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Re: When should ralloc.c be used?

From: Eli Zaretskii
Subject: Re: When should ralloc.c be used?
Date: Fri, 28 Oct 2016 15:25:27 +0300

> Cc: address@hidden, address@hidden, address@hidden
> From: Daniel Colascione <address@hidden>
> Date: Fri, 28 Oct 2016 02:52:19 -0700
> > If reserving a range of addresses doesn't necessarily mean they will
> > be later available for committing, then what is the purpose of
> > reserving them in the first place?  What good does it do?
> Reserving address space is useful for making sure you have a contiguous 
> range of virtual addresses that you can use later.

But if committing more pages from the reserved range is not guaranteed
to succeed, I cannot rely on getting that contiguous range of
addresses, can I?

> > We have in w32heap.c:mmap_realloc code that attempts to commit pages
> > that were previously reserved.  That code does recover from a failure
> > to commit, but such a failure is deemed unusual and causes special
> > warnings under debugger.  I never saw these warnings happen, except
> > when we had bugs in that code.  You seem to say that this is based on
> > false premises, and there's nothing unusual about MEM_COMMIT to fail
> > for the range of pages previously reserved with MEM_RESERVE.
> The MEM_COMMIT failure might be rare in practice --- systems have a lot 
> of memory these days --- but MEM_COMMIT failing for a memory region 
> previously reserved with MEM_RESERVE is perfectly legal.

I can only say that I never saw that happening.


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