Re: [PATCH 3/3] mm: read remote memory without the mmap lock where possible

[Suren Baghdasaryan]

On Fri, Jun 19, 2026 at 6:47 AM Rik van Riel <riel@xxxxxxxxxxx> wrote:
>
> On Tue, 2026-06-16 at 23:19 -0700, Suren Baghdasaryan wrote:
> >
> > I don't think we should be using per-VMA locks if the read spans
> > multiple VMAs. Doing that would risk a possibility of reading
> > inconsistent data since we are locking one VMA at a time. While we
> > load and read VMA, its neighboring VMA can be unmapped and another
> > one
> > can be mapped in its place.
>
> How is that different from userspace overwriting
> data while we are reading it, and us reading half
> new, and half old contents?
>
> We already have nothing synchronizing the contents
> today.
>
> All our locking does is protect the metadata we
> use to find the memory.

Current locking ensures that when reading across VMAs, the structure
of the VMA tree stays consistent during that read. I think that
*structural consistency* of the area we are reading is important to
preserve. Someone can be overriding the data while we are reading it,
but that's *content consistency* and yes, we don't have protection
against that both before and after your change.

>
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