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

[Kirill A. Shutemov]

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov