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

[Baolin Wang]

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, ..."