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

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Date:   Wed, 15 Feb 2023 02:41:57 +0800
From:   Chih-En Lin <shiyn.lin@...il.com>
To:     Pasha Tatashin <pasha.tatashin@...een.com>
Cc:     David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Qi Zheng <zhengqi.arch@...edance.com>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
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Subject: Re: [PATCH v4 00/14] Introduce Copy-On-Write to Page Table

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)?

I'm a little bit confused now.

Thanks,
Chih-En Lin