Re: [dm-devel] [v2 PATCH 0/5] Introduce mempool pages bulk allocator and use it in dm-crypt

[Yang Shi <shy828301@gmail.com>]

On Wed, Feb 15, 2023 at 4:23 AM Mikulas Patocka <mpatocka@redhat.com> wrote:
>
>
>
> On Tue, 14 Feb 2023, Yang Shi wrote:
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> >
> > Changelog:
> > RFC -> v2:
> >   * Added callback variant for page bulk allocator and mempool bulk allocator
> >     per Mel Gorman.
> >   * Used the callback version in dm-crypt driver.
> >   * Some code cleanup and refactor to reduce duplicate code.
> >
> > rfc: https://lore.kernel.org/linux-mm/20221005180341.1738796-1-shy828301@gmail.com/
>
> Hi
>
> This seems like unneeded complication to me. We have alloc_pages(), it can
> allocate multiple pages efficiently, so why not use it?

The alloc_pages() allocates *contiguous* pages, but dm-crypt doesn't
need contiguous pages at all. This may incur unnecessary compaction
overhead to the dm-crypt layer when memory is fragmented. The bulk
allocator is a good fit to this usecase, which allocates multiple
order-0 pages.

In addition, filesystem writeback doesn't guarantee power-of-2 pages
every time IIUC. But alloc_pages() just can allocate power-of-2 pages.

>
> I suggest to modify crypt_alloc_buffer() to use alloc_pages() and if
> alloc_pages() fails (either because the system is low on memory or because
> memory is too fragmented), fall back to the existing code that does
> mempool_alloc().

My PoC patches just did this way, but called bulk allocator. There may
be other potential mepool users as I listed in this cover letter,
which may get benefits from bulk allocator. So introducing a new bulk
mempool API seems better for long run although we just have one user
for now. And it makes other uses easier to gain the benefit by just
calling the new API.

>
> Mikulas
>