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List-Subscribe: List-Unsubscribe: * Mateusz Guzik [250331 13:51]: > On Mon, Mar 31, 2025 at 6:43=E2=80=AFPM Liam R. Howlett wrote: > > > > * Mateusz Guzik [250330 15:43]: > > > On Sun, Mar 30, 2025 at 9:23=E2=80=AFPM Suren Baghdasaryan wrote: > > > > > However, the good news is that mm_count tends to be 1. If both > > > > > mm_count and mm_users are 1, then there is no usefaultfd in use a= nd > > > > > nobody to add it either. > > > > > > > > I'm not sure... IIUC new_userfaultfd() does not take mmap_lock whil= e > > > > calling mmgrab(), therefore I think it can race with the code check= ing > > > > its value. > > > > > > > > Considering the mm struct isn't the only way to find the vmas and there > > are users who use other locks to ensure the mm and vma don't go away > > (rmap, for example). It is reasonable to think that other users may us= e > > the vma lock to avoid mm_struct accesses racing. > > > > Although I don't know of a way this is unsafe today, we are complicatin= g > > the locking story of the mm with this change and no data has been given > > on the benefit. I don't recall any regression caused by the addition o= f > > per-vma locking? > > >=20 > I was not involved in any of this, but I learned about the issue from lwn= : > https://lwn.net/Articles/937943/ >=20 > the new (at the time) per-vma locking was suffering weird crashes in > multithreaded programs and Suren ultimately fixed it by locking parent > vma at a 5% hit, > see fb49c455323f ("fork: lock VMAs of the parent process when > forking"). The patch merely adds vma_start_write(mpnt) in dup_mmap. Ah, thanks for the pointer. I forgot about that part. You are referencing the 5% on 5,000 forks of 10,000 vmas in a tight loop. The kernel build time was not affected. I'm sticking with this being an inconsequential increase on any meaningful benchmark. Reading the commit did trigger my memory about the 5% regression, but I disregarded it as it seems so unlikely in real life that the benefits outweighed the upper bounds of 5% negative. The 5,000 forks of 10,000 vmas benchmark would also be at least a bit faster with Suren's more recent rcu safe vma change [1]. Although, that doesn't rule out changing this to get higher number, I still don't think the complexity of the change is worth it for a contrived benchmark. >=20 > What I'm proposing here remedies the problem for most commonly forking > consumers (single-threaded), assuming it does work. ;) >=20 > To that end see below. >=20 > > > > > > It issues: > > > ctx->mm =3D current->mm; > > > ... > > > mmgrab(ctx->mm); > > > > > > Thus I claim if mm_count is 1 *and* mm_users is 1 *and* we are in > > > dup_mmap(), nobody has a userfaultfd for our mm and there is nobody t= o > > > create it either and the optimization is saved. > > > > mm_count is lazy, so I am not entirely sure we can trust what it says. > > But maybe that's only true of mmgrab_lazy_tlb() now? > > >=20 > warning: I don't know the Linux nomenclature here. I'm going to > outline how I see it. >=20 > There is an idiomatic way of splitting ref counts into two: > - something to prevent the struct itself from getting freed ("hold > count" where I'm from, in this case ->mm_count) > - something to prevent data used by the structure from getting freed > ("use count" where I'm from, in this case ->mm_users) >=20 > mm_users > 0 keeps one ref on ->mm_count >=20 > AFAICS the scheme employed for mm follows the mold. >=20 > So with that mmgrab_lazy_tlb() example, the call bumps the count on first= use. >=20 > Suppose we are left with one thread in the process and a lazy tlb > somewhere as the only consumers. mm_users is 1 because of the only > thread and mm_count is 2 -- one ref for mm_users > 0 and one ref for > lazy tlb. >=20 > Then my proposed check: ->mm_count =3D=3D 1 && mm->mm_users =3D=3D 1 >=20 > ... fails and the optimization is not used. >=20 > Now, per the above, the lock was added to protect against faults > happening in parallel. The only cases I found where this is of note > are remote accesses (e.g., from /proc/pid/cmdline) and userfaultfd. >=20 > I'm not an mm person and this is why I referred to Suren to sort this > out, hoping he would both have interest and enough knowledge about mm > to validate it. >=20 > That is to say I don't *vouch* the idea myself (otherwise I would sign > off on a patch), I am merely bringing it up again long after the dust > has settled. If the idea is a nogo, then so be it, but then it would > be nice to document somewhere why is it so. Thanks for the breakdown here. I am also not sure, but I felt it worth the mention. Reading Documentation/mm/active_mm.rst makes me uncertain of the mm_count =3D=3D 1 and mm_users =3D=3D 1 test. Since mm_count is the number= of 'lazy' users, and it may no longer include the lazy users.. Since there are no real workloads that suffer, is this worth it? [1]. https://lore.kernel.org/all/202503311656.e3596aaf-lkp@intel.com/ Thanks, Liam