Re: [PATCH v1 1/1] s390/uv: Prealloc and use one work page

Harald Freudenberger
2025-03-28T12:51:10Z

On 2025-03-28 11:34, Heiko Carstens wrote:
> On Thu, Mar 27, 2025 at 04:38:24PM +0100, Harald Freudenberger wrote:
>> The pkey handler is calling the uv in some circumstances
>> where no memory allocation is acceptable. As of now only
>> the uv_get_secret_metadata() function allocates memory.
>> As this is exactly one page, lets introduce a pre-allocated
>> work page and protect the concurrent use with a mutex to
>> remove dynamic memory allocation and free. This page may be
>> also used with future extension to the uv code.
>>
>> Signed-off-by: Harald Freudenberger <freude@linux.ibm.com>
>> ---
>> arch/s390/kernel/uv.c | 22 ++++++++++++++++++----
>> 1 file changed, 18 insertions(+), 4 deletions(-)
>
> [adding maintainers, according to get_maintainer.pl]
>
>
>> static int __init uv_init(phys_addr_t stor_base, unsigned long
>> stor_len)
>> {
>> struct uv_cb_init uvcb = {
>> @@ -61,6 +68,12 @@ void __init setup_uv(void)
>> if (!is_prot_virt_host())
>> return;
>>
>> + work_page = (u8 *)__get_free_page(GFP_KERNEL);
>> + if (!work_page) {
>> + pr_warn("Failed to alloc a working memory page\n");
>> + return;
>> + }
>> +
>> uv_stor_base = memblock_alloc_try_nid(
>
> Did you test this? I think this cannot work. When setup_uv() is called
> the buddy allocator is not yet initialized.
> Please use memblock_alloc_or_panic() instead.
>

I only compiled this and I wanted to test this today in my
SEL environment. The patch is a suggestion and should trigger
maybe some feedback.

>> - buf = kzalloc(sizeof(*buf), GFP_KERNEL);
>> - if (!buf)
>> - return -ENOMEM;
>> + mutex_lock(&work_page_lock);
>> + buf = (struct uv_secret_list *)work_page;
>> rc = find_secret(secret_id, buf, secret);
>> - kfree(buf);
>> + mutex_unlock(&work_page_lock);
>
> The commit message does not explain why memory allocation is not
> acceptable. Usually this translates to non-sleepable context. If that
> is the case, then using a mutex would be wrong. This needs to be
> clarified.
>
>> +
>> return rc;
>
> Unrelated whitespace change.
>
> It is up to the kvm folks to decide if the whole approach is ok.