Re: [PATCH] zsmalloc: Fix races between asynchronous zspage free and page migration

[Sultan Alsawaf <sultan@kerneltoast.com>]

On Mon, May 09, 2022 at 05:06:32PM -0700, Andrew Morton wrote:
> Why not simply lock_page() here?  The get_page() alone won't protect
> from all the dire consequences which you have identified?

Hi,

My reasoning is that if the page migrated, then we've got the last reference
to it anyway and there's no point in locking. But more importantly, we'd still
need to take migrate_read_lock() again in order to verify whether or not the
page migrated because of data races stemming from replace_sub_page(), so I don't
think there's much to gain by using lock_page(). When any of the pages in the
zspage migrates, the entire page list is reconstructed and every page's private
storage is rewritten. I had drafted another change that fixes the data races by
trimming out all of that redundant work done in replace_sub_page(), but I wanted
to keep this patch small to make it easier to review and easier to backport.

Sultan