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
