On Tue, Feb 13, 2024 at 10:57 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote: > > On Tue, Feb 13, 2024 at 10:49 AM Liam R. Howlett > <Liam.Howlett@xxxxxxxxxx> wrote: > > > > * Suren Baghdasaryan <surenb@xxxxxxxxxx> [240213 13:25]: > > > On Tue, Feb 13, 2024 at 10:14 AM Lokesh Gidra <lokeshgidra@xxxxxxxxxx> wrote: > > > > > > > > On Tue, Feb 13, 2024 at 9:06 AM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > > > > > > * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240213 06:25]: > > > > > > On Mon, Feb 12, 2024 at 7:33 PM Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> wrote: > > > > > > > > > > > > > > * Lokesh Gidra <lokeshgidra@xxxxxxxxxx> [240212 19:19]: > > > > > > > > All userfaultfd operations, except write-protect, opportunistically use > > > > > > > > per-vma locks to lock vmas. On failure, attempt again inside mmap_lock > > > > > > > > critical section. > > > > > > > > > > > > > > > > Write-protect operation requires mmap_lock as it iterates over multiple > > > > > > > > vmas. > > > > > > > > > > > > > > > > Signed-off-by: Lokesh Gidra <lokeshgidra@xxxxxxxxxx> > > > > > > > > --- > > > > > > > > fs/userfaultfd.c | 13 +- > > > > > > > > include/linux/userfaultfd_k.h | 5 +- > > > > > > > > mm/userfaultfd.c | 392 ++++++++++++++++++++++++++-------- > > > > > > > > 3 files changed, 312 insertions(+), 98 deletions(-) > > > > > > > > > > > > > > > ... > > > > > > > > > > I just remembered an issue with the mmap tree that exists today that you > > > > > needs to be accounted for in this change. > > > > > > > > > > If you hit a NULL VMA, you need to fall back to the mmap_lock() scenario > > > > > today. > > > > > > > > Unless I'm missing something, isn't that already handled in the patch? > > > > We get the VMA outside mmap_lock critical section only via > > > > lock_vma_under_rcu() (in lock_vma() and find_and_lock_vmas()) and in > > > > both cases if we get NULL in return, we retry in mmap_lock critical > > > > section with vma_lookup(). Wouldn't that suffice? > > > > > > I think that case is handled correctly by lock_vma(). > > > > Yeah, it looks good. I had a bit of a panic as I forgot to check that > > and I was thinking of a previous version. I rechecked and v5 looks > > good. > > > > > > > > Sorry for coming back a bit late. The overall patch looks quite good > > > but the all these #ifdef CONFIG_PER_VMA_LOCK seem unnecessary to me. > > > Why find_and_lock_vmas() and lock_mm_and_find_vmas() be called the > > > same name (find_and_lock_vmas()) and in one case it would lock only > > > the VMA and in the other case it takes mmap_lock? Similarly > > > unlock_vma() would in one case unlock the VMA and in the other drop > > > the mmap_lock? That would remove all these #ifdefs from the code. > > > Maybe this was already discussed? > > > > Yes, I don't think we should be locking the mm in lock_vma(), as it > > makes things hard to follow. > > > > We could use something like uffd_prepare(), uffd_complete() but I > > thought of those names rather late in the cycle, but I've already caused > > many iterations of this patch set and that clean up didn't seem as vital > > as simplicity and clarity of the locking code. I anyway have to send another version to fix the error handling that you reported earlier. I can take care of this in that version. mfill_atomic...() functions (annoyingly) have to sometimes unlock and relock. Using prepare/complete in that context seems incompatible. > > Maybe lock_vma_for_uffd()/unlock_vma_for_uffd()? Whatever name is > better I'm fine with it but all these #ifdef's sprinkled around don't > contribute to the readability. I'll wait for an agreement on this because I too don't like using so many ifdef's either. Since these functions are supposed to have prototype depending on mfill/move, how about the following names: uffd_lock_mfill_vma()/uffd_unlock_mfill_vma() uffd_lock_move_vmas()/uffd_unlock_move_vmas() Of course, I'm open to other suggestions as well. > Anyway, I don't see this as a blocker, just nice to have. > > > > > Thanks, > > Liam > > > > -- > > To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@xxxxxxxxxxx. > >
