Re: [PATCH] zsmalloc: return -EAGAIN for zspage migration lock contention

[Sergey Senozhatsky]

On (26/03/18 15:31), Hui Zhu wrote:
> movable_operations::migrate_page() should return -EAGAIN for temporary
> migration failures so the migration core can retry. Other negative
> errors are treated as permanent failures.
> 
> zs_page_migrate() currently returns -EINVAL when zspage_write_trylock()
> fails. That path reflects transient lock contention, not invalid input.
> Returning -EINVAL misclassifies the failure and can reduce migration
> success under contention.

The zspage's reader-lock owner can be preempted for unknown duration,
so I don't know if returning -EAGAIN is safe.  On one hand, migration
attempts are limited by NR_MAX_MIGRATE_PAGES_RETRY, on the other hand,
those attempts are performed basically immediately (?), I'm not sure
if we have good chances for preempted reader-lock owner to get scheduled
and release the lock in time.