Re: [PATCH v3 4/7] mm/shmem, swap: clean up swap entry splitting

From: Baolin Wang
Date: Mon Jun 30 2025 - 05:53:54 EST

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On 2025/6/30 17:16, Kairui Song wrote:

On Mon, Jun 30, 2025 at 2:34 PM Baolin Wang
<baolin.wang@xxxxxxxxxxxxxxxxx> wrote:

On 2025/6/27 14:20, Kairui Song wrote:

From: Kairui Song <kasong@xxxxxxxxxxx>

Instead of keeping different paths of splitting the entry and
recalculating the swap entry and index, do it in one place.

Whenever swapin brought in a folio smaller than the entry, split the
entry. And always recalculate the entry and index, in case it might
read in a folio that's larger than the entry order. This removes
duplicated code and function calls, and makes the code more robust.

Signed-off-by: Kairui Song <kasong@xxxxxxxxxxx>
---
mm/shmem.c | 103 +++++++++++++++++++++--------------------------------
1 file changed, 41 insertions(+), 62 deletions(-)

diff --git a/mm/shmem.c b/mm/shmem.c
index f85a985167c5..5be9c905396e 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2178,8 +2178,12 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
swap_free_nr(swap, nr_pages);
}

-static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
- swp_entry_t swap, gfp_t gfp)
+/*
+ * Split an existing large swap entry. @index should point to one sub mapping
+ * slot within the entry @swap, this sub slot will be split into order 0.
+ */
+static int shmem_split_swap_entry(struct inode *inode, pgoff_t index,
+ swp_entry_t swap, gfp_t gfp)
{
struct address_space *mapping = inode->i_mapping;
XA_STATE_ORDER(xas, &mapping->i_pages, index, 0);
@@ -2250,7 +2254,7 @@ static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
if (xas_error(&xas))
return xas_error(&xas);

- return entry_order;
+ return 0;
}

/*
@@ -2267,11 +2271,11 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
struct address_space *mapping = inode->i_mapping;
struct mm_struct *fault_mm = vma ? vma->vm_mm : NULL;
struct shmem_inode_info *info = SHMEM_I(inode);
+ int error, nr_pages, order, swap_order;
struct swap_info_struct *si;
struct folio *folio = NULL;
bool skip_swapcache = false;
swp_entry_t swap;
- int error, nr_pages, order, split_order;

VM_BUG_ON(!*foliop || !xa_is_value(*foliop));
swap = radix_to_swp_entry(*foliop);
@@ -2321,70 +2325,43 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
goto failed;
}

- /*
- * Now swap device can only swap in order 0 folio, then we
- * should split the large swap entry stored in the pagecache
- * if necessary.
- */
- split_order = shmem_split_large_entry(inode, index, swap, gfp);
- if (split_order < 0) {
- error = split_order;
- goto failed;
- }
-
- /*
- * If the large swap entry has already been split, it is
- * necessary to recalculate the new swap entry based on
- * the old order alignment.
- */
- if (split_order > 0) {
- pgoff_t offset = index - round_down(index, 1 << split_order);
-
- swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
- }
-
/* Here we actually start the io */
folio = shmem_swapin_cluster(swap, gfp, info, index);
if (!folio) {
error = -ENOMEM;
goto failed;
}
- } else if (order > folio_order(folio)) {
- /*
- * Swap readahead may swap in order 0 folios into swapcache
- * asynchronously, while the shmem mapping can still stores
- * large swap entries. In such cases, we should split the
- * large swap entry to prevent possible data corruption.
- */
- split_order = shmem_split_large_entry(inode, index, swap, gfp);
- if (split_order < 0) {
- folio_put(folio);
- folio = NULL;
- error = split_order;
- goto failed;
- }
-
- /*
- * If the large swap entry has already been split, it is
- * necessary to recalculate the new swap entry based on
- * the old order alignment.
- */
- if (split_order > 0) {
- pgoff_t offset = index - round_down(index, 1 << split_order);
-
- 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 need to split an existing large entry if swapin brought in a
+ * smaller folio due to various of reasons.
+ *
+ * And worth noting there is a special case: if there is a smaller
+ * cached folio that covers @swap, but not @index (it only covers
+ * first few sub entries of the large entry, but @index points to
+ * later parts), the swap cache lookup will still see this folio,
+ * And we need to split the large entry here. Later checks will fail,
+ * as it can't satisfy the swap requirement, and we will retry
+ * the swapin from beginning.
+ */
+ swap_order = folio_order(folio);

Nit: 'swap_order' is confusing, and can you just use folio_order() or a
btter name?

Good idea.

+ if (order > swap_order) {
+ error = shmem_split_swap_entry(inode, index, swap, gfp);
+ if (error)
+ goto failed_nolock;
+ }
+
+ index = round_down(index, 1 << swap_order);
+ swap.val = round_down(swap.val, 1 << swap_order);

The round_down() of index and swap value here may cause
shmem_add_to_page_cache() to fail to insert a new folio, because the
swap value stored at that index in the shmem mapping does not match,
leading to another swapin page fault for correction.

For example, shmem stores a large swap entry of order 4 in the range of
index 0-64. When a swapin fault occurs at index = 3, with swap.val =
0x4000, if a split happens and this round_down() logic is applied, then
index = 3, swap.val = 0x4000. However, the actual swap.val should be
0x4003 stored in the shmem mapping. This would cause another swapin fault.

Oops, I missed a swap value fixup in the !SWP_SYNCHRONOUS_IO path
above, it should re-calculate the swap value there. It's fixed in the
final patch but left unhandled here. I'll update this part.

I still prefer my original alignment method, and do you find this will
cause any issues?

"
if (split_order > 0) {
pgoff_t offset = index - round_down(index, 1 << split_order);

swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
}
"

It only fits the cached swapin and uncached swapin, not the cache hit
case. Cache hits may see a larger folio so split didn't happen, but
the round_down is still needed.

IMO, this only fits for the large swap entry split case.

And there is another racy case: before this patch, the split may
happen first, but shmem_swapin_cluster brought in a large folio due to
race in the swap cache layer.

shmem_swapin_cluster() can only allocate order 0 folio, right?

And I'm not sure if split_order is always reliable here, for example
concurrent split may return an inaccurate value here.

We've held the xas lock to ensure the split is reliable, even though concurrent splits may occur, only one split can get the large 'split_order', another will return 0 (since it will see the large swao entry has already been split).

Based on your current patch, would the following modifications be clearer?

diff --git a/mm/shmem.c b/mm/shmem.c
index 5be9c905396e..91c071fb7b67 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2254,7 +2254,7 @@ static int shmem_split_swap_entry(struct inode *inode, pgoff_t index,
if (xas_error(&xas))
return xas_error(&xas);

- return 0;
+ return split_order;
}

/*
@@ -2351,10 +2351,23 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
error = shmem_split_swap_entry(inode, index, swap, gfp);
if (error)
goto failed_nolock;
- }

- index = round_down(index, 1 << swap_order);
- swap.val = round_down(swap.val, 1 << swap_order);
+ /*
+ * If the large swap entry has already been split, it is
+ * necessary to recalculate the new swap entry based on
+ * the old order alignment.
+ */
+ if (split_order > 0) {
+ pgoff_t offset = index - round_down(index, 1 << split_order);
+
+ swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
+ }
+ } else if (order < folio_order(folio)) {
+ /*
+ * TODO; explain the posible race...
+ */
+ swap.val = round_down(swap.val, 1 << folio_order(folio));
+ }

/* We have to do this with folio locked to prevent races */
folio_lock(folio);
@@ -2382,7 +2395,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
goto failed;
}

- error = shmem_add_to_page_cache(folio, mapping, index,
+ error = shmem_add_to_page_cache(folio, mapping,
+ round_down(index, nr_pages),
swp_to_radix_entry(swap), gfp);
if (error)
goto failed;

So I wanted to simplify it: by round_down(folio_order(folio)) we
simply get the index and swap that will be covered by this specific
folio, if the covered range still has the corresponding swap entries
(check and ensured by shmem_add_to_page_cache which holds the
xa_lock), then the folio will be inserted back safely and
successfully.

Next message: Andy Shevchenko: "Re: [PATCH v2 7/8] auxdisplay: Add Titanmec TM16xx 7-segment display controllers driver"
Previous message: Andreas Hindborg: "Re: [PATCH v5 04/10] rust: sync: atomic: Add generic atomics"
In reply to: Kairui Song: "Re: [PATCH v3 4/7] mm/shmem, swap: clean up swap entry splitting"
Next in thread: Kairui Song: "Re: [PATCH v3 4/7] mm/shmem, swap: clean up swap entry splitting"
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