Re: [PATCH v5 14/19] mm, swap: cleanup swap entry management workflow

[Baoquan He]

On 12/20/25 at 03:43am, Kairui Song wrote:
> From: Kairui Song <kasong@xxxxxxxxxxx>
> 
> 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.
                                          ~~~~
                                          freed, typo
> 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
        ~~ s/if/is/ typo

> to lock the swap entry container (e.g. PTL), or a concurrent zap
> may underflow the swap count.
......