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

[Yunsheng Lin <linyunsheng@huawei.com>]

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