Re: [PATCH 02/11] mm/memory: Remove page fault assumption of compound page size

Kirill A. Shutemov
2020-09-11T14:51:44Z

On Wed, Sep 09, 2020 at 03:50:35PM +0100, Matthew Wilcox wrote:
> On Wed, Sep 09, 2020 at 05:29:04PM +0300, Kirill A. Shutemov wrote:
> > On Tue, Sep 08, 2020 at 08:55:29PM +0100, Matthew Wilcox (Oracle) wrote:
> > > A compound page in the page cache will not necessarily be of PMD size,
> > > so check explicitly.
> > >
> > > Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> > > ---
> > > mm/memory.c | 7 ++++---
> > > 1 file changed, 4 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/mm/memory.c b/mm/memory.c
> > > index 602f4283122f..4b35b4e71e64 100644
> > > --- a/mm/memory.c
> > > +++ b/mm/memory.c
> > > @@ -3562,13 +3562,14 @@ static vm_fault_t do_set_pmd(struct vm_fault *vmf, struct page *page)
> > > unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
> > > pmd_t entry;
> > > int i;
> > > - vm_fault_t ret;
> > > + vm_fault_t ret = VM_FAULT_FALLBACK;
> > >
> > > if (!transhuge_vma_suitable(vma, haddr))
> > > - return VM_FAULT_FALLBACK;
> > > + return ret;
> > >
> > > - ret = VM_FAULT_FALLBACK;
> > > page = compound_head(page);
> > > + if (page_order(page) != HPAGE_PMD_ORDER)
> > > + return ret;
> >
> > Maybe also VM_BUG_ON_PAGE(page_order(page) > HPAGE_PMD_ORDER, page)?
> > Just in case.
>
> In the patch where I actually start creating THPs, I limit the order to
> HPAGE_PMD_ORDER, so we're not going to see this today. At some point
> in the future, I can imagine that we allow THPs larger than PMD size,
> and what we'd want alloc_set_pte() to look like is:
>
> if (pud_none(*vmf->pud) && PageTransCompound(page)) {
> ret = do_set_pud(vmf, page);
> if (ret != VM_FAULT_FALLBACK)
> return ret;
> }
> if (pmd_none(*vmf->pmd) && PageTransCompound(page)) {
> ret = do_set_pmd(vmf, page);
> if (ret != VM_FAULT_FALLBACK)
> return ret;
> }
>
> Once we're in that situation, in do_set_pmd(), we'd want to figure out
> which sub-page of the >PMD-sized page to insert. But I don't want to
> write code for that now.
>
> So, what's the right approach if somebody does call alloc_set_pte()
> with a >PMD sized page? It's not exported, so the only two ways to get
> it called with a >PMD sized page is to (1) persuade filemap_map_pages()
> to call it, which means putting it in the page cache or (2) return it
> from vm_ops->fault. If someone actually does that (an interesting
> device driver, perhaps), I don't think hitting it with a BUG is the
> right response. I think it should actually be to map the right PMD-sized
> chunk of the page, but we don't even do that today -- we map the first
> PMD-sized chunk of the page.
>
> With this patch, we'll simply map the appropriate PAGE_SIZE chunk at the
> requested address. So this would be a bugfix for such a demented driver.
> At some point, it'd be nice to handle this with a PMD, but I don't want
> to write that code without a test-case. We could probably simulate
> it with the page cache THP code and be super-aggressive about creating
> order-10 pages ... but this is feeling more and more out of scope for
> this patch set, which today hit 99 patches.

Okay, fair enough. VM_BUG is too strong reaction here as we can make a
reasonable fallback. Maybe WARN_ON_ONCE() would make sense? It would
indicate the place that has to be adjust once we would get abouve
PMD-order pages.

--
Kirill A. Shutemov