linux-kernel - Re: [PATCH] hugetlbfs: add O

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Message-ID: <c0415816-2682-7bf5-2c82-43c3a8941a54@tlen.pl>
Date:   Tue, 22 Oct 2019 09:09:08 +0200
From:   Piotr Sarna <p.sarna@...n.pl>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Michal Hocko <mhocko@...nel.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH] hugetlbfs: add O_TMPFILE support

On 10/21/19 7:17 PM, Mike Kravetz wrote:
> On 10/15/19 4:37 PM, Mike Kravetz wrote:
>> On 10/15/19 3:50 AM, Michal Hocko wrote:
>>> On Tue 15-10-19 11:01:12, Piotr Sarna wrote:
>>>> With hugetlbfs, a common pattern for mapping anonymous huge pages
>>>> is to create a temporary file first.
>>>
>>> Really? I though that this is normally done by shmget(SHM_HUGETLB) or
>>> mmap(MAP_HUGETLB). Or maybe I misunderstood your definition on anonymous
>>> huge pages.
>>>
>>>> Currently libraries like
>>>> libhugetlbfs and seastar create these with a standard mkstemp+unlink
>>>> trick,
>>
>> I would guess that much of libhugetlbfs was writen before MAP_HUGETLB
>> was implemented.  So, that is why it does not make (more) use of that
>> option.
>>
>> The implementation looks to be straight forward.  However, I really do
>> not want to add more functionality to hugetlbfs unless there is specific
>> use case that needs it.
> 
> It was not my intention to shut down discussion on this patch.  I was just
> asking if there was a (new) use case for such a change.  I am checking with
> our DB team as I seem to remember them using the create/unlink approach for
> hugetlbfs in one of their upcoming models.
> 
> Is there a new use case you were thinking about?
> 

Oh, I indeed thought it was a shutdown. The use case I was thinking 
about was in Seastar, where the create+unlink trick is used for creating 
temporary files (in a generic way, not only for hugetlbfs). I simply 
intended to migrate it to a newer approach - O_TMPFILE. However,
for the specific case of hugetlbfs it indeed makes more sense to skip it 
and use mmap's MAP_HUGETLB, so perhaps it's not worth it to patch a 
perfectly good and stable file system just to provide a semi-useful flag 
support. My implementation of tmpfile for hugetlbfs is straightforward 
indeed, but the MAP_HUGETLB argument made me realize that it may not be 
worth the trouble - especially that MAP_HUGETLB is here since 2.6 and 
O_TMPFILE was introduced around v3.11, so the mmap way looks more portable.

tldr: I'd be very happy to get my patch accepted, but the use case I had 
in mind can be easily solved with MAP_HUGETLB, so I don't insist.