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

Baoquan He
2025-12-20T04:02:27Z

On 12/20/25 at 03:43am, Kairui Song wrote:
> From: Kairui Song <kasong@tencent.com>
>
> 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.
......