>> Not really, but I assume performance gain will be minimal and might not >> be worth the trouble. >> >> I'm fairly busy (and not aware of Andreas version), so I can look at >> this, but it will be part of a separate patch because it will go on my >> TODO list. Not mad if someone beats me to it ;) > > Just for the reference: > > https://github.com/aagit/aa/commit/34cd0d78db407af06d35a06b24be8e92593964be Thanks for that reference! > >> >>> >>>>> >>>>> Results of a simple microbenchmark on my Ryzen 9 3900X, comparing the new >>>>> optimization (avoiding write faults) during mprotect() with softdirty >>>>> tracking, where we require a write fault. >>> >>> Are we comparing the mprotect() sequence operations against softdirty >>> clearing operation? Would it make more sense if we compare the same >>> mprotect() sequence to kernels that are before/after this patch applied? >> >> For simplicity I compared on the same kernel, one time exploting the >> optimization and one time disabling the optimization via softdirty. >> >> I can also simply measure without+with. Extra work for me to combine >> outputs :P > > Well, still that's normally how we work on these, don't we? :) > > Still note that the SOFTDIRTY check (I think) was still reverted.. I meant > I kept thinking below check "vma->vm_flags & VM_SOFTDIRTY" should be > "!(vma->vm_flags & VM_SOFTDIRTY)", but again that's separate change so feel > free to ignore as we've discussed, but please make sure even if you want to > compare with softdirty that's taking into account. I wrapped my head around that softdirty check *a lot* and ended up figuring out that it unintuitively is correct. But maybe I ended up confusing myself. Anyhow, this patch merely moves that check. > >> >>> >>>>> >>>>> Running 1000 iterations each >>>>> >>>>> ========================================================== >>>>> Measuring memset() of 4096 bytes >>>>> First write access: >>>>> Min: 169 ns, Max: 8997 ns, Avg: 830 ns >>>>> Second write access: >>>>> Min: 80 ns, Max: 251 ns, Avg: 168 ns >>>>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): >>>>> Min: 180 ns, Max: 290 ns, Avg: 190 ns >>>>> Write access after clearing softdirty: >>>>> Min: 451 ns, Max: 1774 ns, Avg: 470 ns >>>>> -> mprotect = 1.131 * second [avg] >>>>> -> mprotect = 0.404 * softdirty [avg] >>> >>> (I don't understand these two lines.. but maybe I'm the only one?) >> >> Most probably not. >> >> "mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE)" needs 113,1% the >> runtime compared with the "second write" access. >> >> "mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE)" needs 40% of the >> runtime compared with disabling the optimization via softdirty tracking. >> >> I may find time to clean that up a bit more to make it easier to consume >> for humans. > > I see, thanks. Appending the explanation after the test result will also > work for me. > > I'm curious is that 113.1% came from tlb miss? If that's the case, I'd > suggest drop those comparisons if there's a new version, since they're > probably not helping to explain what this patch is changing (avoid page > faluts), and IMHO it can slightly confuse reviewers, if you agree. As we seem to have easier benchmarks from Andrea and Peter, I can just reuse these and make my life easier :) [...] >>>> Looks good in general. Just wondering (out loud) whether it makes more sense >>>> to do all the vm_flags and cp_flags related checks in one of the callers >>>> (mprotect_fixup()?) and propagate whether to try to write-unprotect in >>>> cp_flags (e.g., by introducing new MM_CP_TRY_WRITE_UNPROTECT). >>> >>> I can see why David put it like that, because most of the checks are on >>> ptes not vm_flags. >>> >>> But I also agree on this point, especially if to put it in another way: >>> IMHO it'll be confusing if we keey MM_CP_DIRTY_ACCT==false for all private >>> pages even if we're going to take them into account and do smart unprotect >>> operations. >>> >>> So I'm wondering whether we could still at least move vm_flags check into >>> the mprotect_fixup() as suggested by Nadav, perhaps something like: >>> >>> diff --git a/mm/mprotect.c b/mm/mprotect.c >>> index ba5592655ee3..aefd5fe982af 100644 >>> --- a/mm/mprotect.c >>> +++ b/mm/mprotect.c >>> @@ -583,7 +583,11 @@ mprotect_fixup(struct mmu_gather *tlb, struct vm_area_struct *vma, >>> * held in write mode. >>> */ >>> vma->vm_flags = newflags; >>> - dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot); >>> + if (vma->vm_flags & VM_SHARED) >>> + dirty_accountable = vma_wants_writenotify(vma, vma->vm_page_prot); >>> + else >>> + /* For private mappings, only if it's writable */ >>> + dirty_accountable = vma->vm_flags & VM_WRITE; >>> vma_set_page_prot(vma); >>> >>> change_protection(tlb, vma, start, end, vma->vm_page_prot, >>> >>> Then IIUC we could drop both the VM_WRITE check in change_pte_range(), and >>> also the VM_SHARED check above in can_change_pte_writable(). Not sure >>> whether that'll look slightly cleaner. >> >> I'll give it a shot and most probably rename dirty_accountable to >> something more expressive -- like Nadav proposed, for example. > > Sure. > >> >>> >>> I'm also copying Peter Collingbourne <pcc@xxxxxxxxxx> because afaict he >>> proposed the initial idea (maybe worth some credit in the commit message?), >> >> Do you have a link to that conversation? Either my memory is messing >> with me or I did this without reading that mail (which I think, because >> it simply made sense with PageAnonExclusive at hand). Still, I can add a >> reference to that mail and mention that this was suggested earlier by >> Peter C.. > > I see, no worry then I thought it was coming from that. In this case I'm > not sure whether it's still needed. > > PeterC's v1 was here: > > https://lore.kernel.org/linux-mm/20201212053152.3783250-1-pcc@xxxxxxxxxx/ > > But there're a bunch of versions: > > https://lore.kernel.org/linux-mm/?q=mm%3A+improve+mprotect%28R%7CW%29+efficiency+on+pages+referenced+once No idea why I missed that completely as I tend to read a lot linux-mm. Thanks for the pointers! -- Thanks, David / dhildenb
