairui Song]

From: Kairui Song <kasong@tencent.com>

The current swap-in code assumes that, when a swap entry in shmem mapping
is order 0, its cached folios (if present) must be order 0 too, which
turns out not always correct.

The problem is shmem_split_large_entry is called before verifying the
folio will eventually be swapped in, one possible race is:

    CPU1                          CPU2
shmem_swapin_folio
/* swap in of order > 0 swap entry S1 */
  folio = swap_cache_get_folio
  /* folio = NULL */
  order = xa_get_order
  /* order > 0 */
  folio = shmem_swap_alloc_folio
  /* mTHP alloc failure, folio = NULL */
  <... Interrupted ...>
                                 shmem_swapin_folio
                                 /* S1 is swapped in */
                                 shmem_writeout
                                 /* S1 is swapped out, folio cached */
  shmem_split_large_entry(..., S1)
  /* S1 is split, but the folio covering it has order > 0 now */

Now any following swapin of S1 will hang: `xa_get_order` returns 0, and
folio lookup will return a folio with order > 0.  The
`xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will always
return false causing swap-in to return -EEXIST.

And this looks fragile.  So fix this up by allowing seeing a larger folio
in swap cache, and check the whole shmem mapping range covered by the
swapin have the right swap value upon inserting the folio.  And drop the
redundant tree walks before the insertion.

This will actually improve performance, as it avoids two redundant Xarray
tree walks in the hot path, and the only side effect is that in the
failure path, shmem may redundantly reallocate a few folios causing
temporary slight memory pressure.

And worth noting, it may seems the order and value check before inserting
might help reducing the lock contention, which is not true.  The swap
cache layer ensures raced swapin will either see a swap cache folio or
failed to do a swapin (we have SWAP_HAS_CACHE bit even if swap cache is
bypassed), so holding the folio lock and checking the folio flag is
already good enough for avoiding the lock contention.  The chance that a
folio passes the swap entry value check but the shmem mapping slot has
changed should be very low.

Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
Signed-off-by: Kairui Song <kasong@tencent.com>
Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: <stable@vger.kernel.org>
---
 mm/shmem.c | 30 +++++++++++++++++++++---------
 1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index 334b7b4a61a0..e3c9a1365ff4 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -884,7 +884,9 @@ static int shmem_add_to_page_cache(struct folio *folio,
 				   pgoff_t index, void *expected, gfp_t gfp)
 {
 	XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
-	long nr = folio_nr_pages(folio);
+	unsigned long nr = folio_nr_pages(folio);
+	swp_entry_t iter, swap;
+	void *entry;
 
 	VM_BUG_ON_FOLIO(index != round_down(index, nr), folio);
 	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
@@ -896,14 +898,24 @@ static int shmem_add_to_page_cache(struct folio *folio,
 
 	gfp &= GFP_RECLAIM_MASK;
 	folio_throttle_swaprate(folio, gfp);
+	swap = iter = radix_to_swp_entry(expected);
 
 	do {
 		xas_lock_irq(&xas);
-		if (expected != xas_find_conflict(&xas)) {
-			xas_set_err(&xas, -EEXIST);
-			goto unlock;
+		xas_for_each_conflict(&xas, entry) {
+			/*
+			 * The range must either be empty, or filled with
+			 * expected swap entries. Shmem swap entries are never
+			 * partially freed without split of both entry and
+			 * folio, so there shouldn't be any holes.
+			 */
+			if (!expected || entry != swp_to_radix_entry(iter)) {
+				xas_set_err(&xas, -EEXIST);
+				goto unlock;
+			}
+			iter.val += 1 << xas_get_order(&xas);
 		}
-		if (expected && xas_find_conflict(&xas)) {
+		if (expected && iter.val - nr != swap.val) {
 			xas_set_err(&xas, -EEXIST);
 			goto unlock;
 		}
@@ -2323,7 +2335,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 			error = -ENOMEM;
 			goto failed;
 		}
-	} else if (order != folio_order(folio)) {
+	} else if (order > folio_order(folio)) {
 		/*
 		 * Swap readahead may swap in order 0 folios into swapcache
 		 * asynchronously, while the shmem mapping can still stores
@@ -2348,15 +2360,15 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
 
 			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
 		}
+	} else if (order < folio_order(folio)) {
+		swap.val = round_down(swap.val, 1 << folio_order(folio));
 	}
 
 alloced:
 	/* We have to do this with folio locked to prevent races */
 	folio_lock(folio);
 	if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
-	    folio->swap.val != swap.val ||
-	    !shmem_confirm_swap(mapping, index, swap) ||
-	    xa_get_order(&mapping->i_pages, index) != folio_order(folio)) {
+	    folio->swap.val != swap.val) {
 		error = -EEXIST;
 		goto unlock;
 	}
-- 
2.50.0

Re: [PATCH v3 1/7] mm/shmem, swap: improve cached mTHP handling and fix potential hung

[Baolin Wang]

On 2025/6/27 14:20, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
> 
> The current swap-in code assumes that, when a swap entry in shmem mapping
> is order 0, its cached folios (if present) must be order 0 too, which
> turns out not always correct.
> 
> The problem is shmem_split_large_entry is called before verifying the
> folio will eventually be swapped in, one possible race is:
> 
>      CPU1                          CPU2
> shmem_swapin_folio
> /* swap in of order > 0 swap entry S1 */
>    folio = swap_cache_get_folio
>    /* folio = NULL */
>    order = xa_get_order
>    /* order > 0 */
>    folio = shmem_swap_alloc_folio
>    /* mTHP alloc failure, folio = NULL */
>    <... Interrupted ...>
>                                   shmem_swapin_folio
>                                   /* S1 is swapped in */
>                                   shmem_writeout
>                                   /* S1 is swapped out, folio cached */
>    shmem_split_large_entry(..., S1)
>    /* S1 is split, but the folio covering it has order > 0 now */
> 
> Now any following swapin of S1 will hang: `xa_get_order` returns 0, and
> folio lookup will return a folio with order > 0.  The
> `xa_get_order(&mapping->i_pages, index) != folio_order(folio)` will always
> return false causing swap-in to return -EEXIST.
> 
> And this looks fragile.  So fix this up by allowing seeing a larger folio
> in swap cache, and check the whole shmem mapping range covered by the
> swapin have the right swap value upon inserting the folio.  And drop the
> redundant tree walks before the insertion.
> 
> This will actually improve performance, as it avoids two redundant Xarray
> tree walks in the hot path, and the only side effect is that in the
> failure path, shmem may redundantly reallocate a few folios causing
> temporary slight memory pressure.
> 
> And worth noting, it may seems the order and value check before inserting
> might help reducing the lock contention, which is not true.  The swap
> cache layer ensures raced swapin will either see a swap cache folio or
> failed to do a swapin (we have SWAP_HAS_CACHE bit even if swap cache is
> bypassed), so holding the folio lock and checking the folio flag is
> already good enough for avoiding the lock contention.  The chance that a
> folio passes the swap entry value check but the shmem mapping slot has
> changed should be very low.
> 
> Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> Signed-off-by: Kairui Song <kasong@tencent.com>
> Reviewed-by: Kemeng Shi <shikemeng@huaweicloud.com>
> Cc: <stable@vger.kernel.org>

Thanks for fixing the issue.
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> ---
>   mm/shmem.c | 30 +++++++++++++++++++++---------
>   1 file changed, 21 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 334b7b4a61a0..e3c9a1365ff4 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -884,7 +884,9 @@ static int shmem_add_to_page_cache(struct folio *folio,
>   				   pgoff_t index, void *expected, gfp_t gfp)
>   {
>   	XA_STATE_ORDER(xas, &mapping->i_pages, index, folio_order(folio));
> -	long nr = folio_nr_pages(folio);
> +	unsigned long nr = folio_nr_pages(folio);
> +	swp_entry_t iter, swap;
> +	void *entry;
>   
>   	VM_BUG_ON_FOLIO(index != round_down(index, nr), folio);
>   	VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
> @@ -896,14 +898,24 @@ static int shmem_add_to_page_cache(struct folio *folio,
>   
>   	gfp &= GFP_RECLAIM_MASK;
>   	folio_throttle_swaprate(folio, gfp);
> +	swap = iter = radix_to_swp_entry(expected);
>   
>   	do {
>   		xas_lock_irq(&xas);
> -		if (expected != xas_find_conflict(&xas)) {
> -			xas_set_err(&xas, -EEXIST);
> -			goto unlock;
> +		xas_for_each_conflict(&xas, entry) {
> +			/*
> +			 * The range must either be empty, or filled with
> +			 * expected swap entries. Shmem swap entries are never
> +			 * partially freed without split of both entry and
> +			 * folio, so there shouldn't be any holes.
> +			 */
> +			if (!expected || entry != swp_to_radix_entry(iter)) {
> +				xas_set_err(&xas, -EEXIST);
> +				goto unlock;
> +			}
> +			iter.val += 1 << xas_get_order(&xas);
>   		}
> -		if (expected && xas_find_conflict(&xas)) {
> +		if (expected && iter.val - nr != swap.val) {
>   			xas_set_err(&xas, -EEXIST);
>   			goto unlock;
>   		}
> @@ -2323,7 +2335,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>   			error = -ENOMEM;
>   			goto failed;
>   		}
> -	} else if (order != folio_order(folio)) {
> +	} else if (order > folio_order(folio)) {
>   		/*
>   		 * Swap readahead may swap in order 0 folios into swapcache
>   		 * asynchronously, while the shmem mapping can still stores
> @@ -2348,15 +2360,15 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>   
>   			swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
>   		}
> +	} else if (order < folio_order(folio)) {
> +		swap.val = round_down(swap.val, 1 << folio_order(folio));
>   	}
>   
>   alloced:
>   	/* We have to do this with folio locked to prevent races */
>   	folio_lock(folio);
>   	if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
> -	    folio->swap.val != swap.val ||
> -	    !shmem_confirm_swap(mapping, index, swap) ||
> -	    xa_get_order(&mapping->i_pages, index) != folio_order(folio)) {
> +	    folio->swap.val != swap.val) {
>   		error = -EEXIST;
>   		goto unlock;
>   	}