* Andrii Nakryiko <andrii.nakry...@gmail.com> [250423 18:06]: > On Wed, Apr 23, 2025 at 2:49 PM Suren Baghdasaryan <sur...@google.com> wrote: > > > > On Tue, Apr 22, 2025 at 3:49 PM Andrii Nakryiko > > <andrii.nakry...@gmail.com> wrote: > > > > > > On Fri, Apr 18, 2025 at 10:50 AM Suren Baghdasaryan <sur...@google.com> > > > wrote: > > > > > > > > With maple_tree supporting vma tree traversal under RCU and vma and > > > > its important members being RCU-safe, /proc/pid/maps can be read under > > > > RCU and without the need to read-lock mmap_lock. However vma content > > > > can change from under us, therefore we make a copy of the vma and we > > > > pin pointer fields used when generating the output (currently only > > > > vm_file and anon_name). Afterwards we check for concurrent address > > > > space modifications, wait for them to end and retry. While we take > > > > the mmap_lock for reading during such contention, we do that momentarily > > > > only to record new mm_wr_seq counter. This change is designed to reduce > > > > > > This is probably a stupid question, but why do we need to take a lock > > > just to record this counter? uprobes get away without taking mmap_lock > > > even for reads, and still record this seq counter. And then detect > > > whether there were any modifications in between. Why does this change > > > need more heavy-weight mmap_read_lock to do speculative reads? > > > > Not a stupid question. mmap_read_lock() is used to wait for the writer > > to finish what it's doing and then we continue by recording a new > > sequence counter value and call mmap_read_unlock. This is what > > get_vma_snapshot() does. But your question made me realize that we can > > optimize m_start() further by not taking mmap_read_lock at all. > > Instead of taking mmap_read_lock then doing drop_mmap_lock() we can > > try mmap_lock_speculate_try_begin() and only if it fails do the same > > dance we do in the get_vma_snapshot(). I think that should work. > > Ok, yeah, it would be great to avoid taking a lock in a common case!
We can check this counter once per 4k block and maintain the same 'tearing' that exists today instead of per-vma. Not that anyone said they had an issue with changing it, but since we're on this road anyways I'd thought I'd point out where we could end up. I am concerned about live locking in either scenario, but I haven't looked too deep into this pattern. I also don't love (as usual) the lack of ensured forward progress. It seems like we have four cases for the vm area state now: 1. we want to read a stable vma or set of vmas (per-vma locking) 2. we want to read a stable mm state for reading (the very short named mmap_lock_speculate_try_begin) 3. we ensure a stable vma/mm state for reading (mmap read lock) 4. we are writing - get out of my way (mmap write lock). Cheers, Liam
