On 9/2/26 13:56, 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.
It would be good to include what it looks like with this series. -- Cheers, David
