On 8/23/2023 8:23 PM, Matthew Wilcox wrote:
> On Wed, Aug 23, 2023 at 03:38:13PM +0800, Yin Fengwei wrote:
>> On 8/22/23 04:20, Matthew Wilcox (Oracle) wrote:
>>> - get_page(vmf->page);
>>> - vmf->page->mapping = vmf->vma->vm_file->f_mapping;
>>> - vmf->page->index = vmf->pgoff;
>>> + folio_get(folio);
>>> + rcu_read_unlock();
>>> + folio_lock(folio);
>>> + if (!folio->mapping)
>>> + folio->mapping = vmf->vma->vm_file->f_mapping;
>>>
>>> - ret = 0;
>>> + return VM_FAULT_LOCKED;
>> In __do_fault():
>>
>> if (unlikely(!(ret & VM_FAULT_LOCKED)))
>> lock_page(vmf->page);
>> else
>> VM_BUG_ON_PAGE(!PageLocked(vmf->page), vmf->page);
>>
>> As we lock folio, not sure whether !PageLocked(vmf->page) can be true
>> here. My understanding is yes if vmf->pgoff belongs to tail pages. Did
>> I can miss something here?
>
> There's only one lock bit per folio; there's no lock bit for individual
> pages. When we check PageLocked() on a tail page, it redirects to the
> head page.
>
> __PAGEFLAG(Locked, locked, PF_NO_TAIL)
>
> #define PF_NO_TAIL(page, enforce) ({ \
> VM_BUG_ON_PGFLAGS(enforce && PageTail(page), page); \
> PF_POISONED_CHECK(compound_head(page)); })
>
> #define TESTPAGEFLAG(uname, lname, policy) \
> static __always_inline int Page##uname(struct page *page) \
> { return test_bit(PG_##lname, &policy(page, 0)->flags); }
>
> and that expands out to:
>
> static __always_inline int PageLocked(struct page *page)
> { return test_bit(PG_##locked, &compound_head(page)->flags); }
>
Ah. Here is the trick. Thanks a lot for detail explanation.
Regards
Yin, Fengwei
