linux-kernel - Re: [RFC PATCH] Take mmap_min_addr into account while choosing unmapped address for x86-64.

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Date:   Wed, 28 Feb 2018 08:17:59 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Ilya Smith <blackzert@...il.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
        x86@...nel.org, kirill.shutemov@...ux.intel.com,
        dsafonov@...tuozzo.com, hughd@...gle.com,
        gregkh@...uxfoundation.org, craigb@...gle.com, oleg@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] Take mmap_min_addr into account while choosing
 unmapped address for x86-64.

On Tue 27-02-18 16:27:29, Ilya Smith wrote:
> > 
> > mmap_min_addr handling is a bit mess... As you say, we would return
> > EPERM rather than ENOMEM which can be confusing but depleting the
> > address space like that is quite unlikely on 64b unless I am missing.
> > It is good to be in sync here with the generic implementation though,
> > IMO.
> > 
> 
> If we take a look on mm/mmap.c:
> #ifndef HAVE_ARCH_UNMAPPED_AREA_TOPDOWN
> unsigned long
> arch_get_unmapped_area_topdown(
> …
> 	if (len > TASK_SIZE - mmap_min_addr)
> 		return -ENOMEM;
> …
> 	info.low_limit = max(PAGE_SIZE, mmap_min_addr);
> 
> And this one looks like a generic implementation.
> But for many other architectures like arch/parisc/kernel/sys_parisc.c 
> or arch/x86/kernel/sys_x86_64.c
> 
> 	info.low_limit = PAGE_SIZE;

Yeah, this is what I meant when saying that mmap_min_addr is a bit of a
mess. I am wondering whether the low_limit should be checked inside
vm_unmapped_area. We would still need some mmap_min_addr handling at
arch_get_unmapped_area_topdown layer which is still suboptimal but I do
not see an easy way around without reworking how the arch specific parts
are implemented currently.
-- 
Michal Hocko
SUSE Labs