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

["Kirill A. Shutemov" <kirill@shutemov.name>]

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.

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.

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

-- 
  Kiryl Shutsemau / Kirill A. Shutemov