Re: zswap z3fold + memory offline = infinite loop

[Vitaly Wool <vitaly.wool@konsulko.com>]

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On Wed, May 13, 2020, 2:36 AM Qian Cai <cai@lca.pw> wrote:

> Put zswap z3fold pages into the memory and then offline those memory would
> trigger an infinite loop here in
>
> __offline_pages() --> do_migrate_range() because there is no error
> handling,
>
>                         if (pfn) {
>                                 /*
>                                  * TODO: fatal migration failures should
> bail
>                                  * out
>                                  */
>                                 do_migrate_range(pfn, end_pfn);
>
> There, isolate_movable_page() will always return -EBUSY  because,
>
>         if (!mapping->a_ops->isolate_page(page, mode))
>                 goto out_no_isolated;
>
> i.e., z3fold_page_isolate() will always return false because,
>
> zhdr->mapped_count == 2
>

So who mapped these pages? The whole zswap operation presumes that objects
are mapped for a short while to run some I/O and so, most of the time
zhdr->mapped_count would be 0.

Removing that check in ->isolate() is not a big deal, but ->migratepage()
shall not allow actual migration anyway if there are mapped objects.

~Vitaly

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