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

Chuck Lever
2025-02-18T14:18:34Z

On 2/18/25 4:16 AM, Yunsheng Lin wrote:
> 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?

NFSD is in fs/nfsd/, and it is the major consumer of
net/sunrpc/svc_xprt.c.

>> 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

I still don't see what is broken about the current API.

Anyway, any changes in svc_alloc_arg() will need to be run through the
upstream NFSD CI suite before they are merged.

--
Chuck Lever