Re: [LSF/MM TOPIC] Non standard size THP

From: Kirill A. Shutemov
Date: Wed Feb 13 2019 - 08:28:08 EST

On Wed, Feb 13, 2019 at 05:06:47AM -0800, Matthew Wilcox wrote:
> On Tue, Feb 12, 2019 at 11:33:31AM +0300, Kirill A. Shutemov wrote:
> > To consider it seriously we need to understand what it means for
> > split_huge_p?d()/split_huge_page()? How khugepaged will deal with this?
> >
> > In particular, I'm worry to expose (to user or CPU) page table state in
> > the middle of conversion (huge->small or small->huge). Handling this on
> > page table level provides a level atomicity that you will not have.
> We could do an RCU-style trick where (eg) for merging 16 consecutive
> entries together, we allocate a new PTE leaf, take the mmap_sem for write,
> copy the page table over, update the new entries, then put the new leaf
> into the PMD level. Then iterate over the old PTE leaf again, and set
> any dirty bits in the new leaf which were set during the race window.
> Does that cover all the problems?

Probably, but it will kill scalability. Taking mmap_sem for write to
handle page fault or MADV_DONTNEED will not make anybody happy.

Kirill A. Shutemov