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. 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. 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. >
