On Mon, Jun 13, 2022 at 10:10:16PM +0200, David Hildenbrand wrote: > On 13.06.22 21:43, Peter Xu wrote: > > Hi, David, > > > > On Fri, Jun 10, 2022 at 11:42:06AM -0700, Nadav Amit wrote: > >> On Jun 10, 2022, at 11:14 AM, David Hildenbrand <david@xxxxxxxxxx> wrote: > >> > >>> Similar to our MM_CP_DIRTY_ACCT handling for shared, writable mappings, we > >>> can try mapping anonymous pages writable if they are exclusive, > >>> the PTE is already dirty, and no special handling applies. Mapping the > >>> PTE writable is essentially the same thing the write fault handler would do > >>> in this case. > >>> > >>> Special handling is required for uffd-wp and softdirty tracking, so take > >>> care of that properly. Also, leave PROT_NONE handling alone for now; > >>> in the future, we could similarly extend the logic in do_numa_page() or > >>> use pte_mk_savedwrite() here. Note that we'll now also check for uffd-wp in > >>> case of VM_SHARED -- which is harmless and prepares for uffd-wp support for > >>> shmem. > >>> > >>> While this improves mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE) > >>> performance, it should also be a valuable optimization for uffd-wp, when > >>> un-protecting. > >>> > >>> Applying the same logic to PMDs (anonymous THP, anonymous hugetlb) is > >>> probably not worth the trouble, but could similarly be added if there is > >>> demand. > > > > My memory was that Andrea had a version that used to have thp optimized > > too. It'll be a slight pity to lose it if still possible, then we treat > > thp and small pages the same logic and be all fair. Or is there any other > > challenge that we're facing? > > 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 > > > > >>> > >>> 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. > > > > >>> > >>> 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. > > > > >>> ---------------------------------------------------------- > >>> Measuring single byte access per page of 4096 bytes > >>> First write access: > >>> Min: 761 ns, Max: 1152 ns, Avg: 784 ns > >>> Second write access: > >>> Min: 130 ns, Max: 181 ns, Avg: 137 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 150 ns, Max: 1553 ns, Avg: 155 ns > >>> Write access after clearing softdirty: > >>> Min: 169 ns, Max: 1783 ns, Avg: 432 ns > >>> -> mprotect = 1.131 * second [avg] > >>> -> mprotect = 0.359 * softdirty [avg] > >>> ========================================================== > >>> Measuring memset() of 16384 bytes > >>> First write access: > >>> Min: 1594 ns, Max: 3497 ns, Avg: 2143 ns > >>> Second write access: > >>> Min: 250 ns, Max: 381 ns, Avg: 260 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 290 ns, Max: 1643 ns, Avg: 300 ns > >>> Write access after clearing softdirty: > >>> Min: 1242 ns, Max: 8987 ns, Avg: 1297 ns > >>> -> mprotect = 1.154 * second [avg] > >>> -> mprotect = 0.231 * softdirty [avg] > >>> ---------------------------------------------------------- > >>> Measuring single byte access per page of 16384 bytes > >>> First write access: > >>> Min: 1953 ns, Max: 2945 ns, Avg: 2008 ns > >>> Second write access: > >>> Min: 130 ns, Max: 912 ns, Avg: 142 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 160 ns, Max: 240 ns, Avg: 166 ns > >>> Write access after clearing softdirty: > >>> Min: 1112 ns, Max: 1513 ns, Avg: 1126 ns > >>> -> mprotect = 1.169 * second [avg] > >>> -> mprotect = 0.147 * softdirty [avg] > >>> ========================================================== > >>> Measuring memset() of 65536 bytes > >>> First write access: > >>> Min: 7524 ns, Max: 15650 ns, Avg: 7680 ns > >>> Second write access: > >>> Min: 251 ns, Max: 1323 ns, Avg: 648 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 270 ns, Max: 1282 ns, Avg: 736 ns > >>> Write access after clearing softdirty: > >>> Min: 4558 ns, Max: 12524 ns, Avg: 4623 ns > >>> -> mprotect = 1.136 * second [avg] > >>> -> mprotect = 0.159 * softdirty [avg] > >>> ---------------------------------------------------------- > >>> Measuring single byte access per page of 65536 bytes > >>> First write access: > >>> Min: 7083 ns, Max: 9027 ns, Avg: 7241 ns > >>> Second write access: > >>> Min: 140 ns, Max: 201 ns, Avg: 156 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 190 ns, Max: 451 ns, Avg: 197 ns > >>> Write access after clearing softdirty: > >>> Min: 3707 ns, Max: 5119 ns, Avg: 3958 ns > >>> -> mprotect = 1.263 * second [avg] > >>> -> mprotect = 0.050 * softdirty [avg] > >>> ========================================================== > >>> Measuring memset() of 524288 bytes > >>> First write access: > >>> Min: 58470 ns, Max: 87754 ns, Avg: 59353 ns > >>> Second write access: > >>> Min: 5180 ns, Max: 6863 ns, Avg: 5318 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 5871 ns, Max: 9358 ns, Avg: 6028 ns > >>> Write access after clearing softdirty: > >>> Min: 35797 ns, Max: 41338 ns, Avg: 36710 ns > >>> -> mprotect = 1.134 * second [avg] > >>> -> mprotect = 0.164 * softdirty [avg] > >>> ---------------------------------------------------------- > >>> Measuring single byte access per page of 524288 bytes > >>> First write access: > >>> Min: 53751 ns, Max: 59431 ns, Avg: 54506 ns > >>> Second write access: > >>> Min: 781 ns, Max: 2194 ns, Avg: 1123 ns > >>> Write access after mprotect(PROT_READ)+mprotect(PROT_READ|PROT_WRITE): > >>> Min: 161 ns, Max: 1282 ns, Avg: 622 ns > >>> Write access after clearing softdirty: > >>> Min: 30888 ns, Max: 34565 ns, Avg: 31229 ns > >>> -> mprotect = 0.554 * second [avg] > >>> -> mprotect = 0.020 * softdirty [avg] > >>> > >>> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > >>> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> > >>> Cc: Nadav Amit <nadav.amit@xxxxxxxxx> > >>> Cc: Dave Hansen <dave.hansen@xxxxxxxxx> > >>> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx> > >>> Cc: Peter Xu <peterx@xxxxxxxxxx> > >>> Cc: Yang Shi <shy828301@xxxxxxxxx> > >>> Cc: Hugh Dickins <hughd@xxxxxxxxxx> > >>> Cc: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx> > >>> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> > >>> --- > >>> > >>> v1 -> v2: > >>> * Rebased on v5.19-rc1 > >>> * Rerun benchmark > >>> * Fix minor spelling issues in subject+description > >>> * Drop IS_ENABLED(CONFIG_MEM_SOFT_DIRTY) check > >>> * Move pte_write() check into caller > >>> > >>> --- > >>> mm/mprotect.c | 67 ++++++++++++++++++++++++++++++++++++++++++--------- > >>> 1 file changed, 55 insertions(+), 12 deletions(-) > >>> > >>> diff --git a/mm/mprotect.c b/mm/mprotect.c > >>> index ba5592655ee3..728772bf41c7 100644 > >>> --- a/mm/mprotect.c > >>> +++ b/mm/mprotect.c > >>> @@ -38,6 +38,45 @@ > >>> > >>> #include "internal.h" > >>> > >>> +static inline bool can_change_pte_writable(struct vm_area_struct *vma, > >>> + unsigned long addr, pte_t pte, > >>> + unsigned long cp_flags) > >>> +{ > >>> + struct page *page; > >>> + > >>> + if ((vma->vm_flags & VM_SHARED) && !(cp_flags & MM_CP_DIRTY_ACCT)) > >>> + /* > >>> + * MM_CP_DIRTY_ACCT is only expressive for shared mappings; > >>> + * without MM_CP_DIRTY_ACCT, there is nothing to do. > >>> + */ > >>> + return false; > >>> + > >>> + if (pte_protnone(pte) || !pte_dirty(pte)) > >>> + return false; > >>> + > >>> + /* Do we need write faults for softdirty tracking? */ > >>> + if ((vma->vm_flags & VM_SOFTDIRTY) && !pte_soft_dirty(pte)) > >>> + return false; > >>> + > >>> + /* Do we need write faults for uffd-wp tracking? */ > >>> + if (userfaultfd_pte_wp(vma, pte)) > >>> + return false; > >>> + > >>> + if (!(vma->vm_flags & VM_SHARED)) { > >>> + /* > >>> + * We can only special-case on exclusive anonymous pages, > >>> + * because we know that our write-fault handler similarly would > >>> + * map them writable without any additional checks while holding > >>> + * the PT lock. > >>> + */ > >>> + page = vm_normal_page(vma, addr, pte); > >>> + if (!page || !PageAnon(page) || !PageAnonExclusive(page)) > >>> + return false; > >>> + } > >>> + > >>> + return true; > >>> +} > >>> + > >> > >> 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 Thanks, -- Peter Xu
