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charset="us-ascii" X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: E3BB440002 X-Stat-Signature: zxhfydi5p9ihdjzzcpi7zc86kicnjut4 X-Rspam-User: X-HE-Tag: 1789001910-103845 X-HE-Meta: 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 DWqooNr5 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 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Fri, Sep 04, 2026, Jason Gunthorpe wrote: > On Fri, Sep 04, 2026 at 10:24:21PM +0000, David Matlack wrote: > > > The proposal here (which is inspired by the KVM UAPI) is to ensure every > > LUO ABI struct has 2 properties: > > > > 1. A field to encode options/features (e.g. u64 flags). > > 2. A way way to grow without breaking backward compatibility (e.g. so > > we can add new fields). > > > > Each flag can mean whatever it needs to. e.g. It can indicate the > > precence of one or more fields (i.e. new fields in the struct), or it > > can mean a field now has a different meaning (i.e. union in the struct). > > > > This would enable adding support for new features without breaking > > backward compatibility. Downstream users would have to ensure their > > kernel does not start using a new feature while it can still rollback to > > a version that does not support the new feature. > > This was never the biggest problem. The main issue was the functional > behaviors of the kernel that cannot be represented simply as data in a > struct with some flag bits. > > Like for instance kernel A supports memfd folio sizes far larger than > kernel B because we fixed MAX_ORDER. You can't fix that just with > simplistic flags. Can you elaborate on why the folio sizes matter? Honest question, because I don't understand why the serialization format wouldn't express things as "N contiguous pages starting at PFN X". Then the implementation would rebuild its folios as appropriate. I could see things like HugeTLB not working if someone booted the kernel with support for only 1GiB pages and then tried to feed it payload with sub-1GiB ranges. But to me, those sorts of things fall into the "well yeah, don't do that" category.