On Thu Oct 24, 2024 at 12:49 PM CEST, Daniel Gomez wrote: > On Wed Oct 23, 2024 at 11:27 AM CEST, David Hildenbrand wrote: > > On 23.10.24 10:04, Baolin Wang wrote: > > > > > > > > > On 2024/10/22 23:31, David Hildenbrand wrote: > > >> On 22.10.24 05:41, Baolin Wang wrote: > > >>> > > >>> > > >>> On 2024/10/21 21:34, Daniel Gomez wrote: > > >>>> 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? > > >>> > > >>> IMHO, as I discussed with Kirill, we still need maintain compatibility > > >>> with the 'huge=' mount option. This means that if 'huge=never' is set > > >>> for tmpfs, huge page allocation will still be prohibited (which can > > >>> address Hugh's request?). However, if 'huge=' is not set, we can > > >>> allocate large folios based on the write size. > > So, in order to make tmpfs behave like other filesystems, we need to > allocate large folios by default. Not setting 'huge=' is the same as > setting it to 'huge=never' as per documentation. But 'huge=' is meant to > control THP, not large folios, so it should not have a conflict here, or > else, what case are you thinking? > > So, to make tmpfs behave like other filesystems, we need to allocate > large folios by default. According to the documentation, not setting > 'huge=' is the same as setting 'huge=never.' However, 'huge=' is > intended to control THP, not large folios, so there shouldn't be > a conflict in this case. Can you clarify what specific scenario or > conflict you're considering here? Perhaps when large folios order is the > same as PMD-size? Sorry for duplicate paragraph. > > > >> > > >> I consider allocating large folios in shmem/tmpfs on the write path less > > >> controversial than allocating them on the page fault path -- especially > > >> as long as we stay within the size to-be-written. > > >> > > >> I think in RHEL THP on shmem/tmpfs are disabled as default (e.g., > > >> shmem_enabled=never). Maybe because of some rather undesired > > >> side-effects (maybe some are historical?): I recall issues with VMs with > > >> THP+ memory ballooning, as we cannot reclaim pages of folios if > > >> splitting fails). I assume most of these problematic use cases don't use > > >> tmpfs as an ordinary file system (write()/read()), but mmap() the whole > > >> thing. > > >> > > >> Sadly, I don't find any information about shmem/tmpfs + THP in the RHEL > > >> documentation; most documentation is only concerned about anon THP. > > >> Which makes me conclude that they are not suggested as of now. > > >> > > >> I see more issues with allocating them on the page fault path and not > > >> having a way to disable it -- compared to allocating them on the write() > > >> path. > > > > > > I may not understand your issues. IIUC, you can disable allocating huge > > > pages on the page fault path by using the 'huge=never' mount option or > > > setting shmem_enabled=deny. No? > > > > That's what I am saying: if there is some way to disable it that will > > keep working, great. > > I agree. That aligns with what I recall Hugh requested. However, I > believe if that is the way to go, we shouldn't limit it to tmpfs. > Otherwise, why should tmpfs be prevented from allocating large folios if > other filesystems in the system are allowed to allocate them? I think, > if we want to disable large folios we should make it more generic, > something similar to Ryan's proposal [1] for controlling folio sizes. > > [1] https://lore.kernel.org/all/20240717071257.4141363-1-ryan.roberts@xxxxxxx/ > > That said, there has already been disagreement on this point here [2]. > > [2] https://lore.kernel.org/all/ZvVRiJYfaXD645Nh@xxxxxxxxxxxxxxxxxxxx/
