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Date: Thu, 12 Apr 2018 09:40:23 +0300 (MSK)
From: Alexander Monakov <amonakov@...ras.ru>
To: musl@...ts.openwall.com
Subject: Re: [RFC PATCH] reduce severity of ldso reclaim_gaps hack

On Thu, 12 Apr 2018, Rich Felker wrote:
> > > This does not seem necessary. Free chunks in the last bin can be
> > > larger than MMAP_THRESHOLD; they're just broken up to satisfy
> > > allocations. Of course it's unlikely to happen anyway.
> > 
> > Do such oversized chunks appear in normal operation? This seems non-obvious,
> > so a comment pointing that out would probably be helpful.
> 
> The only way I could see it happening is on an arch ABI that allows
> very large pages (and has the ELF load segments aligned accordingly,
> as x86_64 does). In this case if the kernel/hardware only supported
> large (e.g. 2MB) pages, you'd pretty much always end up with >1.5MB of
> reclaimed space per DSO. IMO this is an awful kernel/hardware
> constraint to have, very wasteful, but it's exactly the situation
> where you'd most care about the gaps getting reclaimed for something
> useful.

What I meant to ask is: apart from chunks created via reclaim_gaps, can
such oversized chunks appear as a result of malloc-family calls invoked
by the program?

Alexander

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