Re: [PATCH v4 0/5] implement lightweight guard pages

[Aleksandr Mikhalitsyn]

On Wed, Mar 19, 2025 at 4:02 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
>
> On Wed, Mar 19, 2025 at 03:52:56PM +0100, David Hildenbrand wrote:
> > On 19.03.25 15:50, Alexander Mikhalitsyn wrote:
> > > On Mon, Oct 28, 2024 at 02:13:26PM +0000, Lorenzo Stoakes wrote:
>
> [snip]
>
>
> > >
> > > Dear Lorenzo,
> > > Dear colleagues,
> > >
> > > sorry about raising an old thread.
> > >
>
> No worries!
>
> > > It looks like this feature is now used in glibc [1]. And we noticed failures in CRIU [2]
> > > CI on Fedora Rawhide userspace. Now a question is how we can properly detect such
> > > "guarded" pages from user space. As I can see from MADV_GUARD_INSTALL implementation,
> > > it does not modify VMA flags anyhow, but only page tables. It means that /proc/<pid>/maps
> > > and /proc/<pid>/smaps interfaces are useless in this case. (Please, correct me if I'm missing
> > > anything here.)
>
> Sorry to hear that.

No problem at all ;)

>
> > >
> > > I wonder if you have any ideas / suggestions regarding Checkpoint/Restore here. We (CRIU devs) are happy
> > > to develop some patches to bring some uAPI to expose MADV_GUARDs, but before going into this we decided
> > > to raise this question in LKML.
>
> There's no need.
>
> >
> >
> > See [1] and [2]
> >
> > [1]
> > https://lkml.kernel.org/r/cover.1740139449.git.lorenzo.stoakes@xxxxxxxxxx
> > [2] https://lwn.net/Articles/1011366/
>
> As per David, there is already a feature heading for 6.15 which will allow
> this to be exposed by /proc/$pid/pagemap.

Yeah, that's indeed very helpful!

>
> In addition, I plan to add a 'maybe has guard regions' flag that can be
> observed in smaps to assist narrowing down which VMAs to check.
>
> However unfortunately due to the nature of the feature there is no getting
> around the need to traverse page tables.
>
> That thread (and LWN article :) go into extensive detail as to why. In
> essence - it's the basis of its design to express this information at the
> page table level only, and any attempt to encode this at the VMA level
> (other than a 'maybe' flag) would eliminate the purpose of the feature.

Thank you very much for these explanations!
I'll read the LWN article too.

>
> Let me know if there's any way I can help!

Keep in contact! ;)

Kind regards,
Alex

>
> Cheers, Lorenzo
>
> >
> >
> > --
> > Cheers,
> >
> > David / dhildenb
> >