linux-kernel - Re: [RFC PATCH v3 0/4] Support large folios for tmpfs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <8e48cf24-83e1-486e-b89c-41edb7eeff3e@linux.alibaba.com>
Date: Mon, 21 Oct 2024 14:24:18 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>
Cc: Matthew Wilcox <willy@...radead.org>, akpm@...ux-foundation.org,
 hughd@...gle.com, david@...hat.com, wangkefeng.wang@...wei.com,
 21cnbao@...il.com, ryan.roberts@....com, ioworker0@...il.com,
 da.gomez@...sung.com, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v3 0/4] Support large folios for tmpfs

On 2024/10/17 19:26, Kirill A. Shutemov wrote:
> On Thu, Oct 17, 2024 at 05:34:15PM +0800, Baolin Wang wrote:
>> + Kirill
>>
>> On 2024/10/16 22:06, Matthew Wilcox wrote:
>>> On Thu, Oct 10, 2024 at 05:58:10PM +0800, Baolin Wang wrote:
>>>> Considering that tmpfs already has the 'huge=' option to control the THP
>>>> allocation, it is necessary to maintain compatibility with the 'huge='
>>>> option, as well as considering the 'deny' and 'force' option controlled
>>>> by '/sys/kernel/mm/transparent_hugepage/shmem_enabled'.
>>>
>>> No, it's not.  No other filesystem honours these settings.  tmpfs would
>>> not have had these settings if it were written today.  It should simply
>>> ignore them, the way that NFS ignores the "intr" mount option now that
>>> we have a better solution to the original problem.
>>>
>>> To reiterate my position:
>>>
>>>    - When using tmpfs as a filesystem, it should behave like other
>>>      filesystems.
>>>    - When using tmpfs to implement MAP_ANONYMOUS | MAP_SHARED, it should
>>>      behave like anonymous memory.
>>
>> I do agree with your point to some extent, but the ‘huge=’ option has
>> existed for nearly 8 years, and the huge orders based on write size may not
>> achieve the performance of PMD-sized THP in some scenarios, such as when the
>> write length is consistently 4K. So, I am still concerned that ignoring the
>> 'huge' option could lead to compatibility issues.
> 
> Yeah, I don't think we are there yet to ignore the mount option.

OK.

> Maybe we need to get a new generic interface to request the semantics
> tmpfs has with huge= on per-inode level on any fs. Like a set of FADV_*
> handles to make kernel allocate PMD-size folio on any allocation or on
> allocations within i_size. I think this behaviour is useful beyond tmpfs.
> 
> Then huge= implementation for tmpfs can be re-defined to set these
> per-inode FADV_ flags by default. This way we can keep tmpfs compatible
> with current deployments and less special comparing to rest of
> filesystems on kernel side.

I did a quick search, and I didn't find any other fs that require 
PMD-sized huge pages, so I am not sure if FADV_* is useful for 
filesystems other than tmpfs. Please correct me if I missed something.

> If huge= is not set, tmpfs would behave the same way as the rest of
> filesystems.

So if 'huge=' is not set, tmpfs write()/fallocate() can still allocate 
large folios based on the write size? If yes, that means it will change 
the default huge behavior for tmpfs. Because previously having 'huge=' 
is not set means the huge option is 'SHMEM_HUGE_NEVER', which is similar 
to what I mentioned:
"Another possible choice is to make the huge pages allocation based on 
write size as the *default* behavior for tmpfs, ..."