linux-kernel - Re: [PATCH] Fix get_unmapped_area and fsync for hugetlb shm segments

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Message-ID: <m1k5xsa6do.fsf@ebiederm.dsl.xmission.com>
Date:	Wed, 07 Mar 2007 18:09:07 -0700
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Bill Irwin <bill.irwin@...cle.com>
Cc:	Adam Litke <agl@...ibm.com>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [PATCH] Fix get_unmapped_area and fsync for hugetlb shm segments

Bill Irwin <bill.irwin@...cle.com> writes:

> On Wed, Mar 07, 2007 at 04:03:17PM -0700, Eric W. Biederman wrote:
>> I think the right answer is most likely to add an extra file method or
>> two so we can remove the need for is_file_hugepages.
>> There are still 4 calls to is_file_hugepages in ipc/shm.c and
>> 2 calls in mm/mmap.c not counting the one in is_file_shm_hugepages.
>> The special cases make it difficult to properly wrap hugetlbfs files
>> with another file, which is why we have the weird special case above.
>
> It's not clear to me that the core can be insulated from hugetlb's
> distinct pagecache and memory mapping granularities in a Linux-native
> manner, but if you come up with something new or manage to get the
> known methods past Linus, akpm, et al, more power to you.

I will agree with that there are limits on what can be achieved.
However looking at where we have tests for is_file_hugepages most of
those tests don't appear to be inherently anything to do with huge
pages, so it wouldn't surprise me if we couldn't generalize things a
little more.

> I'm not entirely sure what you're up to, but I'm mostly here to sanction
> others' design notions since my own are far too extreme, and, of course,
> review and ack patches, take bugreports and write fixes (not that I've
> managed to get to any of them first in a long while, if ever), and so on.
> I say killing the is_whatever_hugepages() checks with whatever abstraction
> is good, since I don't like them myself, provided it's sane. Go for it.

Mostly I had reference counting and consistency problems with
ipc/shm.c that had horrible leak potential when I exited a ipc
namespace.  Implementing everything as stacked files made the code
simpler and more maintainable. (shm_nattach stopped being a special
case yea!)

I'm happy to stop here but if someone cares to proceed with removing
is_file_hugepages I want to encourage that.  I don't see any other
cleanups short of that are really worth doing.

Everything in ipc/shm.c could be considered a weird special case, so
I'm not going to worry about it too much.  Although removing those
special cases is good.

There is some odd accounting logic in mm/mmap.c based on
is_file_hugepages and there is the get_unmapped_area case.  For
get_unmapped_area I see no reason to presume that the only kind of
file that must live at a specific address are huge pages (even if that
is the only kind of file where we have that case today).  So
generalizing that check should be relatively straight forward.

Eric
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