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

Yunsheng Lin
2025-02-18T09:16:05Z

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