Hi: On 2021/2/4 20:36, Matthew Wilcox wrote: > On Wed, Feb 03, 2021 at 03:41:37AM -0500, Miaohe Lin wrote: >> The developer will have trouble figuring out why the BUG actually triggered >> when there is a complex expression in the VM_BUG_ON. Because we can only >> identify the condition triggered BUG via line number provided by VM_BUG_ON. >> Optimize this by spliting such a complex expression into two simple >> conditions. > >> pmd_t pmd; >> VM_BUG_ON(address & ~HPAGE_PMD_MASK); >> - VM_BUG_ON(!pmd_present(*pmdp) || (!pmd_trans_huge(*pmdp) && >> - !pmd_devmap(*pmdp))); >> + VM_BUG_ON(!pmd_present(*pmdp)); >> + /* Below assumes pmd_present() is true */ >> + VM_BUG_ON(!pmd_trans_huge(*pmdp) && !pmd_devmap(*pmdp)); > > This is not a complex condition. We're in the huge PMD handling case > and we're looking at a PMD which either isn't present or isn't huge. > It might be useful to print out the PMD in such a case, but splitting > this into the two cases of pmd-not-present and pmd-isn't-huge isn't > particularly useful. >
Many thanks for your time. You are right that it would be more helpful if we had a VM_BUG_ON_PMD() that we could print the pmd's value and permit diagnosis from that. I think splitting this into the two cases is the best we can do now while lacking of such helper. > I think you know that, or you wouldn't feel the need to put in a > comment explaining it! > . > Thanks again.
