On 2024/8/26 06:31, Hugh Dickins wrote:
On Mon, 12 Aug 2024, Baolin Wang wrote:
Now the swap device can only swap-in order 0 folio, even though a large
folio is swapped out. This requires us to split the large entry previously
saved in the shmem pagecache to support the swap in of small folios.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/shmem.c | 100 +++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 100 insertions(+)
diff --git a/mm/shmem.c b/mm/shmem.c
index 345e25425e37..996062dc196b 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1990,6 +1990,81 @@ 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, int new_order, gfp_t gfp)
+{
+ struct address_space *mapping = inode->i_mapping;
+ XA_STATE_ORDER(xas, &mapping->i_pages, index, new_order);
+ void *alloced_shadow = NULL;
+ int alloced_order = 0, i;
gfp needs to be adjusted: see fix patch below.
Ah, good catch. Thank you Hugh.
+
+ for (;;) {
+ int order = -1, split_order = 0;
+ void *old = NULL;
+
+ xas_lock_irq(&xas);
+ old = xas_load(&xas);
+ if (!xa_is_value(old) || swp_to_radix_entry(swap) != old) {
+ xas_set_err(&xas, -EEXIST);
+ goto unlock;
+ }
+
+ order = xas_get_order(&xas);
+
+ /* Swap entry may have changed before we re-acquire the lock */
+ if (alloced_order &&
+ (old != alloced_shadow || order != alloced_order)) {
+ xas_destroy(&xas);
+ alloced_order = 0;
+ }
+
+ /* Try to split large swap entry in pagecache */
+ if (order > 0 && order > new_order) {
I have not even attempted to understand all the manipulations of order and
new_order and alloced_order and split_order. And further down it turns out
that this is only ever called with new_order 0.
You may be wanting to cater for more generality in future, but for now
please cut this down to the new_order 0 case, and omit that parameter.
It will be easier for us to think about the xa_get_order() races if
the possibilities are more limited.
Sure. I will drop the 'new_order' with following fix.
+ if (!alloced_order) {
+ split_order = order;
+ goto unlock;
+ }
+ xas_split(&xas, old, order);
+
+ /*
+ * Re-set the swap entry after splitting, and the swap
+ * offset of the original large entry must be continuous.
+ */
+ for (i = 0; i < 1 << order; i += (1 << new_order)) {
+ pgoff_t aligned_index = round_down(index, 1 << order);
+ swp_entry_t tmp;
+
+ tmp = swp_entry(swp_type(swap), swp_offset(swap) + i);
+ __xa_store(&mapping->i_pages, aligned_index + i,
+ swp_to_radix_entry(tmp), 0);
+ }
So that is done under xas lock: good. But is the intermediate state
visible to RCU readers, and could that be a problem?
In xas_split(), the multi-index entry has been split into smaller
entries, and each of these smaller entries has been set with the old
swap value. During the process of __xa_store(), these entries will be
re-set to the new swap value. Although RCU readers might observe the old
swap value, I have not seen any problems until now (may be I missed
something).
For concurrent shmem swap-in cases, there are some checks in
shmem_swapin_folio() (including folio->swap.val and shmem_confirm_swap()
validation ) to ensure the correctness of the swap values.
For the shmem_partial_swap_usage(), we may get racy swap usages, but it
is not a problem form its comments:
" * This is safe to call without i_rwsem or the i_pages lock thanks to RCU,
* as long as the inode doesn't go away and racy results are not a
problem."
For shmem truncation, when removing the racy swap entry from shmem page
cache, it will use xa_cmpxchg_irq() to sync the correct swap state.
[PATCH] mm: shmem: split large entry if the swapin folio is not large
fix 2
Now we only split large folio to order 0, so drop the 'new_order'
parameter.
Signed-off-by: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
---
mm/shmem.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/mm/shmem.c b/mm/shmem.c
index d8038a66b110..f00b7b99ad09 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -1998,10 +1998,10 @@ static void shmem_set_folio_swapin_error(struct
inode *inode, pgoff_t index,
}
static int shmem_split_large_entry(struct inode *inode, pgoff_t index,
- swp_entry_t swap, int new_order,
gfp_t gfp)
+ swp_entry_t swap, gfp_t gfp)
{
struct address_space *mapping = inode->i_mapping;
- XA_STATE_ORDER(xas, &mapping->i_pages, index, new_order);
+ XA_STATE_ORDER(xas, &mapping->i_pages, index, 0);
void *alloced_shadow = NULL;
int alloced_order = 0, i;
@@ -2026,7 +2026,7 @@ static int shmem_split_large_entry(struct inode
*inode, pgoff_t index,
}
/* Try to split large swap entry in pagecache */
- if (order > 0 && order > new_order) {
+ if (order > 0) {
if (!alloced_order) {
split_order = order;
goto unlock;
@@ -2037,7 +2037,7 @@ static int shmem_split_large_entry(struct inode
*inode, pgoff_t index,
* Re-set the swap entry after splitting, and
the swap
* offset of the original large entry must be
continuous.
*/
- for (i = 0; i < 1 << order; i += (1 << new_order)) {
+ for (i = 0; i < 1 << order; i++) {
pgoff_t aligned_index =
round_down(index, 1 << order);
swp_entry_t tmp;
@@ -2123,7 +2123,7 @@ static int shmem_swapin_folio(struct inode *inode,
pgoff_t index,
* should split the large swap entry stored in the
pagecache
* if necessary.
*/
- split_order = shmem_split_large_entry(inode, index,
swap, 0, gfp);
+ split_order = shmem_split_large_entry(inode, index,
swap, gfp);
if (split_order < 0) {
error = split_order;
goto failed;
