Thank you for your contribution! Sashiko AI review found 3 potential issue(s) to consider: - [High] Global TLB flush broadcast inside global page table lock causing severe lock contention. - [High] PMD fault materialization failure incorrectly returns VM_FAULT_OOM instead of falling back to PTE. - [High] __vmemmap_materialize_page dereferences a potentially NULL PTE pointer if the DAX device is concurrently hot-unplugged. --
commit 9e2a623911317e382af87ef795b2fc34f1686554 Author: Muchun Song <[email protected]> fsdax: materialize pmem vmemmap metadata on faults This patch opts pmem FS-DAX into the shared read-only vmemmap mode and materializes private metadata before inserting a PFN into a userspace mapping. It shifts private vmemmap allocation from device registration to the first DAX fault for each metadata page. [ ... ] > diff --git a/fs/dax.c b/fs/dax.c > index 1fbba0d21c13d..89377301d51b0 100644 > --- a/fs/dax.c > +++ b/fs/dax.c [ ... ] > @@ -1877,6 +1878,10 @@ static vm_fault_t dax_fault_iter(struct vm_fault *vmf, > if (err) > return pmd ? VM_FAULT_FALLBACK : dax_fault_return(err); > > + err = vmemmap_materialize_page(pfn_to_page(pfn), pmd ? PMD_ORDER : 0); [Severity: High] Is there a risk of severe lock contention and RCU stalls here? When a PMD fault triggers vmemmap_materialize_page(), it loops 512 times. Inside that loop, __vmemmap_materialize_page() acquires the global init_mm.page_table_lock and performs a synchronous TLB flush: mm/sparse-vmemmap.c:__vmemmap_materialize_page() { ... set_pte_at(&init_mm, addr, pte, mk_pte(candidate, PAGE_KERNEL)); flush_tlb_kernel_range(addr, addr + PAGE_SIZE); put_page(template); out: spin_unlock(&init_mm.page_table_lock); } Could this cause the system to become unusable under load due to sequential global IPI broadcasts while holding the global kernel page table lock? [Severity: High] Is there a race condition with DAX device hot-unplug here? The prior call to dax_iomap_direct_access() acquires and then releases the DAX SRCU read lock. By the time we call vmemmap_materialize_page(), there is no lock preventing the DAX device from being hot-unplugged, which tears down the vmemmap page tables. If that happens concurrently, __vmemmap_materialize_page() will find pmd_none(*pmd) is true in virt_to_kpte(), resulting in a NULL pte pointer. Will the subsequent ptep_get(pte) dereference this NULL pointer and panic the kernel? > + if (err) > + return dax_fault_return(err); [Severity: High] Does returning dax_fault_return(err) on a -ENOMEM error incorrectly trigger an OOM kill for PMD faults? dax_fault_return(err) translates -ENOMEM to VM_FAULT_OOM. If this allocation fails during a PMD fault, shouldn't it return VM_FAULT_FALLBACK instead, allowing the kernel to gracefully split the huge page and retry at the PTE level (similar to how dax_iomap_direct_access() handles PMD fallback just above this)? > + > *entry = dax_insert_entry(xas, vmf, iter, *entry, pfn, entry_flags); [ ... ] -- Sashiko AI review ยท https://sashiko.dev/#/patchset/[email protected]?part=4
