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charset=UTF-8 Content-Transfer-Encoding: 7bit X-Stat-Signature: 6c4wbdwf5amtmkbihei1x49hnehgg844 X-Rspamd-Server: rspam09 X-Rspam-User: X-Rspamd-Queue-Id: 9C843A000C X-HE-Tag: 1773759932-8857 X-HE-Meta: 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 maW1yfsS M1GfCUlsZvRg3SwaRc6PhSSGRwwNiWziif1//AMUQLv40uMVjTnEmt10PeRMCGpl5yNNNRTTZUMSgxZ6VusL59Oe/ZqrhskK79FE7H3QaokvMY6PqkIXSjUebHhl4+KAeGDwue5YJjNR7HSq23kOQQuxO0Q== Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 17/03/2026 12:43, Kevin Brodsky wrote: > On 17/03/2026 12:45, Ryan Roberts wrote: >> On 17/03/2026 09:29, Kevin Brodsky wrote: >>> On 17/03/2026 10:13, Ryan Roberts wrote: >>>>>>> Another option would be to initially map by pte then collapse to >>>>>>> block mappings >>>>>>> once we have determined that all cpus support BBML2_NOABORT. We >>>>>>> originally opted >>>>>>> not to do that because it's a tax on symetric systems. But we could >>>>>>> throw in the >>>>>>> towel if it's the least bad solution we can come up with for solving >>>>>>> this. I >>>>>>> think it might help some of Kevin's use cases too? >>>>>> May be an option too. When we discussed this there was no usecase for >>>>>> direct mapping collapse. But if we can have multiple usecases, it may >>>>>> be worth it. >>>> I could imagine that if user space creates and destroys lots of secretmem areas, >>>> then it will completely split the linear map to ptes and that will never recover >>>> currently. So I think in the long term, having the ability to collapse would be >>>> useful. I just don't particularly like forcing symetric systems to map by pte >>>> initially (which is slow) only to collapse later (which will cost even more >>>> time). But it does feel inherrently more robust. >>> Now that you spell it out, I'm realising this would actually make things >>> pretty complicated for protected page tables. In that series, page >>> tables for the linear map are allocated by a separate memblock-based >>> allocator [1], tracking the allocated ranges to set their pkey later. >>> There's a strong assumption that these page tables are never freed. >>> >>> If we initially PTE-mapped the linear map and then later collapsed it, >>> that assumption clearly wouldn't hold. >> Sorry I don't understand why the assumptions change? All I'm proposing is walkng >> the linear map to find compatible PTEs and collapsing them into the biggest >> possible blocks. The pages aren't being freed, they are just being mapped >> differently (which can be done live for BBML2_NOABORT). PTEs with different >> pkeys would be considered incompatible, so we would end up with a boundary in >> the leaf mappings at that point. > > I'm not sure I'm following, if all entries in a PTE page are compatible, > then surely we just convert the parent PMD entry to become a leaf and > then free the PTE page? And same idea one level above. Ahh - good point! That totally passed me by before. But I'm not sure it's the end of the world... We would end up with about 0.2% (4K/2M if I've done my maths correctly?) of the linear map sub-optimally mapped. Personally I don't think that would be the end of the world. > >> >>> It could be handled by poking >>> holes in the tracked ranges, but it gets ugly and increases fragmentation. >> You'd still want page tables to be allocated from contiguous physical (and >> virtual) memory so that the boundaries where pkeys change are minimized. > > Yes that's for sure, that's why I'm concerned with individual pages > being freed in a middle of a block. > >> I guess I've misunderstood something... > > I might have too :/ > > - Kevin