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wrote: > On 29.10.25 19:02, Lorenzo Stoakes wrote: > > On Wed, Oct 29, 2025 at 05:19:54PM +0100, David Hildenbrand wrote: > > > > > > > Why is a tlb_remove_table_sync_one() needed in huge_pmd_unshare()? > > > > > > > > > > > > Because nothing else on that path is guaranteed to send any IPIs > > > > > > before the page table becomes reusable in another process. > > > > > > > > > > I feel that David's suggestion of just disallowing the use of shared page > > > > > tables like this (I mean really does it actually come up that much?) is the > > > > > right one then. > > > > > > > > Yeah, I also like that suggestion. > > > > > > I started hacking on this (only found a bit of time this week), and in > > > essence, we'll be using the mmu_gather when unsharing to collect the pages > > > and handle the TLB flushing etc. > > > > > > (TLB flushing in that hugetlb area is a mess) > > > > > > It almost looks like a cleanup. > > > > > > Having that said, it will take a bit longer to finish it and, of course, I > > > first have to test it then to see if it even works. > > > > > > But it looks doable. :) > > > > Ohhhh nice :) > > > > I look forward to it! > > As shared offline already, it looked simple, but there is one nasty corner > case: if we never reuse a shared page table, who will take care of unmapping > all pages? Right. That is nasty... :) > > I played with various ideas, but it just ended up looking more complicated > and possibly even slower. Yeah... > > So what I am currently looking into is simply reducing (batching) the number > of IPIs. As in the IPIs we are now generating in tlb_remove_table_sync_one()? Or something else? As this bug is only an issue when we don't use IPIs for pgtable freeing right (e.g. CONFIG_MMU_GATHER_RCU_TABLE_FREE is set), as otherwise tlb_remove_table_sync_one() is a no-op? > > In essence, we only have to send one IPI when unsharing multiple page > tables, and we only have to send one when we are the last one sharing the > page table (before it can get reused). Right, hopefully that significantly cuts down on the amount genrated. > > While at it, I'm looking into making also the TLB flushing easier to > understand here. Good idea ;) > > I'm hacking on a prototype and should likely have something to test this > week. Thanks! > > [I guess what I am doing now is aligned with Jann's initial ideas to > optimize this ] > > -- > Cheers > > David Cheers, Lorenzo