linux-kernel - Re: [PATCH v2] mm: split thp synchronously on MADV

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Date:   Thu, 25 Nov 2021 09:39:18 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Shakeel Butt <shakeelb@...gle.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, Peter Xu <peterx@...hat.com>
Cc:     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
Subject: Re: [PATCH v2] mm: split thp synchronously on MADV_DONTNEED and
 munmap

On 25.11.21 03:45, Shakeel Butt wrote:
> Many applications do sophisticated management of their heap memory for
> better performance but with low cost. We have a bunch of such
> applications running on our production and examples include caching and
> data storage services. These applications keep their hot data on the
> THPs for better performance and release the cold data through
> MADV_DONTNEED to keep the memory cost low.
> 
> The kernel defers the split and release of THPs until there is memory
> pressure. This complicates the memory management of these sophisticated
> applications which then needs to look into low level kernel handling of
> THPs to better gauge their headroom for expansion.
> 
> More specifically these applications monitor their cgroup usage to decide
> if they can expand the memory footprint or release some (unneeded/cold)
> buffer. They uses madvise(MADV_DONTNEED) to release the memory which
> basically puts the THP into defer list. These deferred THPs are still
> charged to the cgroup which leads to bloated usage read by the application
> and making wrong decisions. In addition these applications are very
> latency sensitive and would prefer to not face memory reclaim due to
> non-deterministic nature of reclaim.
> 
> Internally we added a cgroup interface to trigger the split of deferred
> THPs for that cgroup but this is hacky and exposing kernel internals to
> users. This patch solves this problem in a more general way for the users
> by splitting the THPS synchronously on MADV_DONTNEED. This patch does
> the same for munmap() too.
> 

I'll have to defer diving into the code.

Just a comment: It might be good to add that there are still cases where
splitting the compound page can fail -- for example, if the page is
still pinned/referenced.

So if you have a THP and intended to only pin/reference e.g., the first
4k of it (e.g., O_DIRECT, io_uring fixed buffers), MADV_DONTNEED/unmap
e.g., the last 4k of it will not split synchronously.

In addition to explicit user action on a compound page; I remember there
might be other kernel-internal temporary references that could
theoretically block splitting, but maybe most of them are at least for
now limited to !compound pages.

-- 
Thanks,

David / dhildenb