On 4/23/26 22:10, Peter Xu wrote: > On Thu, Apr 23, 2026 at 09:25:30PM +0200, David Hildenbrand (Arm) wrote: >>> >>> The other thing is, as I mentioned in the other email, I still don't know >>> how the current RW protection would work for anonymous. I don't yet think >>> the user swapper can read the anon page with RW-protected pgtables. So far >>> my understanding is maybe you only care about shmem so it's fine, but it'll >>> always be great to confirm with you. >> >> I wonder if uffdio_move could be used for a swapper implementation instead? > > If RW is justified to be useful first, maybe. > > I had a gut feeling Kirill's use case doesn't use anon at all, then if > nobody needs it we can still decide to not support anon. > >> >> If we ever have to read from a protnone page, maybe we could teach ptrace >> access >> to do it, or have something that can read from prot_none areas -- like >> uffdio_copy, which can write to prot-none areas. > > Somethinig like swap_access() in my proposal can also partly achieve that.
Looks more like the hammer for the nail here: we could fault the page in just fine, while keeping it mapped prot_none and keeping the uffd-rwp pte bit set. I was rather thinking of some uffd-specific thing that can read from a uffd-rwp protected pte without trigger uffd. > > https://lore.kernel.org/all/[email protected]/ > > There, it was only about reading from swap so far, though. But that one > might be easier to be extended to read PROT_NONE and directly put data into > buffer user specified (ps: in my local tree impl I named it maccess() to > pair with mincore(), but it doesn't really matter; it doesn't even need to > be a syscall..). > > To me, the interfacing is not a major issue. The major question I have is > why RW protection can help in swap system impl when we already have uffd-wp. > > So I want to make sure the use case can't be implemented by uffd-wp already. > Because that's really what we might do for QEMU. There has to be some added value indeed. -- Cheers, David
