On 07/11/2017 02:02 PM, Andrea Arcangeli wrote: > On Tue, Jul 11, 2017 at 11:23:19AM -0700, Mike Kravetz wrote: >> I was surprised as well when a JVM developer pointed this out. >> >> From the old e-mail thread, here is original use case: >> shmget(IPC_PRIVATE, 31498240, 0x1c0|0600) = 11337732 >> shmat(11337732, 0, 0) = 0x40299000 >> shmctl(11337732, IPC_RMID, 0) = 0 >> mremap(0x402a9000, 0, 65536, MREMAP_MAYMOVE|MREMAP_FIXED, 0) = 0 >> mremap(0x402a9000, 0, 65536, MREMAP_MAYMOVE|MREMAP_FIXED, 0x100000) = 0x100000 >> >> The JVM team wants to do something similar. They are using >> mmap(MAP_ANONYMOUS|MAP_SHARED) to create the initial mapping instead >> of shmget/shmat. As Vlastimil mentioned previously, one would not >> expect a shared mapping for parts of the JVM heap. I am working >> to get clarification from the JVM team. > > Why don't they use memfd_create instead? That's made so that the fd is > born anon unlinked so when the last reference is dropped all memory > associated with it is automatically freed. No need of IC_RMID and then > they can use mmap instead of mremap(len=0) to get a double map of it. Wow! I did not even know about memfd_create until you mentioned it. That would certainly work for 'normal' pages. > If they use mmap(MAP_ANONYMOUS|MAP_SHARED) it's not hugetlbfs, that > would have been the only issue. > > Using hugetlbfs for JVM wouldn't be really flexible, better they try > to leverage THP on SHM or the hugetlbfs reservation gets in the way of > efficient use of the unused memory for memory allocations that don't > have a definitive size (i.e. JVM forks or more JVM are run in > parallel). Well, the JVM has had a config option for the use of hugetlbfs for quite some time. I assume they have already had to deal with these issues. What prompted this discussion is that they want the mremap mirroring/ duplication functionality extended to support hugetlbfs. This is pretty straight forward. But, I wanted to have a discussion about whether the mremap(old_size == 0) functionality should be formally documented first. Do note that if you actually create/mount a hugetlbfs filesystem and use a fd in that filesystem you can get the desired functionality. However, they want to avoid this extra step if possible and use mmap(anon, hugetlb). I'm guessing that if memfd_create supported hugetlbfs, that would also meet their needs. Any thoughts about extending memfd_create support to hugetlbfs? I can't think of any big issues. In fact, 'under the covers' there actually is a hugetlbfs file created for anon mappings. However, that is not exposed to the user. >> Yes. I think this should be a separate patch. As mentioned earlier, >> mremap today creates a new/additional private mapping if called in this >> way with old_size == 0. To me, this is a bug. > > Kernel by sheer luck should stay stable, but the result is weird and > it's unlikely intentional. Yes, that is why I think it is a bug. Not that kernel is unstable, but rather the unintentional/unexpected result. > memfd_create doesn't have such issue, the new mmap MAP_PRIVATE will > get the file pages correctly after a new mmap (even if there were cows > in the old MAP_PRIVATE mmap). > >> One reason for the RFC was to determine if people thought we should: >> 1) Just document the existing old_size == 0 functionality >> 2) Create a more explicit interface such as a new mremap flag for this >> functionality >> >> I am waiting to see what direction people prefer before making any >> man page updates. > > I guess old_size == 0 would better be dropped if possible, if > memfd_create fits perfectly your needs as I supposed above. If it's > not dropped then it's not very far from allowing mmap of /proc/self/mm > again (removed around so far as 2.3.x?). Yes, in my google'ing it appears the first users of mremap(old_size == 0) previously used mmap of /proc/self/mm. If memfd_create can be extended to support hugetlbfs, then I might suggest dropping the memfd_create(old_size == 0) support. Just a thought. -- Mike Kravetz -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
