Re: [PATCH 14/19] mm, swap: sanitize swap entry management workflow

[YoungJun Park]

On Wed, Oct 29, 2025 at 11:58:40PM +0800, Kairui Song wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>

Hello Kairui!

> The current swap entry allocation/freeing workflow has never had a clear
> definition. This makes it hard to debug or add new optimizations.
> 
> This commit introduces a proper definition of how swap entries would be
> allocated and freed. Now, most operations are folio based, so they will
> never exceed one swap cluster, and we now have a cleaner border between
> swap and the rest of mm, making it much easier to follow and debug,
> especially with new added sanity checks. Also making more optimization
> possible.
> 
> Swap entry will be mostly allocated and free with a folio bound.
> The folio lock will be useful for resolving many swap ralated races.
> 
> Now swap allocation (except hibernation) always starts with a folio in
> the swap cache, and gets duped/freed protected by the folio lock:
> 
> - folio_alloc_swap() - The only allocation entry point now.
>   Context: The folio must be locked.
>   This allocates one or a set of continuous swap slots for a folio and
>   binds them to the folio by adding the folio to the swap cache. The
>   swap slots' swap count start with zero value.
> 
> - folio_dup_swap() - Increase the swap count of one or more entries.
>   Context: The folio must be locked and in the swap cache. For now, the
>   caller still has to lock the new swap entry owner (e.g., PTL).
>   This increases the ref count of swap entries allocated to a folio.
>   Newly allocated swap slots' count has to be increased by this helper
>   as the folio got unmapped (and swap entries got installed).
> 
> - folio_put_swap() - Decrease the swap count of one or more entries.
>   Context: The folio must be locked and in the swap cache. For now, the
>   caller still has to lock the new swap entry owner (e.g., PTL).
>   This decreases the ref count of swap entries allocated to a folio.
>   Typically, swapin will decrease the swap count as the folio got
>   installed back and the swap entry got uninstalled
> 
>   This won't remove the folio from the swap cache and free the
>   slot. Lazy freeing of swap cache is helpful for reducing IO.
>   There is already a folio_free_swap() for immediate cache reclaim.
>   This part could be further optimized later.
> 
> The above locking constraints could be further relaxed when the swap
> table if fully implemented. Currently dup still needs the caller
> to lock the swap entry container (e.g. PTL), or a concurrent zap
> may underflow the swap count.
> 
> Some swap users need to interact with swap count without involving folio
> (e.g. forking/zapping the page table or mapping truncate without swapin).
> In such cases, the caller has to ensure there is no race condition on
> whatever owns the swap count and use the below helpers:
> 
> - swap_put_entries_direct() - Decrease the swap count directly.
>   Context: The caller must lock whatever is referencing the slots to
>   avoid a race.
> 
>   Typically the page table zapping or shmem mapping truncate will need
>   to free swap slots directly. If a slot is cached (has a folio bound),
>   this will also try to release the swap cache.
> 
> - swap_dup_entry_direct() - Increase the swap count directly.
>   Context: The caller must lock whatever is referencing the entries to
>   avoid race, and the entries must already have a swap count > 1.
> 
>   Typically, forking will need to copy the page table and hence needs to
>   increase the swap count of the entries in the table. The page table is
>   locked while referencing the swap entries, so the entries all have a
>   swap count > 1 and can't be freed.
> 
> Hibernation subsystem is a bit different, so two special wrappers are here:
> 
> - swap_alloc_hibernation_slot() - Allocate one entry from one device.
> - swap_free_hibernation_slot() - Free one entry allocated by the above
> helper.

During the code review, I found something to be verified.
It is not directly releavant your patch, 
I send the email for checking it right and possible fix on this patch.

on the swap_alloc_hibernation_slot function
nr_swap_pages is decreased. but as I think it is decreased on swap_range_alloc.

The nr_swap_pages are decremented as the callflow as like the below.

cluster_alloc_swap_entry -> alloc_swap_scan_cluster
-> closter_alloc_range -> swap_range_alloc

Introduced on
4f78252da887ee7e9d1875dd6e07d9baa936c04f
mm: swap: move nr_swap_pages counter decrement  from folio_alloc_swap() to swap_range_alloc()

 #ifdef CONFIG_HIBERNATION
 /* Allocate a slot for hibernation */
 swp_entry_t swap_alloc_hibernation_slot(int type)
 {
....
                       local_unlock(&percpu_swap_cluster.lock);
                        if (offset) {
                                entry = swp_entry(si->type, offset);
                                atomic_long_dec(&nr_swap_pages); // here


Thank you,
Youngjun Park