Re: RFC for new feature to move pages from one vma to another without split

[Lokesh Gidra]

On Tue, Jun 6, 2023 at 4:18 PM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Tue, Jun 6, 2023 at 1:15 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
> >
> > On 4/13/23 17:36, Peter Xu wrote:
> > > On Thu, Apr 13, 2023 at 10:10:44AM +0200, David Hildenbrand wrote:
> > >> So instead, we consider the whole address space as a virtual, anon file,
> > >> starting at offset 0. The pgoff of a VMA is then simply the offset in that
> > >> virtual file (easily computed from the start of the VMA), and VMA merging is
> > >> just the same as for an ordinary file.
> > >
> > > Interesting point, thanks!
> >
> > FYI, I've advised a master thesis exploring how to update page->index during
> > mremap() to keep things mergeable:
> >
> > https://dspace.cuni.cz/bitstream/handle/20.500.11956/176288/120426800.pdf
> >
> > I think the last RFC posting was:
> > https://lore.kernel.org/all/20220516125405.1675-1-matenajakub@xxxxxxxxx/
> >
> > It was really tricky for the general case. Maybe it would be more feasible
> > for the limited case Lokesh describes, if we could be sure the pages that
> > are moved aren't mapped anywhere else.

It's great that mremap is being improved for mereabilitly. However,
IIUC, it would still cause unnecessary splits and merges in the
private anonymous case. Also, mremap works with mmap_sem exclusively
held, thereby impacting scalability of concurrent mremap calls.

IMHO, Andrea's userfaultfd REMAP patch is a better alternative as it
doesn't have these downsides.

>
> Lokesh asked me to pick up this work and prepare patches for
> upstreaming. I'll start working on them after I finish with per-vma
> lock support for swap and userfaultd (targeting later this week).
> Thanks for all the input folks!

Thanks so much, Suren!