Re: [PATCH v3 14/15] mm, swap: implement dynamic allocation of swap table

[Chris Mason <clm@meta.com>]

On 9/15/25 12:24 PM, Kairui Song wrote:
> On Mon, Sep 15, 2025 at 11:55 PM Chris Mason <clm@meta.com> wrote:
>>
>> On Thu, 11 Sep 2025 00:08:32 +0800 Kairui Song <ryncsn@gmail.com> wrote:

[ ... ]
             spin_lock(&si->global_cluster_lock);
>>> +     /*
>>> +      * Back to atomic context. First, check if we migrated to a new
>>> +      * CPU with a usable percpu cluster. If so, try using that instead.
>>> +      * No need to check it for the spinning device, as swap is
>>> +      * serialized by the global lock on them.
>>> +      *
>>> +      * The is_usable check is a bit rough, but ensures order 0 success.
>>> +      */
>>> +     offset = this_cpu_read(percpu_swap_cluster.offset[order]);
>>> +     if ((si->flags & SWP_SOLIDSTATE) && offset) {
>>> +             pcp_ci = swap_cluster_lock(si, offset);
>>> +             if (cluster_is_usable(pcp_ci, order) &&
>>> +                 pcp_ci->count < SWAPFILE_CLUSTER) {
>>> +                     ci = pcp_ci;
>>                        ^^^^^^^^^^^^^
>> ci came from the caller, and in the case of isolate_lock_cluster() they
>> had just removed it from a list.  We overwrite ci and return something
>> different.
> 
> Yes, that's expected. See the comment above. We have just dropped
> local lock so it's possible that we migrated to another CPU which has
> its own percpu cache ci (percpu_swap_cluster.offset).
> 
> To avoid fragmentation, drop the isolated ci and use the percpu ci
> instead. But you are right that I need to add the ci back to the list,
> or it will be leaked. Thanks!

Yeah, the comment helped a lot (thank you).  It was just the leak I was
worried about ;)

-chris