linux-kernel - Re: [PATCH] mm: split thp synchronously on MADV

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Date:   Fri, 26 Nov 2021 17:39:45 +0800
From:   Peter Xu <peterx@...hat.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
        Matthew Wilcox <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] mm: split thp synchronously on MADV_DONTNEED

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