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SubjectRe: [PATCH v1 2/5] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory
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On 30.03.21 18:21, Jann Horn wrote:
> On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@redhat.com> wrote:
>>>> +long faultin_vma_page_range(struct vm_area_struct *vma, unsigned long start,
>>>> + unsigned long end, bool write, int *locked)
>>>> +{
>>>> + struct mm_struct *mm = vma->vm_mm;
>>>> + unsigned long nr_pages = (end - start) / PAGE_SIZE;
>>>> + int gup_flags;
>>>> +
>>>> + VM_BUG_ON(!PAGE_ALIGNED(start));
>>>> + VM_BUG_ON(!PAGE_ALIGNED(end));
>>>> + VM_BUG_ON_VMA(start < vma->vm_start, vma);
>>>> + VM_BUG_ON_VMA(end > vma->vm_end, vma);
>>>> + mmap_assert_locked(mm);
>>>> +
>>>> + /*
>>>> + * FOLL_HWPOISON: Return -EHWPOISON instead of -EFAULT when we hit
>>>> + * a poisoned page.
>>>> + * FOLL_POPULATE: Always populate memory with VM_LOCKONFAULT.
>>>> + * !FOLL_FORCE: Require proper access permissions.
>>>> + */
>>>> + gup_flags = FOLL_TOUCH | FOLL_POPULATE | FOLL_MLOCK | FOLL_HWPOISON;
>>>> + if (write)
>>>> + gup_flags |= FOLL_WRITE;
>>>> +
>>>> + /*
>>>> + * See check_vma_flags(): Will return -EFAULT on incompatible mappings
>>>> + * or with insufficient permissions.
>>>> + */
>>>> + return __get_user_pages(mm, start, nr_pages, gup_flags,
>>>> + NULL, NULL, locked);
>>>
>>> You mentioned in the commit message that you don't want to actually
>>> dirty all the file pages and force writeback; but doesn't
>>> POPULATE_WRITE still do exactly that? In follow_page_pte(), if
>>> FOLL_TOUCH and FOLL_WRITE are set, we mark the page as dirty:
>>
>> Well, I mention that POPULATE_READ explicitly doesn't do that. I
>> primarily set it because populate_vma_page_range() also sets it.
>>
>> Is it safe to *not* set it? IOW, fault something writable into a page
>> table (where the CPU could dirty it without additional page faults)
>> without marking it accessed? For me, this made logically sense. Thus I
>> also understood why populate_vma_page_range() set it.
>
> FOLL_TOUCH doesn't have anything to do with installing the PTE - it
> essentially means "the caller of get_user_pages wants to read/write
> the contents of the returned page, so please do the same things you
> would do if userspace was accessing the page". So in particular, if
> you look up a page via get_user_pages() with FOLL_WRITE|FOLL_TOUCH,
> that tells the MM subsystem "I will be writing into this page directly
> from the kernel, bypassing the userspace page tables, so please mark
> it as dirty now so that it will be properly written back later". Part
> of that is that it marks the page as recently used, which has an
> effect on LRU pageout behavior, I think - as far as I understand, that
> is why populate_vma_page_range() uses FOLL_TOUCH.
>
> If you look at __get_user_pages(), you can see that it is split up
> into two major parts: faultin_page() for creating PTEs, and
> follow_page_mask() for grabbing pages from PTEs. faultin_page()
> ignores FOLL_TOUCH completely; only follow_page_mask() uses it.
>
> In a way I guess maybe you do want the "mark as recently accessed"
> part that FOLL_TOUCH would give you without FOLL_WRITE? But I think
> you very much don't want the dirtying that FOLL_TOUCH|FOLL_WRITE leads
> to. Maybe the ideal approach would be to add a new FOLL flag to say "I
> only want to mark as recently used, I don't want to dirty". Or maybe
> it's enough to just leave out the FOLL_TOUCH entirely, I don't know.

Any thoughts why populate_vma_page_range() does it?

--
Thanks,

David / dhildenb

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