Re: [PATCH v3 09/15] mm/shmem, swap: remove redundant error handling for replacing folio

Baolin Wang
2025-09-12T08:22:14Z

On 2025/9/11 00:08, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
>
> Shmem may replace a folio in the swap cache if the cached one doesn't
> fit the swapin's GFP zone. When doing so, shmem has already double
> checked that the swap cache folio is locked, still has the swap cache
> flag set, and contains the wanted swap entry. So it is impossible to
> fail due to an XArray mismatch. There is even a comment for that.
>
> Delete the defensive error handling path, and add a WARN_ON instead:
> if that happened, something has broken the basic principle of how the
> swap cache works, we should catch and fix that.
>
> Signed-off-by: Kairui Song <kasong@tencent.com>
> Reviewed-by: David Hildenbrand <david@redhat.com>
> ---
> mm/shmem.c | 42 ++++++++++++------------------------------
> 1 file changed, 12 insertions(+), 30 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 410f27bc4752..5f395fab489c 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -1661,13 +1661,13 @@ int shmem_writeout(struct folio *folio, struct swap_iocb **plug,
> }
>
> /*
> - * The delete_from_swap_cache() below could be left for
> + * The swap_cache_del_folio() below could be left for
> * shrink_folio_list()'s folio_free_swap() to dispose of;
> * but I'm a little nervous about letting this folio out of
> * shmem_writeout() in a hybrid half-tmpfs-half-swap state
> * e.g. folio_mapping(folio) might give an unexpected answer.
> */
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> goto redirty;
> }
> if (nr_pages > 1)
> @@ -2045,7 +2045,7 @@ static struct folio *shmem_swap_alloc_folio(struct inode *inode,
> new->swap = entry;
>
> memcg1_swapin(entry, nr_pages);
> - shadow = get_shadow_from_swap_cache(entry);
> + shadow = swap_cache_get_shadow(entry);

Again, there are still some issues with the patch split. The swapcache
related APIs replacement should be placed in Patch 8, otherwise there
will be buidling errors after applying Patch 8.

With this issue fixed:
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>

> if (shadow)
> workingset_refault(new, shadow);
> folio_add_lru(new);
> @@ -2121,35 +2121,17 @@ static int shmem_replace_folio(struct folio **foliop, gfp_t gfp,
> /* Swap cache still stores N entries instead of a high-order entry */
> xa_lock_irq(&swap_mapping->i_pages);
> for (i = 0; i < nr_pages; i++) {
> - void *item = xas_load(&xas);
> -
> - if (item != old) {
> - error = -ENOENT;
> - break;
> - }
> -
> - xas_store(&xas, new);
> + WARN_ON_ONCE(xas_store(&xas, new));
> xas_next(&xas);
> }
> - if (!error) {
> - mem_cgroup_replace_folio(old, new);
> - shmem_update_stats(new, nr_pages);
> - shmem_update_stats(old, -nr_pages);
> - }
> xa_unlock_irq(&swap_mapping->i_pages);
>
> - if (unlikely(error)) {
> - /*
> - * Is this possible? I think not, now that our callers
> - * check both the swapcache flag and folio->private
> - * after getting the folio lock; but be defensive.
> - * Reverse old to newpage for clear and free.
> - */
> - old = new;
> - } else {
> - folio_add_lru(new);
> - *foliop = new;
> - }
> + mem_cgroup_replace_folio(old, new);
> + shmem_update_stats(new, nr_pages);
> + shmem_update_stats(old, -nr_pages);
> +
> + folio_add_lru(new);
> + *foliop = new;
>
> folio_clear_swapcache(old);
> old->private = NULL;
> @@ -2183,7 +2165,7 @@ static void shmem_set_folio_swapin_error(struct inode *inode, pgoff_t index,
> nr_pages = folio_nr_pages(folio);
> folio_wait_writeback(folio);
> if (!skip_swapcache)
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> /*
> * Don't treat swapin error folio as alloced. Otherwise inode->i_blocks
> * won't be 0 when inode is released and thus trigger WARN_ON(i_blocks)
> @@ -2422,7 +2404,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
> folio->swap.val = 0;
> swapcache_clear(si, swap, nr_pages);
> } else {
> - delete_from_swap_cache(folio);
> + swap_cache_del_folio(folio);
> }
> folio_mark_dirty(folio);
> swap_free_nr(swap, nr_pages);