Re: [PATCH 2/4] mm: mempool: introduce page bulk allocator

[Yang Shi <shy828301@gmail.com>]

On Thu, Oct 13, 2022 at 5:38 AM Mel Gorman <mgorman@techsingularity.net> wrote:
>
> On Wed, Oct 05, 2022 at 11:03:39AM -0700, Yang Shi wrote:
> > Since v5.13 the page bulk allocator was introduced to allocate order-0
> > pages in bulk.  There are a few mempool allocator callers which does
> > order-0 page allocation in a loop, for example, dm-crypt, f2fs compress,
> > etc.  A mempool page bulk allocator seems useful.  So introduce the
> > mempool page bulk allocator.
> >
> > It introduces the below APIs:
> >   - mempool_init_pages_bulk()
> >   - mempool_create_pages_bulk()
> > They initialize the mempool for page bulk allocator.  The pool is filled
> > by alloc_page() in a loop.
> >
> >   - mempool_alloc_pages_bulk_list()
> >   - mempool_alloc_pages_bulk_array()
> > They do bulk allocation from mempool.
> > They do the below conceptually:
> >   1. Call bulk page allocator
> >   2. If the allocation is fulfilled then return otherwise try to
> >      allocate the remaining pages from the mempool
> >   3. If it is fulfilled then return otherwise retry from #1 with sleepable
> >      gfp
> >   4. If it is still failed, sleep for a while to wait for the mempool is
> >      refilled, then retry from #1
> > The populated pages will stay on the list or array until the callers
> > consume them or free them.
> > Since mempool allocator is guaranteed to success in the sleepable context,
> > so the two APIs return true for success or false for fail.  It is the
> > caller's responsibility to handle failure case (partial allocation), just
> > like the page bulk allocator.
> >
> > The mempool typically is an object agnostic allocator, but bulk allocation
> > is only supported by pages, so the mempool bulk allocator is for page
> > allocation only as well.
> >
> > Signed-off-by: Yang Shi <shy828301@gmail.com>
>
> Overall, I think it's an ok approach and certainly a good use case for
> the bulk allocator.
>
> The main concern that I have is that the dm-crypt use case doesn't really
> want to use lists as such and it's just a means for collecting pages to pass
> to bio_add_page(). bio_add_page() is working with arrays but you cannot
> use that array directly as any change to how that array is populated will
> then explode. Unfortunately, what you have is adding pages to a list to
> take them off the list and put them in an array and that is inefficient.

Yeah, I didn't think of a better way to pass the pages to dm-crypt.

>
> How about this
>
> 1. Add a callback to __alloc_pages_bulk() that takes a page as a
>    parameter like bulk_add_page() or whatever.
>
> 2. For page_list == NULL && page_array == NULL, the callback is used
>
> 3. Add alloc_pages_bulk_cb() that passes in the name of a callback
>    function
>
> 4. In the dm-crypt case, use the callback to pass the page to bio_add_page
>    for the new page allocated.

Thank you so much for the suggestion. But I have a hard time
understanding how these work together. Do you mean call bio_add_page()
in the callback? But bio_add_page() needs other parameters. Or I
misunderstood you?

>
> It's not free because there will be an additional function call for every
> page bulk allocated but I suspect that's cheaper than adding a pile of
> pages to a list just to take them off again. It also avoids adding a user
> for the bulk allocator list interface that does not even want a list.
>
> It might mean that there is additional cleanup work for __alloc_pages_bulk
> to abstract away whether a list, array or cb is used but nothing
> impossible.
>
> --
> Mel Gorman
> SUSE Labs