Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

[Chih-En Lin]

On Tue, Feb 14, 2023 at 01:52:16PM -0500, Pasha Tatashin wrote:
> On Tue, Feb 14, 2023 at 1:42 PM Chih-En Lin <shiyn.lin@xxxxxxxxx> wrote:
> >
> > On Tue, Feb 14, 2023 at 11:30:26AM -0500, Pasha Tatashin wrote:
> > > > > The thing with THP is, that during fork(), we always allocate a backup PTE
> > > > > table, to be able to PTE-map the THP whenever we have to. Otherwise we'd
> > > > > have to eventually fail some operations we don't want to fail -- similar to
> > > > > the case where break_cow_pte() could fail now due to -ENOMEM although we
> > > > > really don't want to fail (e.g., change_pte_range() ).
> > > > >
> > > > > I always considered that wasteful, because in many scenarios, we'll never
> > > > > ever split a THP and possibly waste memory.
> > > > >
> > > > > Optimizing that for THP (e.g., don't always allocate backup THP, have some
> > > > > global allocation backup pool for splits + refill when close-to-empty) might
> > > > > provide similar fork() improvements, both in speed and memory consumption
> > > > > when it comes to anonymous memory.
> > > >
> > > > When collapsing huge pages, do/can they reuse those PTEs for backup?
> > > > So, we don't have to allocate the PTE or maintain the pool.
> > >
> > > It might not work for all pages, as collapsing pages might have had
> > > holes in the user page table, and there were no PTE tables.
> >
> > So if there have holes in the user page table, after we doing the
> > collapsing and then splitting. Do those holes be filled? Assume it is,
> > then, I think it's the reason why it's not work for all the pages.
> >
> > But, after those operations, Will the user get the additional and
> > unexpected memory (which is from the huge page filling)?
> 
> Yes, more memory is going to be allocated for a process in such THP
> collapse case. This is similar to madvise huge pages, and touching the
> first byte may allocate 2M.

Thanks for the explanation.
Yeah, It seems like the reuse case can't work for all the pages.

Thanks,
Chih-En Lin