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

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Message-ID: <D51IU4N746MI.FDS6C7GYO4RP@samsung.com>
Date: Mon, 21 Oct 2024 15:34:03 +0200
From: Daniel Gomez <da.gomez@...sung.com>
To: "Kirill A. Shutemov" <kirill@...temov.name>, Baolin Wang
	<baolin.wang@...ux.alibaba.com>
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>,
	<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 Mon Oct 21, 2024 at 10:54 AM CEST, Kirill A. Shutemov wrote:
> On Mon, Oct 21, 2024 at 02:24:18PM +0800, Baolin Wang wrote:
>> 
>> 
>> 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.
>
> What do you mean by "require"? THPs are always opportunistic.
>
> IIUC, we don't have a way to hint kernel to use huge pages for a file on
> read from backing storage. Readahead is not always the right way.
>
>> > 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, ..."
>
> I am more worried about breaking existing users of huge pages. So changing
> behaviour of users who don't specify huge is okay to me.

I think moving tmpfs to allocate large folios opportunistically by
default (as it was proposed initially) doesn't necessary conflict with
the default behaviour (huge=never). We just need to clarify that in
the documentation.

However, and IIRC, one of the requests from Hugh was to have a way to
disable large folios which is something other FS do not have control
of as of today. Ryan sent a proposal to actually control that globally
but I think it didn't move forward. So, what are we missing to go back
to implement large folios in tmpfs in the default case, as any other fs
leveraging large folios?