Re: [PATCH RFC v2 0/4] Add support for sharing page tables across processes (Previously mshare)

[Matthew Wilcox]

On Mon, Jul 31, 2023 at 06:48:47PM +0200, David Hildenbrand wrote:
> On 31.07.23 18:38, Matthew Wilcox wrote:
> > On Mon, Jul 31, 2023 at 06:30:22PM +0200, David Hildenbrand wrote:
> > > Assume we do do the page table sharing at mmap time, if the flags are right.
> > > Let's focus on the most common:
> > > 
> > > mmap(memfd, PROT_READ | PROT_WRITE, MAP_SHARED)
> > > 
> > > And doing the same in each and every process.
> > 
> > That may be the most common in your usage, but for a database, you're
> > looking at two usage scenarios.  Postgres calls mmap() on the database
> > file itself so that all processes share the kernel page cache.
> > Some Commercial Databases call mmap() on a hugetlbfs file so that all
> > processes share the same userspace buffer cache.  Other Commecial
> > Databases call shmget() / shmat() with SHM_HUGETLB for the exact
> > same reason.
> 
> I remember you said that postgres might be looking into using shmem as well,
> maybe I am wrong.

No, I said that postgres was also interested in sharing page tables.
I don't think they have any use for shmem.

> memfd/hugetlb/shmem could all be handled alike, just "arbitrary filesystems"
> would require more work.

But arbitrary filesystems was one of the origin use cases; where the
database is stored on a persistent memory filesystem, and neither the
kernel nor userspace has a cache.  The Postgres & Commercial Database
use-cases collapse into the same case, and we want to mmap the files
directly and share the page tables.

> > This is why I proposed mshare().  Anyone can use it for anything.
> > We have such a diverse set of users who want to do stuff with shared
> > page tables that we should not be tying it to memfd or any other
> > filesystem.  Not to mention that it's more flexible; you can map
> > individual 4kB files into it and still get page table sharing.
> 
> That's not what the current proposal does, or am I wrong?

I think you're wrong, but I haven't had time to read the latest patches.

> Also, I'm curious, is that a real requirement in the database world?

I don't know.  It's definitely an advantage that falls out of the design
of mshare.