Date: Tue, 1 Apr 2014 20:40:57 +0400 From: Vasily Kulikov <segoon@...nwall.com> To: musl@...ts.openwall.com Subject: Re: Proposed approach for malloc to deal with failing brk On Sun, Mar 30, 2014 at 20:41 -0400, Rich Felker wrote: > We want brk. This is not because "brk is faster than mmap", but > because it takes a lot of work to replicate what brk does using mmap, > and there's no hope of making a complex dance of multiple syscalls > equally efficient. My best idea for emulating brk was to mmap a huge > PROT_NONE region and gradually mprotect it to PROT_READ|PROT_WRITE, What problem do you try to solve via PROT_NONE -> PROT_WRITE? Why not simply instantly mmap it as PROT_WRITE? Linux will not allocate physical pages until the first access, so you don't lose physical memory when it is not actually used. > but it turns out this is what glibc does for per-thread arenas and > it's really slow, probably because it involves splitting one VMA and > merging into another. Yes, both VMA split/merge and PTE/etc. changes. -- Vasily
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