Re: [PATCH] mm: split thp synchronously on MADV_DONTNEED

[Peter Xu]

On Fri, Nov 26, 2021 at 10:16:58AM +0100, David Hildenbrand wrote:
> Would this really apply to MADV_DONTNEED on shmem, and would deferred
> splitting apply on shmem? I'm constantly confused about shmem vs. anon,
> but I would have assumed that shmem is fd-based and we wouldn't end up
> in rmap_walk_anon. For shmem, the pagecache would contain the THP which
> would stick around and deferred splits don't even apply.

Good point..  when split on shmem we just clear pmd, so yeah I don't think
we'll ever add it into the deferred list.

> User space can trigger a split manually using some MADV hackery. But it
> can only be used for the use case here, where we actually want to zap a
> page.
> 
> 1. MADV_FREE a single 4k page in the range. This will split the PMD->PTE
>    and the compound page.

Seems to be a very implicit but working solution indeed.

> 2. MADV_DONTNEED either the complete range or the single 4k page.

Is this what this patch is working on?

Thanks,

-- 
Peter Xu