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 > >
