Re: [PATCH v5 3/6] userfaultfd: convert copy_huge_page_from_user() to copy_folio_from_user()

zhangpeng (AS)
2023-04-08T04:44:38Z

On 2023/4/7 10:28, Vishal Moola wrote:

> On Fri, Mar 31, 2023 at 2:41 AM Peng Zhang <zhangpeng362@huawei.com> wrote:
>> From: ZhangPeng <zhangpeng362@huawei.com>
>>
>> Replace copy_huge_page_from_user() with copy_folio_from_user().
>> copy_folio_from_user() does the same as copy_huge_page_from_user(), but
>> takes in a folio instead of a page. Convert page_kaddr to kaddr in
>> copy_folio_from_user() to do indenting cleanup.
>>
>> Signed-off-by: ZhangPeng <zhangpeng362@huawei.com>
>> Reviewed-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
>> ---
>> include/linux/mm.h | 7 +++----
>> mm/hugetlb.c | 5 ++---
>> mm/memory.c | 26 ++++++++++++--------------
>> mm/userfaultfd.c | 6 ++----
>> 4 files changed, 19 insertions(+), 25 deletions(-)
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index e249208f8fbe..cf4d773ca506 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -3682,10 +3682,9 @@ extern void copy_user_huge_page(struct page *dst, struct page *src,
>> unsigned long addr_hint,
>> struct vm_area_struct *vma,
>> unsigned int pages_per_huge_page);
>> -extern long copy_huge_page_from_user(struct page *dst_page,
>> - const void __user *usr_src,
>> - unsigned int pages_per_huge_page,
>> - bool allow_pagefault);
>> +long copy_folio_from_user(struct folio *dst_folio,
>> + const void __user *usr_src,
>> + bool allow_pagefault);
>>
>> /**
>> * vma_is_special_huge - Are transhuge page-table entries considered special?
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 7e4a80769c9e..aade1b513474 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -6217,9 +6217,8 @@ int hugetlb_mfill_atomic_pte(pte_t *dst_pte,
>> goto out;
>> }
>>
>> - ret = copy_huge_page_from_user(&folio->page,
>> - (const void __user *) src_addr,
>> - pages_per_huge_page(h), false);
>> + ret = copy_folio_from_user(folio, (const void __user *) src_addr,
>> + false);
>>
>> /* fallback to copy_from_user outside mmap_lock */
>> if (unlikely(ret)) {
>> diff --git a/mm/memory.c b/mm/memory.c
>> index 808f354bce65..4976422b6979 100644
>> --- a/mm/memory.c
>> +++ b/mm/memory.c
>> @@ -5868,35 +5868,33 @@ void copy_user_huge_page(struct page *dst, struct page *src,
>> process_huge_page(addr_hint, pages_per_huge_page, copy_subpage, &arg);
>> }
>>
>> -long copy_huge_page_from_user(struct page *dst_page,
>> - const void __user *usr_src,
>> - unsigned int pages_per_huge_page,
>> - bool allow_pagefault)
>> +long copy_folio_from_user(struct folio *dst_folio,
>> + const void __user *usr_src,
>> + bool allow_pagefault)
>> {
>> - void *page_kaddr;
>> + void *kaddr;
>> unsigned long i, rc = 0;
>> - unsigned long ret_val = pages_per_huge_page * PAGE_SIZE;
>> + unsigned int nr_pages = folio_nr_pages(dst_folio);
>> + unsigned long ret_val = nr_pages * PAGE_SIZE;
>> struct page *subpage;
>>
>> - for (i = 0; i < pages_per_huge_page; i++) {
>> - subpage = nth_page(dst_page, i);
>> - page_kaddr = kmap_local_page(subpage);
>> + for (i = 0; i < nr_pages; i++) {
>> + subpage = folio_page(dst_folio, i);
>> + kaddr = kmap_local_page(subpage);
>> if (!allow_pagefault)
>> pagefault_disable();
>> - rc = copy_from_user(page_kaddr,
>> - usr_src + i * PAGE_SIZE, PAGE_SIZE);
>> + rc = copy_from_user(kaddr, usr_src + i * PAGE_SIZE, PAGE_SIZE);
>> if (!allow_pagefault)
>> pagefault_enable();
>> - kunmap_local(page_kaddr);
>> + kunmap_local(kaddr);
>>
>> ret_val -= (PAGE_SIZE - rc);
>> if (rc)
>> break;
>>
>> - flush_dcache_page(subpage);
>> -
>> cond_resched();
>> }
>> + flush_dcache_folio(dst_folio);
>> return ret_val;
>> }
> Moving the flush_dcache_page() outside the loop to be
> flush_dcache_folio() changes the behavior of the function.
>
> Initially, if it fails to copy the entire page, the function breaks out
> of the loop and returns the number of unwritten bytes without
> flushing the page from the cache. Now if it fails, it will still flush
> out the page it failed on, as well as any later pages it may not
> have gotten to yet.

Agreed. If it fails, could we just not flush the folio?
Like this:
long copy_folio_from_user(...)
{
...
for (i = 0; i < nr_pages; i++) {
...
rc = copy_from_user(kaddr, usr_src + i * PAGE_SIZE, PAGE_SIZE);
...
ret_val -= (PAGE_SIZE - rc);
if (rc)
- break;
+ return ret_val;
cond_resched();
}
flush_dcache_folio(dst_folio);
return ret_val;
}

Thanks for your review.

Best Regards,
Peng