netdev - Re: [PATCH 2/5] mm/page_alloc: Add a bulk page allocator

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Date:   Sat, 13 Mar 2021 19:33:43 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     Chuck Lever III <chuck.lever@...cle.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-Net <netdev@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linux NFS Mailing List <linux-nfs@...r.kernel.org>
Subject: Re: [PATCH 2/5] mm/page_alloc: Add a bulk page allocator

On Sat, Mar 13, 2021 at 04:56:31PM +0000, Chuck Lever III wrote:
> IME lists are indeed less CPU-efficient, but I wonder if that
> expense is insignificant compared to serialization primitives like
> disabling and re-enabling IRQs, which we are avoiding by using
> bulk page allocation.

Cache misses are a worse problem than serialisation.  Paul McKenney had
a neat demonstration where he took a sheet of toilet paper to represent
an instruction, and then unrolled two rolls of toilet paper around the
lecture theatre to represent an L3 cache miss.  Obviously a serialising
instruction is worse than an add instruction, but i'm thinking maybe
50-100 sheets of paper, not an entire roll?

Anyway, I'm not arguing against a bulk allocator, nor even saying this
is a bad interface.  It just maybe could be better.

> My initial experience with the current interface left me feeling
> uneasy about re-using the lru list field. That seems to expose an
> internal API feature to consumers of the page allocator. If we
> continue with a list-centric bulk allocator API I hope there can
> be some conveniently-placed documentation that explains when it is
> safe to use that field. Or perhaps the field should be renamed.

Heh.  Spoken like a filesystem developer who's never been exposed to the
->readpages API (it's almost dead).  It's fairly common in the memory
management world to string pages together through the lru list_head.
Slab does it, as does put_pages_list() in mm/swap.c.  It's natural for
Mel to keep using this pattern ... and I dislike it intensely.

> I have a mild preference for an array-style interface because that's
> more natural for the NFSD consumer, but I'm happy to have a bulk
> allocator either way. Purely from a code-reuse point of view, I
> wonder how many consumers of alloc_pages_bulk() will be like
> svc_alloc_arg(), where they need to fill in pages in an array. Each
> such consumer would need to repeat the logic to convert the returned
> list into an array. We have, for instance, release_pages(), which is
> an array-centric page allocator API. Maybe a helper function or two
> might prevent duplication of the list conversion logic.
> 
> And I agree with Mel that passing a single large array seems more
> useful then having to build code at each consumer call-site to
> iterate over smaller page_vecs until that array is filled.

So how about this?

You provide the interface you'd _actually_ like to use (array-based) and
implement it on top of Mel's lru-list implementation.  If it's general
enough to be used by Jesper's use-case, we lift it to page_alloc.c.
If we go a year and there are no users of the lru-list interface, we
can just change the implementation.