On Mon, Jan 13, 2025 at 9:11 AM Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote: > > On Mon, Jan 13, 2025 at 08:58:37AM -0800, Suren Baghdasaryan wrote: > > On Mon, Jan 13, 2025 at 4:14 AM Lorenzo Stoakes > > <lorenzo.stoakes@xxxxxxxxxx> wrote: > > > > > > A nit on subject, I mean this is part of what this series does, and hey - > > > we have only so much text to put in here - but isn't this both > > > reimplementing per-VMA lock as a refcount _and_ importantly allocating VMAs > > > using the RCU typesafe mechanism? > > > > > > Do we have to do both in one series? Can we split this out? I mean maybe > > > that's just churny and unnecessary, but to me this series is 'allocate VMAs > > > RCU safe and refcount VMA lock' or something like this. Maybe this is > > > nitty... but still :) > > > > There is "motivational dependency" because one of the main reasons I'm > > converting the vm_lock into vm_refcnt is to make it easier to add > > SLAB_TYPESAFE_BY_RCU (see my last reply to Hillf). But technically we > > can leave the SLAB_TYPESAFE_BY_RCU out of this series if that makes > > thighs easier. That would be the 2 patches at the end: > > Right yeah... maybe it's better to do it in one hit. > > > > > mm: prepare lock_vma_under_rcu() for vma reuse possibility > > mm: make vma cache SLAB_TYPESAFE_BY_RCU > > > > I made sure that each patch is bisectable, so there should not be a > > problem with tracking issues. > > > > > > > > One general comment here - this is a really major change in how this stuff > > > works, and yet I don't see any tests anywhere in the series. > > > > Hmm. I was diligently updating the tests to reflect the replacement of > > vm_lock with vm_refcnt and adding assertions for detach/attach cases. > > This actually reminds me that I missed updading vm_area_struct in > > vma_internal.h for the member regrouping patch; will add that. I think > > the only part that did not affect tests is SLAB_TYPESAFE_BY_RCU but I > > was not sure what kind of testing I can add for that. Any suggestions > > would be welcomed. > > And to be clear I'm super grateful you did that :) thanks, be good to > change the member regrouping thing also. > > But that doesn't change the fact that this series has exactly zero tests > for it. And for something so broad, it feels like a big issue, we really > want to be careful with something so big here. > > You've also noticed that I've cleverly failed to _actually_ suggest > SLAB_TYPESAFE_BY_RCU tests, and mea culpa - it's super hard to think of how > to test that. > > Liam has experience doing RCU testing this for the maple tree stuff, but it > wasn't pretty and wasn't easy and would probably require massive rework to > expose this stuff to some viable testing environment, or in other words - > is unworkable. > > HOWEVER, I feel like maybe we could try to create scenarios where we might > trigger reuse bugs? > > Perhaps some userland code, perhaps even constrained by cgroup, that maps a > ton of stuff and unmaps in a loop in parallel? > > Perhaps create scenarios with shared memory where we up refcounts a lot too? I have this old spf_test (https://github.com/surenbaghdasaryan/spf_test/blob/main/spf_test.c) which I often use to weed out vma locking issues because it starts multiple threads doing mmap + page faults. Perhaps we can repackage it into a test/benchmark for testing contention on mmap/vma locks? > > Anyway, this is necessarily nebulous without further investigation, what I > was thinking more concretely is: > > Using the VMA userland testing: > > 1. Assert reference count correctness across locking scenarios and various > VMA operations. > 2. Assert correct detached/not detached state across different scenarios. > > This won't quite be complete as not everything is separated out quite > enough to allow things like process tear down/forking etc. to be explicitly > tested but you can unit tests the VMA bits at least. > > One note on this, I intend to split the vma.c file into multiple files in > tools/testing/vma/ so if you add tests here it'd be worth probably putting > them into a new file. > > I'm happy to help with this if you need any assistance, feel free to ping! As a starting point I was thinking of changing vma_assert_attached()/vma_assert_detached() and vma_mark_attached()/vma_mark_detached() to return a bool and use WARN_ON_ONCE() (to address your concern about asserts being dependent on CONFIG_DEBUG_VM) like this: static inline bool vma_assert_detached() { return !WARN_ON_ONCE(atomic_read(&vma->vm_refcnt)); } static inline bool vma_mark_attached(struct vm_area_struct *vma) { vma_assert_write_locked(vma); if (!vma_assert_detached(vma)) return false; atomic_set(&vma->vm_refcnt, 1); return true; } With that we can add correctness checks in the tools/testing/vma/vma.c for different states, for example in the alloc_and_link_vma() we can check that after vma_link() the vma is indeed attached: ASSERT_TRUE(vma_assert_attached(vma)); This might not cover all states but is probably a good starting point. WDYT? > > Sorry to put this on you so late in the series, I realise it's annoying, > but I feel like things have changed a lot and obviously aggregated with two > series in one in effect and these are genuine concerns that at this stage I > feel like we need to try to at least make some headway on. > > > > > > > > > I know it's tricky to write tests for this, but the new VMA testing > > > environment should make it possible to test a _lot_ more than we previously > > > could. > > > > > > However due to some (*ahem*) interesting distribution of where functions > > > are, most notably stuff in kernel/fork.c, I guess we can't test > > > _everything_ there effectively. > > > > > > But I do feel like we should be able to do better than having absolutely no > > > testing added for this? > > > > Again, I'm open to suggestions for SLAB_TYPESAFE_BY_RCU testing but > > for the rest I thought the tests were modified accordingly. > > See above ^ > > > > > > > > > I think there's definitely quite a bit you could test now, at least in > > > asserting fundamentals in tools/testing/vma/vma.c. > > > > > > This can cover at least detached state asserts in various scenarios. > > > > Ok, you mean to check that VMA re-attachment/re-detachment would > > trigger assertions? I'll look into adding tests for that. > > Yeah this is one, see above :) > > > > > > > > > But that won't cover off the really gnarly stuff here around RCU slab > > > allocation, and determining precisely how to test that in a sensible way is > > > maybe less clear. > > > > > > But I'd like to see _something_ here please, this is more or less > > > fundamentally changing how all VMAs are allocated and to just have nothing > > > feels unfortunate. > > > > Again, I'm open to suggestions on what kind of testing I can add for > > SLAB_TYPESAFE_BY_RCU change. > > See above > > > > > > > > > I'm already nervous because we've hit issues coming up to v9 and we're not > > > 100% sure if a recent syzkaller is related to these changes or not, I'm not > > > sure how much we can get assurances with tests but I'd like something. > > > > If you are referring to the issue at [1], I think David ran the > > syzcaller against mm-stable that does not contain this patchset and > > the issue still triggered (see [2]). This of course does not guarantee > > that this patchset has no other issues :) I'll try adding tests for > > re-attaching, re-detaching and welcome ideas on how to test > > SLAB_TYPESAFE_BY_RCU transition. > > Thanks, > > Suren. > > OK that's reassuring! > > > > > [1] https://lore.kernel.org/all/6758f0cc.050a0220.17f54a.0001.GAE@xxxxxxxxxx/ > > [2] https://lore.kernel.org/all/67823fba.050a0220.216c54.001c.GAE@xxxxxxxxxx/ > > > > > > > > Thanks! > > > > > > On Fri, Jan 10, 2025 at 08:25:47PM -0800, Suren Baghdasaryan wrote: > > > > Back when per-vma locks were introduces, vm_lock was moved out of > > > > vm_area_struct in [1] because of the performance regression caused by > > > > false cacheline sharing. Recent investigation [2] revealed that the > > > > regressions is limited to a rather old Broadwell microarchitecture and > > > > even there it can be mitigated by disabling adjacent cacheline > > > > prefetching, see [3]. > > > > Splitting single logical structure into multiple ones leads to more > > > > complicated management, extra pointer dereferences and overall less > > > > maintainable code. When that split-away part is a lock, it complicates > > > > things even further. With no performance benefits, there are no reasons > > > > for this split. Merging the vm_lock back into vm_area_struct also allows > > > > vm_area_struct to use SLAB_TYPESAFE_BY_RCU later in this patchset. > > > > This patchset: > > > > 1. moves vm_lock back into vm_area_struct, aligning it at the cacheline > > > > boundary and changing the cache to be cacheline-aligned to minimize > > > > cacheline sharing; > > > > 2. changes vm_area_struct initialization to mark new vma as detached until > > > > it is inserted into vma tree; > > > > 3. replaces vm_lock and vma->detached flag with a reference counter; > > > > 4. regroups vm_area_struct members to fit them into 3 cachelines; > > > > 5. changes vm_area_struct cache to SLAB_TYPESAFE_BY_RCU to allow for their > > > > reuse and to minimize call_rcu() calls. > > > > > > > > Pagefault microbenchmarks show performance improvement: > > > > Hmean faults/cpu-1 507926.5547 ( 0.00%) 506519.3692 * -0.28%* > > > > Hmean faults/cpu-4 479119.7051 ( 0.00%) 481333.6802 * 0.46%* > > > > Hmean faults/cpu-7 452880.2961 ( 0.00%) 455845.6211 * 0.65%* > > > > Hmean faults/cpu-12 347639.1021 ( 0.00%) 352004.2254 * 1.26%* > > > > Hmean faults/cpu-21 200061.2238 ( 0.00%) 229597.0317 * 14.76%* > > > > Hmean faults/cpu-30 145251.2001 ( 0.00%) 164202.5067 * 13.05%* > > > > Hmean faults/cpu-48 106848.4434 ( 0.00%) 120641.5504 * 12.91%* > > > > Hmean faults/cpu-56 92472.3835 ( 0.00%) 103464.7916 * 11.89%* > > > > Hmean faults/sec-1 507566.1468 ( 0.00%) 506139.0811 * -0.28%* > > > > Hmean faults/sec-4 1880478.2402 ( 0.00%) 1886795.6329 * 0.34%* > > > > Hmean faults/sec-7 3106394.3438 ( 0.00%) 3140550.7485 * 1.10%* > > > > Hmean faults/sec-12 4061358.4795 ( 0.00%) 4112477.0206 * 1.26%* > > > > Hmean faults/sec-21 3988619.1169 ( 0.00%) 4577747.1436 * 14.77%* > > > > Hmean faults/sec-30 3909839.5449 ( 0.00%) 4311052.2787 * 10.26%* > > > > Hmean faults/sec-48 4761108.4691 ( 0.00%) 5283790.5026 * 10.98%* > > > > Hmean faults/sec-56 4885561.4590 ( 0.00%) 5415839.4045 * 10.85%* > > > > > > > > Changes since v8 [4]: > > > > - Change subject for the cover letter, per Vlastimil Babka > > > > - Added Reviewed-by and Acked-by, per Vlastimil Babka > > > > - Added static check for no-limit case in __refcount_add_not_zero_limited, > > > > per David Laight > > > > - Fixed vma_refcount_put() to call rwsem_release() unconditionally, > > > > per Hillf Danton and Vlastimil Babka > > > > - Use a copy of vma->vm_mm in vma_refcount_put() in case vma is freed from > > > > under us, per Vlastimil Babka > > > > - Removed extra rcu_read_lock()/rcu_read_unlock() in vma_end_read(), > > > > per Vlastimil Babka > > > > - Changed __vma_enter_locked() parameter to centralize refcount logic, > > > > per Vlastimil Babka > > > > - Amended description in vm_lock replacement patch explaining the effects > > > > of the patch on vm_area_struct size, per Vlastimil Babka > > > > - Added vm_area_struct member regrouping patch [5] into the series > > > > - Renamed vma_copy() into vm_area_init_from(), per Liam R. Howlett > > > > - Added a comment for vm_area_struct to update vm_area_init_from() when > > > > adding new members, per Vlastimil Babka > > > > - Updated a comment about unstable src->shared.rb when copying a vma in > > > > vm_area_init_from(), per Vlastimil Babka > > > > > > > > [1] https://lore.kernel.org/all/20230227173632.3292573-34-surenb@xxxxxxxxxx/ > > > > [2] https://lore.kernel.org/all/ZsQyI%2F087V34JoIt@xsang-OptiPlex-9020/ > > > > [3] https://lore.kernel.org/all/CAJuCfpEisU8Lfe96AYJDZ+OM4NoPmnw9bP53cT_kbfP_pR+-2g@xxxxxxxxxxxxxx/ > > > > [4] https://lore.kernel.org/all/20250109023025.2242447-1-surenb@xxxxxxxxxx/ > > > > [5] https://lore.kernel.org/all/20241111205506.3404479-5-surenb@xxxxxxxxxx/ > > > > > > > > Patchset applies over mm-unstable after reverting v8 > > > > (current SHA range: 235b5129cb7b - 9e6b24c58985) > > > > > > > > Suren Baghdasaryan (17): > > > > mm: introduce vma_start_read_locked{_nested} helpers > > > > mm: move per-vma lock into vm_area_struct > > > > mm: mark vma as detached until it's added into vma tree > > > > mm: introduce vma_iter_store_attached() to use with attached vmas > > > > mm: mark vmas detached upon exit > > > > types: move struct rcuwait into types.h > > > > mm: allow vma_start_read_locked/vma_start_read_locked_nested to fail > > > > mm: move mmap_init_lock() out of the header file > > > > mm: uninline the main body of vma_start_write() > > > > refcount: introduce __refcount_{add|inc}_not_zero_limited > > > > mm: replace vm_lock and detached flag with a reference count > > > > mm: move lesser used vma_area_struct members into the last cacheline > > > > mm/debug: print vm_refcnt state when dumping the vma > > > > mm: remove extra vma_numab_state_init() call > > > > mm: prepare lock_vma_under_rcu() for vma reuse possibility > > > > mm: make vma cache SLAB_TYPESAFE_BY_RCU > > > > docs/mm: document latest changes to vm_lock > > > > > > > > Documentation/mm/process_addrs.rst | 44 ++++---- > > > > include/linux/mm.h | 156 ++++++++++++++++++++++------- > > > > include/linux/mm_types.h | 75 +++++++------- > > > > include/linux/mmap_lock.h | 6 -- > > > > include/linux/rcuwait.h | 13 +-- > > > > include/linux/refcount.h | 24 ++++- > > > > include/linux/slab.h | 6 -- > > > > include/linux/types.h | 12 +++ > > > > kernel/fork.c | 129 +++++++++++------------- > > > > mm/debug.c | 12 +++ > > > > mm/init-mm.c | 1 + > > > > mm/memory.c | 97 ++++++++++++++++-- > > > > mm/mmap.c | 3 +- > > > > mm/userfaultfd.c | 32 +++--- > > > > mm/vma.c | 23 ++--- > > > > mm/vma.h | 15 ++- > > > > tools/testing/vma/linux/atomic.h | 5 + > > > > tools/testing/vma/vma_internal.h | 93 ++++++++--------- > > > > 18 files changed, 465 insertions(+), 281 deletions(-) > > > > > > > > -- > > > > 2.47.1.613.gc27f4b7a9f-goog > > > >
