On Mon, Sep 21, 2026 at 10:11:04PM +0100, Muhammad Usama Anjum wrote: > On 21/09/2026 11:55 am, Yeoreum Yun wrote: > > Using ptep_get() and its counterparts in common code is suboptimal on > > kernel configurations with generic compile-time folded page tables. > > By default, ptep_get() and its friends expands to READ_ONCE(), > > forcing the compiler to emit a load even when the value is not used > > afterwards. > > > > This issue was recently reported by Christophe Leroy [1] for ppc32 > > preventing futher code conversion to ptep_get()/pmdp_get()/... helper > > and the same behavior can also be observed on arm64 when built with > > 2- or 3-level page tables > > > > e.g) perf_get_page_size() in arm64 with CONFIG_PGTABLE_LEVEL=3: > > > > 00000000000052a0 <perf_get_page_size>: > > ... > > 52dc: d53b4234 mrs x20, DAIF > > 52e0: d50343df msr DAIFSet, #0x3 > > ... > > 52fc: d35e9a69 ubfx x9, x19, #30, #9 /* > > pud_offset_lockless() */ > > 5300: f9403508 ldr x8, [x8, #0x68] > > 5304: f869790a ldr x10, [x8, x9, lsl #3] /* pudp_get() */ > > 5308: f90007ea str x10, [sp, #0x8] > > 530c: f8697908 ldr x8, [x8, x9, lsl #3] /* pudp_get() */ > > ... > > 5360: 90000009 adrp x9, 0x5000 <perf_prepare_sample+0x548> > > 5364: 92746908 and x8, x8, #0x7ffffff000 > > 5368: d3557675 ubfx x21, x19, #21, #9 /* > > pmd_offset_lockless() */ > > ... > > 5394: f8757ac8 ldr x8, [x22, x21, lsl #3] /* pmdp_get() */ > > > > Though PGTABLE_LEVEL=3, since the pudp_get() still remain with > > READ_ONCE(), there's redundant load for the pud which is folded. > > > > To prevent generating suboptimal code, make pXdp_get() return a dummy > > entry for compile-time folded page tables, make the helpers such as > > pXd_offset()/pXd_offset_lockless(), set_pXd() validate dummy entries > > at compile time to catch the wrong usage and prohibit calls to > > pXd_page() in pgtable-nopXd.h. > > > > This series does not change the behaviour of existing code that directly > > manipulates folded page-table levels using set_pgd(), pgd_page_vaddr(), and > > related helpers. Those helpers continue to behave as before. > > > > The new restrictions only apply to code that adopts the pXdp_get()-based > > access model for compile-time folded page tables. > > > > As the pXdp_get() can return *dummy* entry, some of code using > > the stack value where saves the pXdp_get() could be a problematic: > > > > 1. Passing address of stack value where saves the pXdp_get() result > > to pXd_offset() for example: > > > > pud_t *pudp, pud; > > pmd_t *pmdp; > > > > pud = pudp_get(pudp, address); > > pmdp = pmd_offset(&pud, pud, address); > > > > (e.g. host_pfn_mapping_level() in loongarch). > > > > 2. Using the pXdp_get() result to use as argument of pXd_val() and > > to check prot without checking pgtable is folded. > > for example, x86's effective_prot(). > > > > 3. Using set_pXd() with pXdp_get() will set problematic dummy entry > > in folded page table like: > > > > set_pXd(pxdp, pXdp_get(pxdp_k)); > > > > 4. Using pgd_page_vaddr() to get the first-level pgtable. > > passing dummy pxdp_get() for pgd_page_vaddr() will return wrong > > address. Therefore, make pgd_page_vaddr() and pXd_pgtable() to > > trigger the error for improper usage with folded dummy entry in the > > generic compile-time folded pgtable. > > > > Thanksfully, above cases are rare since (1) most of usage using > > pXd_offset() with result of upper pXd_offset(), (2) it's extreamely > > rare to use pXd_val() for non-leaf entry in the kernel, > > (3) is to handle the vmalloc_fault or set the first level of page table > > and (4) to setup early page table and etc. > > > > Therefore, properly handle this uncommon and problematic pattern, and > > document the current design of compile-time folded page tables. > > > > This patch is based on mm-unstable. > > > > Future work > > =========== > > - print_bad_page_map() and show_pte() still prints dummy values > > instead of printing the same content for all generic compile-time > > folded page tables. We might want to skip printing dummy values later. > > > > - We currently catch abuse of dummy values on the stack at compile-time by > > relying on constant propagation by the compiler. Usama's work [3] on > > using > > distinct types for sw vs. hw PTEs could help here as well." > > > > - Clean up vmalloc fault handling by synchronizing the vmalloc entry on > > 32-bit architectures. This code is almost identical across architectures. > > > > - Unfortunately, the current design of compile-time folded page tables > > appears > > to be internally consistent but confusing. For example, > > when CONFIG_PGTABLE_LEVELS is 2, p4d, pud, and pmd are expected to > > be folded into pgd. However, the architecture code uses set_pmd() > > to update the top-level page-table entry, even though it includes > > pgtable-nopmd.h. > > > > In the future, it would be good to eliminate this source of confusion, > > possibly by treating all folded upper levels consistently as dummy > > wrappers > > around the highest real page-table level: > > > > NOPGD > > --> +------+ P4D > > | ptr0 |-------> +------+ PUD > > +------+ | ptr0 |-------> +-----+ > > | ptr1 |- | ptr | -------> ... > > | ptr2 | \ | ptr | > > | ptr3 | \ ... > > ... \ > > \ PUD > > +----> +-----+ > > | ptr | -------> ... > > | ptr | > > ... > > > > Link: [1] > > https://lore.kernel.org/all/[email protected]/ > > Link: [2] > > https://lore.kernel.org/all/[email protected]/ > > Link: [3] > > https://lore.kernel.org/r/[email protected] > > I applied all 21 patches to the declared base and built and booted the > kernels before and after on x86_64 and arm64 using virtme-ng. > > Tested-by: Muhammad Usama Anjum <[email protected]>
Thanks for your testing ;) [...] -- Sincerely, Yeoreum Yun
