netdev - Re: [RFC] mm: alloc_pages_bulk: remove assumption of populating only NULL elements

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Message-ID: <cc6fc730-e5f4-485b-b0b6-ec70374b3ab1@huawei.com>
Date: Tue, 18 Feb 2025 17:16:00 +0800
From: Yunsheng Lin <linyunsheng@...wei.com>
To: Chuck Lever <chuck.lever@...cle.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>, Jeff Layton <jlayton@...nel.org>, Neil Brown
	<neilb@...e.de>, Olga Kornievskaia <okorniev@...hat.com>, Dai Ngo
	<Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>
CC: Luiz Capitulino <luizcap@...hat.com>, Mel Gorman
	<mgorman@...hsingularity.net>, <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: [RFC] mm: alloc_pages_bulk: remove assumption of populating only
 NULL elements

On 2025/2/17 22:20, Chuck Lever wrote:
> On 2/17/25 7:31 AM, Yunsheng Lin wrote:
>> As mentioned in [1], it seems odd to check NULL elements in
>> the middle of page bulk allocating,
> 
> I think I requested that check to be added to the bulk page allocator.
> 
> When sending an RPC reply, NFSD might release pages in the middle of

It seems there is no usage of the page bulk allocation API in fs/nfsd/
or fs/nfs/, which specific fs the above 'NFSD' is referring to?

> the rq_pages array, marking each of those array entries with a NULL
> pointer. We want to ensure that the array is refilled completely in this
> case.
> 

I did some researching, it seems you requested that in [1]?
It seems the 'holes are always at the start' for the case in that
discussion too, I am not sure if the case is referring to the caller
in net/sunrpc/svc_xprt.c? If yes, it seems caller can do a better
job of bulk allocating pages into a whole array sequentially without
checking NULL elements first before doing the page bulk allocation
as something below:

+++ b/net/sunrpc/svc_xprt.c
@@ -663,9 +663,10 @@ static bool svc_alloc_arg(struct svc_rqst *rqstp)
                pages = RPCSVC_MAXPAGES;
        }

-       for (filled = 0; filled < pages; filled = ret) {
-               ret = alloc_pages_bulk(GFP_KERNEL, pages, rqstp->rq_pages);
-               if (ret > filled)
+       for (filled = 0; filled < pages; filled += ret) {
+               ret = alloc_pages_bulk(GFP_KERNEL, pages - filled,
+                                      rqstp->rq_pages + filled);
+               if (ret)
                        /* Made progress, don't sleep yet */
                        continue;

@@ -674,7 +675,7 @@ static bool svc_alloc_arg(struct svc_rqst *rqstp)
                        set_current_state(TASK_RUNNING);
                        return false;
                }
-               trace_svc_alloc_arg_err(pages, ret);
+               trace_svc_alloc_arg_err(pages, filled);
                memalloc_retry_wait(GFP_KERNEL);
        }
        rqstp->rq_page_end = &rqstp->rq_pages[pages];


1. https://lkml.iu.edu/hypermail/linux/kernel/2103.2/09060.html