linux-kernel - Re: [PATCH v5 3/6] userfaultfd: convert copy_huge_page_from_user() to copy_folio_from

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Date:   Tue, 11 Apr 2023 09:30:49 +0800
From:   "Yin, Fengwei" <fengwei.yin@...el.com>
To:     Mike Kravetz <mike.kravetz@...cle.com>,
        "zhangpeng (AS)" <zhangpeng362@...wei.com>
CC:     Vishal Moola <vishal.moola@...il.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <akpm@...ux-foundation.org>,
        <willy@...radead.org>, <sidhartha.kumar@...cle.com>,
        <muchun.song@...ux.dev>, <wangkefeng.wang@...wei.com>,
        <sunnanyong@...wei.com>
Subject: Re: [PATCH v5 3/6] userfaultfd: convert copy_huge_page_from_user() to
 copy_folio_from_user()



On 4/11/2023 5:26 AM, Mike Kravetz wrote:
> On 04/08/23 12:43, zhangpeng (AS) wrote:
>> On 2023/4/7 10:28, Vishal Moola wrote:
>>
>>> On Fri, Mar 31, 2023 at 2:41 AM Peng Zhang <zhangpeng362@...wei.com> wrote:
>>>> From: ZhangPeng <zhangpeng362@...wei.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@...wei.com>
>>>> Reviewed-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
>>>> ---
>>>> -                               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?
> 
> I believe that should be OK.  If returning an error, nobody should be
> depending on any part of the page being present or not in the cache.
Maybe we should flush_dcache because this function returns the 
bytes copied successfully? flushing cache to make sure the copied
pieces to RAM for sure.

For the range not copied yet, flushing cache or not doesn't make
difference. Thanks.

Regards
Yin, Fengwei