linux-kernel - Re: [PATCH v2] mm: alloc_pages_bulk: remove assumption of populating only NULL elements

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Message-id: <174129565467.33508.7106343513316364028@noble.neil.brown.name>
Date: Fri, 07 Mar 2025 08:14:14 +1100
From: "NeilBrown" <neilb@...e.de>
To: "Yunsheng Lin" <linyunsheng@...wei.com>
Cc: "Qu Wenruo" <wqu@...e.com>, "Yishai Hadas" <yishaih@...dia.com>,
 "Jason Gunthorpe" <jgg@...pe.ca>,
 "Shameer Kolothum" <shameerali.kolothum.thodi@...wei.com>,
 "Kevin Tian" <kevin.tian@...el.com>,
 "Alex Williamson" <alex.williamson@...hat.com>, "Chris Mason" <clm@...com>,
 "Josef Bacik" <josef@...icpanda.com>, "David Sterba" <dsterba@...e.com>,
 "Gao Xiang" <xiang@...nel.org>, "Chao Yu" <chao@...nel.org>,
 "Yue Hu" <zbestahu@...il.com>, "Jeffle Xu" <jefflexu@...ux.alibaba.com>,
 "Sandeep Dhavale" <dhavale@...gle.com>, "Carlos Maiolino" <cem@...nel.org>,
 "Darrick J. Wong" <djwong@...nel.org>,
 "Andrew Morton" <akpm@...ux-foundation.org>,
 "Jesper Dangaard Brouer" <hawk@...nel.org>,
 "Ilias Apalodimas" <ilias.apalodimas@...aro.org>,
 "David S. Miller" <davem@...emloft.net>, "Eric Dumazet" <edumazet@...gle.com>,
 "Jakub Kicinski" <kuba@...nel.org>, "Paolo Abeni" <pabeni@...hat.com>,
 "Simon Horman" <horms@...nel.org>, "Trond Myklebust" <trondmy@...nel.org>,
 "Anna Schumaker" <anna@...nel.org>, "Chuck Lever" <chuck.lever@...cle.com>,
 "Jeff Layton" <jlayton@...nel.org>, "Olga Kornievskaia" <okorniev@...hat.com>,
 "Dai Ngo" <Dai.Ngo@...cle.com>, "Tom Talpey" <tom@...pey.com>,
 "Luiz Capitulino" <luizcap@...hat.com>,
 "Mel Gorman" <mgorman@...hsingularity.net>,
 "Dave Chinner" <david@...morbit.com>, kvm@...r.kernel.org,
 virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-btrfs@...r.kernel.org, linux-erofs@...ts.ozlabs.org,
 linux-xfs@...r.kernel.org, linux-mm@...ck.org, netdev@...r.kernel.org,
 linux-nfs@...r.kernel.org
Subject: Re: [PATCH v2] mm: alloc_pages_bulk: remove assumption of populating
 only NULL elements

On Thu, 06 Mar 2025, Yunsheng Lin wrote:
> On 2025/3/6 7:41, NeilBrown wrote:
> > On Wed, 05 Mar 2025, Yunsheng Lin wrote:
> >>
> >> For the existing btrfs and sunrpc case, I am agreed that there
> >> might be valid use cases too, we just need to discuss how to
> >> meet the requirements of different use cases using simpler, more
> >> unified and effective APIs.
> > 
> > We don't need "more unified".
> 
> What I meant about 'more unified' is how to avoid duplicated code as
> much as possible for two different interfaces with similar‌ functionality.
> 
> The best way I tried to avoid duplicated code as much as possible is
> to defragment the page_array before calling the alloc_pages_bulk()
> for the use case of btrfs and sunrpc so that alloc_pages_bulk() can
> be removed of the assumption populating only NULL elements, so that
> the API is simpler and more efficient.
> 
> > 
> > If there are genuinely two different use cases with clearly different
> > needs - even if only slightly different - then it is acceptable to have
> > two different interfaces.  Be sure to choose names which emphasise the
> > differences.
> 
> The best name I can come up with for the use case of btrfs and sunrpc
> is something like alloc_pages_bulk_refill(), any better suggestion about
> the naming?

I think alloc_pages_bulk_refill() is a good name.

So:
- alloc_pages_bulk() would be given an uninitialised array of page
  pointers and a required count and would return the number of pages
  that were allocated
- alloc_pages_bulk_refill() would be given an initialised array of page
  pointers some of which might be NULL.  It would attempt to allocate
  pages for the non-NULL pointers and return the total number of
  allocated pages in the array - just like the current
  alloc_pages_bulk().

sunrpc could usefully use both of these interfaces.

alloc_pages_bulk() could be implemented by initialising the array and
then calling alloc_pages_bulk_refill().  Or alloc_pages_bulk_refill()
could be implemented by compacting the pages and then calling
alloc_pages_bulk().
If we could duplicate the code and have two similar but different
functions.

The documentation for _refill() should make it clear that the pages
might get re-ordered.

Having looked at some of the callers I agree that the current interface
is not ideal for many of them, and that providing a simpler interface
would help.

Thanks,
NeilBrown