On Fri, Feb 27, 2026 at 4:14 AM Usama Arif <[email protected]> wrote: > > > > On 26/02/2026 21:01, Nico Pache wrote: > > On Thu, Feb 26, 2026 at 4:33 AM Usama Arif <[email protected]> wrote: > >> > >> When the kernel creates a PMD-level THP mapping for anonymous pages, it > >> pre-allocates a PTE page table via pgtable_trans_huge_deposit(). This > >> page table sits unused in a deposit list for the lifetime of the THP > >> mapping, only to be withdrawn when the PMD is split or zapped. Every > >> anonymous THP therefore wastes 4KB of memory unconditionally. On large > >> servers where hundreds of gigabytes of memory are mapped as THPs, this > >> adds up: roughly 200MB wasted per 100GB of THP memory. This memory > >> could otherwise satisfy other allocations, including the very PTE page > >> table allocations needed when splits eventually occur. > >> > >> This series removes the pre-deposit and allocates the PTE page table > >> lazily — only when a PMD split actually happens. Since a large number > >> of THPs are never split (they are zapped wholesale when processes exit or > >> munmap the full range), the allocation is avoided entirely in the common > >> case. > >> > >> The pre-deposit pattern exists because split_huge_pmd was designed as an > >> operation that must never fail: if the kernel decides to split, it needs > >> a PTE page table, so one is deposited in advance. But "must never fail" > >> is an unnecessarily strong requirement. A PMD split is typically triggered > >> by a partial operation on a sub-PMD range — partial munmap, partial > >> mprotect, partial mremap and so on. > >> Most of these operations already have well-defined error handling for > >> allocation failures (e.g., -ENOMEM, VM_FAULT_OOM). Allowing split to > >> fail and propagating the error through these existing paths is the natural > >> thing to do. Furthermore, split failing requires an order-0 allocation for > >> a page table to fail, which is extremely unlikely. > >> > >> Designing functions like split_huge_pmd as operations that cannot fail > >> has a subtle but real cost to code quality. It forces a pre-allocation > >> pattern - every THP creation path must deposit a page table, and every > >> split or zap path must withdraw one, creating a hidden coupling between > >> widely separated code paths. > >> > >> This also serves as a code cleanup. On every architecture except powerpc > >> with hash MMU, the deposit/withdraw machinery becomes dead code. The > >> series removes the generic implementations in pgtable-generic.c and the > >> s390/sparc overrides, replacing them with no-op stubs guarded by > >> arch_needs_pgtable_deposit(), which evaluates to false at compile time > >> on all non-powerpc architectures. > > > > Hi Usama, > > > > Thanks for tackling this, it seems like an interesting problem. Im > > trying to get more into reviewing, so bare with me I may have some > > stupid comments or questions. Where I can really help out is with > > testing. I will build this for all RH-supported architectures and run > > some automated test suites and performance metrics. I'll report back > > if I spot anything. > > > > Cheers! > > -- Nico > > > > Thanks for the build and looking into reviewing this. All comments > and questions are welcome! I had only tested on x86, and I had a look > at the link you shared so its great to know that powerPC and s390 are fine.
Good news: as you noted all the builds succeeded, and the sanity tests dont show any signs of an immediate issue across the architectures. I'll proceed to debug kernels, and then performance testing. I will try to start reviewing the actual code changes in depth next week :) Cheers, -- Nico >
