RFR[S] 8005165 Platform-independent C++ vtables for CDS (original) (raw)
Ioi Lam ioi.lam at oracle.com
Mon Mar 6 19:16:41 UTC 2017
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Hi Thomas,
On 3/6/17 8:51 AM, Thomas Stüfe wrote:
Hi Coleen and Ioi,
I had to port C++ code to platforms with terrible compilers for a time in my life, that is why I like code to be as portable as possible. That said, you are right in your argumentation, the SafeFetch solution is not terribly elegant and Ioi's original way of determining the vtable size is cleaner. I did some checks on some of our architectures with a test similar to Ioi's and on a first glance it seems to work for simple cases (single and public inheritance) on ppc (AIX) and Linux ia64. Although the vtables seemed to me to contain function descriptors, not real pointers to code, so this is something to keep in mind. But if the live vtable are copied, the function descriptors they contain should point to valid code too, so it should not matter. Just to remember to not expect every slot in the array to be a valid code pointer. I also notice on our current Solaris build the first slot in the vtable is the RTTI pointer, not a function address. This slot is different in the TesterA and TesterB classes, so I had to skip over it. I think this is safe because Metadata has about 10 virtual methods, so skipping over the first one should not cause any harm. So, in short, I remove my objection to Ioi's original solution, as far as that matters. I still think we rely on a lot here: Contiguous vtable containing absolute memory addresses, vtable pointer at start of object and vtable entries to be ordered from base->derived class. So I wonder how much effort it would be (now or in the future as a separate change) to have a fallback where - at loading time - instead of copying vtables the vtable pointers in the objects were fixed up to point to the new live vtables? I know this would be more expensive and potentially defeat the point of shared classes. But maybe not, it depends on how many objects are there, no?
I think for portability, perhaps we can add a porting layer so the vtables can be copied in a different way? I think this is better than patching each object at run time.
Also, I would suggest not adding such a layer now, but do it when such a need actually arises.
Thanks
- Ioi
Kind Regards, Thomas On Sun, Mar 5, 2017 at 4:17 PM, <coleen.phillimore at oracle.com_ _<mailto:coleen.phillimore at oracle.com>> wrote:
Ioi, Some comments inline (where no comments, insert "ok") :) On 3/2/17 10:37 PM, Ioi Lam wrote: Hi Coleen, Thanks for the comments. I have updated the webrev. See in-line for responses. http://cr.openjdk.java.net/~iklam/jdk10/8005165-platform-independent-cds-vtable.v03/ <http://cr.openjdk.java.net/%7Eiklam/jdk10/8005165-platform-independent-cds-vtable.v03/> On 3/2/17 8:48 AM, coleen.phillimore at oracle.com <mailto:coleen.phillimore at oracle.com> wrote: Ioi I like the concept of this a lot but have some stylistic comments to help people reading this code later. http://cr.openjdk.java.net/~iklam/jdk10/8005165-platform-independent-cds-vtable.v02/src/share/vm/memory/metaspaceShared.cpp.udiff.html <http://cr.openjdk.java.net/%7Eiklam/jdk10/8005165-platform-independent-cds-vtable.v02/src/share/vm/memory/metaspaceShared.cpp.udiff.html> s/vtab/vtable/g and s/Vtab/Vtable/ please. It doesn't save many characters, especially in CppVtableInfo/Testers Done. + // Start at slot 1, because slot 0 may be RTTI (on Solaris/Sparc) + int i; + for (i=1; ; i++) { Since you're using 'i' later, can you rename it to something descriptive. Or have another variable "vtablelength" to use later. This looks like an old style for loop. Done Can the functions for CppVtableInfo be declared outside of the class declaration? They don't need to be inline and then the debug code for testing the vtable size can be not in the middle of the class declaration. Then you can move the Tester classes to inside the same #ifndef PRODUCT block. Can you put #endif // PRODUCT when the ifdef covers several lines of code? Done vtabof could be more descriptive, like cppvtablefor(). I changed to vtableof(). Because the class name is already CppVtableCloner, repeating the word "cpp" seems repetitive to me. Was PrintSharedSpaces was never converted to UL? Right. I've filed https://bugs.openjdk.java.net/browse/JDK-8176132 <https://bugs.openjdk.java.net/browse/JDK-8176132> (-XX:+PrintSharedSpaces should be converted to use Unified Logging.) + int n = MAXVTABLESIZE; Can you propagate MAXVTABLESIZE to the places where it's used. n isn't descriptive. This starts out with maxvtablesize and then changes the size. Reusing 'n' makes this really hard to follow. Not having a comment that we only allocate enough slots for the vtable makes it hard too. + // allocate CppVtableInfo in the MD section + info = (CppVtabInfo*)mdtop; + info->setvtabsize(n); // initially set to maxvtablesize + + // allocate temporary local instance of the metadata type T + T tmp; + intptrt* srcvtab = vtabof(tmp); + intptrt* dstvtab = info->vtab(); + Fixed. Something like that for comments. dstvtab is the destinationvtable in the MD section. I've dropped the md prefix from the functions that deal with the vtables, since they shouldn't care whether it's the "MD" section or not. Now it looks like this: // Allocate and initialize the C++ vtables, starting from top, but do not go past end. intptrt* MetaspaceShared::allocatecppvtableclones(intptrt* top, intptrt* end) { assert(DumpSharedSpaces, "dump-time only"); // Layout (each slot is a intptrt): // [number of slots in the first vtable = n1] // [ slots for the first vtable] // [number of slots in the first second = n2] // [ slots for the second vtable] // ... // The order of the vtables is the same as the CPPVTABPATCHTYPESDO macro. CPPVTABLEPATCHTYPESDO(ALLOCCPPVTABLECLONE); return top; } + for (int i=0; i<n; i++) {_ _+ const intptrt bad = intptrt(0xdeadbeef);_ _+ intptrt num = SafeFetchN(&srcvtab[i], bad);_ _+ if (num == bad_ _+ // || i > 120 /* uncomment this line to test */ + ) { + info->setvtabsize(i-1); + break; + } + dstvtab[i] = num; + } I dont understand this code. You get deadbeef for a bad value if SafeFetchN gets a fault but why would it get a fault at the end of the metadata's vtable? Couldn't it just run onto the next vtable? I think your original way of counting vtable entries might be better (sorry I didn't have time to study that thread). I've modified the comments to this. Does it make sense to you? // It is not always safe to call memcpy(), because srcvtable might be shorter than // MAXVTABLESIZE, and the C++ linker might have placed the vtable at the very // end of the last page of libjvm.so. Crossing over to the next page might // cause a page fault. My fear is the JVM would suddenly start crashing because the order of .o files have changed on the linker's command line, or if you enable some special linker optimization flags. It's better safe than sorry. This wasn't exactly what I was not understanding. I didn't see that you are copying 120 entries from the old vtable and junk memory beyond the old vtable, unless you get a segv, in which case you copy less. I don't think you should copy random memory into the vtable in the archive. This doesn't seem secure, even with the segv protection. Since we already have assumptions about C++ vtable layout in the code and it's mostly specified by various ABIs, and you have the assert code, I think I would prefer that you copy only the vtable entries into the archive. I guess Thomas Stuefe had a different opinion. I've read the original thread. Two points: If new C++ compiler implementations add a discontigous vtable, both the SafeFetchN and subclass additional virtual function at end implementation will fail. I don't think C++ implementations would do this and a contiguous vtable as first in the instance has been standard for years. If our metadata adds multiple inheritance, the same issue would be a problem for both implementations, as well as for the implementation we have before Ioi's fix. Ioi's subclass adding virtual function method would work for any esoteric C++ implementations in my memory, except the vptr for the old DECC++ compiler was after the nonstatic data members (which would fail with all of our implementations). Since the code is there anyway for debug purposes, we're not saving code by implementing SafeFetchN. The SafeFetchN implementation isn't obvious at all what it's doing, and requires better comments, especially if you don't know already what SafeFetchN does. It looks really cryptic. The poisoned values also bothered me in that they overload other poisoned values in other parts of the jvm. Ioi, could you make all methods of CppVtableCloner out of line? The other changes look good, although I might have more requests for comments. Thanks, Coleen Would be nice to have comments here too!! + intptrt* start = mdtop; This doesn't do anything (?) Fixed. This was left over code. + MetaspaceShared::zerocppvtableclonesforwriting(); Why not zero the destination vtable in allocate? Or does patching the vtable pointers call virtual functions? You could prevent that so you don't need this code. I added this comment: // During patching, some virtual methods may be called, so at this point // the vtables must contain valid methods (as filled in by CppVtableCloner::allocate). MetaspaceShared::patchcppvtablepointers(); // The vtable clones contain addresses of the current process. // We don't want to write these addresses into the archive. MetaspaceShared::zerocppvtableclonesforwriting(); + // Restore the vtable in case we invoke any virtual methods. + MetaspaceShared::clonecppvtables((intptrt*)vtbllist); Can this be restorecppvtables since that's what it's doing. The first is after the dump and the second call is at UseSharedSpaces. A couple of comments in this clonecppvtables --> restorecppvtables would be nice. eg: I prefer to use the word clone. Otherwise when you just say "vtable" it's not clear whether you're talking about the original one (made by the c++ linker), or the cloned one in the CDS archive. + static intptrt* clonevtable(const char* name, intptrt* p) { + T tmp; // Allocate temporary dummy metadata object to get vtable initialized + CppVtabInfo* info = (CppVtabInfo*)p; + int n = info->vtabsize(); + intptrt* srcvtab = vtabof(tmp); + intptrt* dstvtab = info->vtab(); + + // We already checked (and, if necessary, adjusted n) when the vtables were allocated, so we are + // safe to do memcpy. + if (PrintSharedSpaces) { + tty->printcr("%s copying %d vtable entries", name, n); + } + memcpy(dstvtab, srcvtab, sizeof(intptrt) * n); + return dstvtab + n; + } Done. I changed the wording T tmp; // Allocate temporary dummy metadata object to get to the original vtable. As we are not really "initializing a vtable" here. Same with 'patch'. It'd be so much faster and easier to read this code with more comments please. http://cr.openjdk.java.net/~iklam/jdk10/8005165-platform-independent-cds-vtable.v02/src/share/vm/oops/constantPool.hpp.udiff.html <http://cr.openjdk.java.net/%7Eiklam/jdk10/8005165-platform-independent-cds-vtable.v02/src/share/vm/oops/constantPool.hpp.udiff.html> Why are these testers here? I updated the comment: // Used by CDS. These classes need to access the private ConstantPool() constructor. template friend class CppVtableTesterA; template friend class CppVtableTesterB; template friend class CppVtableCloner; Thanks - Ioi On 3/1/17 3:25 AM, Ioi Lam wrote: https://bugs.openjdk.java.net/browse/JDK-8005165 <https://bugs.openjdk.java.net/browse/JDK-8005165> http://cr.openjdk.java.net/~iklam/jdk10/8005165-platform-independent-cds-vtable.v02/ <http://cr.openjdk.java.net/%7Eiklam/jdk10/8005165-platform-independent-cds-vtable.v02/> Hi, This is the official review (follow up of the "Determining the size of C++ vtables" thread onhotspot-dev at openjdk.java.net <mailto:onhotspot-dev at openjdk.java.net>). The new code has the same assumption as the existing code in JDK 10: for a C++ object that contains virtual methods (e.g., ConstantPool), we assume the first intptrt slot of the object is a vptr, which points to a vtable, which consists of no more than 150 intptrt's. ConstantPool*p -->[ vptr ] -------> [ vtable slot 0 ] [ field #0 ] [ vtable slot 1 ] [ field #1 ] [ vtable slot 2 ] [ field #2 ] [ vtable slot 3 ] [ .... ] [ vtable slot 4] [ vtable slot 5 ] [ ... ] + In the existing code, we were pointing the vtable slots to code that's generated by HotSpot. + In the new code, we copy the vtable slots from an existing vtable (generated by the C++ linker). Per Thomas Stüfe's advice, I don't try to determine the size of the vtable (as that would add one more compiler requirement where new virtual methods added by a subclass must be placed at a higher offset in the vtable). Instead, I have added code in non-product builds to ensure that the vtables are no longer than 150 entries. You can run with "-XX:+PrintSharedSpaces -Xshare:dump" to print out the actual size of the vtables for your particular platform: ConstantPool has 12 virtual methods InstanceKlass has 113 virtual methods InstanceClassLoaderKlass has 113 virtual methods InstanceMirrorKlass has 113 virtual methods InstanceRefKlass has 113 virtual methods Method has 12 virtual methods ObjArrayKlass has 114 virtual methods TypeArrayKlass has 114 virtual methods As mentioned in the code comments, if you have an esoteric C++ compiler, the verifysufficientsize() function will probably fail, but hopefully that would give you some hints for porting this code. To avoid accidentally touching an unmapped page, the code uses SafeFetchN for copying the vtable contents, and would shrink the vtable to less than 150 entries if necessary. I can't test this for real, but I've added some code to simulate an error: for (int i=0; i<n; i++) {_ _const intptrt bad = intptrt(0xdeadbeef);_ _intptrt num = SafeFetchN(&srcvtab[i], bad);_ _if (num == bad_ _// || i > 120 /* uncomment this line to test */ ) { info->setvtabsize(i-1); break; } dstvtab[i] = num; } Results: + Removed 850 lines of CPU-dependent code + CDS image is about 50K smaller + Previously Metadata objects must live in the read-write section in the CDS archive, because their vptr was updated at run time. Now vptr is no longer updated, so ConstantPool can be moved to the read-only section (see JDK-8171392). Thanks - Ioi
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