Re: [PATCH v1 2/5] mm/madvise: introduce MADV_POPULATE_(READ|WRITE) to prefault/prealloc memory

[David Hildenbrand]

On 30.03.21 18:31, David Hildenbrand wrote:
> On 30.03.21 18:30, David Hildenbrand wrote:
> > On 30.03.21 18:21, Jann Horn wrote:
> > > On Tue, Mar 30, 2021 at 5:01 PM David Hildenbrand <david@xxxxxxxxxx> 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?
>
> Sorry, I missed the explanation above - thanks!

Looking into the details, adjusting the FOLL_TOUCH logic won't make too 
much of a difference for MADV_POPULATE_WRITE I guess. AFAIKs, the 
biggest impact of FOLL_TOUCH is actually with FOLL_FORCE - which we are 
not using, but populate_vma_page_range() is.


If a page was not faulted in yet, 
faultin_page(FOLL_WRITE)->handle_mm_fault(FAULT_FLAG_WRITE) will already 
mark the PTE/PMD/... dirty and accessed. One example is 
handle_pte_fault(). We will mark the page accessed again via FOLL_TOUCH, 
which doesn't seem to be strictly required.


If the page was already faulted in, we have three cases:

1. Page faulted in writable. The page should already be dirty (otherwise 
we would be in trouble I guess). We will mark it accessed.

2. Page faulted in readable. handle_mm_fault() will fault it in writable 
and set the page dirty.

3. Page faulted in readable and we have FOLL_FORCE. We mark the page 
dirty and accessed.


So doing a MADV_POPULATE_WRITE, whereby we prefault page tables 
writable, doesn't seem to fly without marking the pages dirty. That's 
one reason why I included MADV_POPULATE_READ.

We could

a) Drop FOLL_TOUCH. We are not marking the page accessed, which would 
mean it gets evicted rather earlier than later.

b) Introduce FOLL_ACCESSED which won't do the dirtying. But then, the 
pages are already dirty as explained above, so there isn't a real 
observable change.

c) Keep it as is: Mark the page accessed and dirty. As it's already 
dirty, that does not seem to be a real issue.

Am I missing something obvious? Thanks!

--
Thanks,

David / dhildenb