Re: [PATCH v2] mm: split thp synchronously on MADV_DONTNEED and munmap

[Shakeel Butt <shakeelb@google.com>]

On Thu, Nov 25, 2021 at 12:39 AM David Hildenbrand <david@redhat.com> wrote:
>
> 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.
>

Hi David,

Thanks for your time (and apologies) but I have to rescind this patch
for now due to mistaken performance impact. Let's move the discussion
to the other thread and decide next steps there.

thanks,
Shakeel