On Tue, Feb 03, 2015 at 11:19:12AM -0800, Shaohua Li wrote: > There was a similar patch posted before, but it doesn't get merged. I'd like > to try again if there are more discussions. > http://marc.info/?l=linux-mm&m=141230769431688&w=2 > > mremap can be used to accelerate realloc. The problem is mremap will > punch a hole in original VMA, which makes specific memory allocator > unable to utilize it. Jemalloc is an example. It manages memory in 4M > chunks. mremap a range of the chunk will punch a hole, which other > mmap() syscall can fill into. The 4M chunk is then fragmented, jemalloc > can't handle it. > > This patch adds a new flag for mremap. With it, mremap will not punch the > hole. page tables of original vma will be zapped in the same way, but > vma is still there. That is original vma will look like a vma without > pagefault. Behavior of new vma isn't changed. > > For private vma, accessing original vma will cause > page fault and just like the address of the vma has never been accessed. > So for anonymous, new page/zero page will be fault in. For file mapping, > new page will be allocated with file reading for cow, or pagefault will > use existing page cache. > > For shared vma, original and new vma will map to the same file. We can > optimize this without zaping original vma's page table in this case, but > this patch doesn't do it yet. > > Since with MREMAP_NOHOLE, original vma still exists. pagefault handler > for special vma might not able to handle pagefault for mremap'd area. > The patch doesn't allow vmas with VM_PFNMAP|VM_MIXEDMAP flags do NOHOLE > mremap. Any comments on this? There are real requirements on this feature. jemalloc/tcmalloc are good examples here. Thanks, Shaohua -- 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>