On 03/07/2020 12:35 PM, Christophe Leroy wrote: > > > Le 07/03/2020 à 01:56, Anshuman Khandual a écrit : >> >> >> On 03/07/2020 06:04 AM, Qian Cai wrote: >>> >>> >>>> On Mar 6, 2020, at 7:03 PM, Anshuman Khandual <Anshuman.Khandual@xxxxxxx> wrote: >>>> >>>> Hmm, set_pte_at() function is not preferred here for these tests. The idea >>>> is to avoid or atleast minimize TLB/cache flushes triggered from these sort >>>> of 'static' tests. set_pte_at() is platform provided and could/might trigger >>>> these flushes or some other platform specific synchronization stuff. Just >>> >>> Why is that important for this debugging option? >> >> Primarily reason is to avoid TLB/cache flush instructions on the system >> during these tests that only involve transforming different page table >> level entries through helpers. Unless really necessary, why should it >> emit any TLB/cache flush instructions ? > > What's the problem with thoses flushes ? > >> >>> >>>> wondering is there specific reason with respect to the soft lock up problem >>>> making it necessary to use set_pte_at() rather than a simple WRITE_ONCE() ? >>> >>> Looks at the s390 version of set_pte_at(), it has this comment, >>> vmaddr); >>> >>> /* >>> * Certain architectures need to do special things when PTEs >>> * within a page table are directly modified. Thus, the following >>> * hook is made available. >>> */ >>> >>> I can only guess that powerpc could be the same here. >> >> This comment is present in multiple platforms while defining set_pte_at(). >> Is not 'barrier()' here alone good enough ? Else what exactly set_pte_at() >> does as compared to WRITE_ONCE() that avoids the soft lock up, just trying >> to understand. >> > > > Argh ! I didn't realise that you were writing directly into the page tables. When it works, that's only by chance I guess. > > To properly set the page table entries, set_pte_at() has to be used: > - On powerpc 8xx, with 16k pages, the page table entry must be copied four times. set_pte_at() does it, WRITE_ONCE() doesn't. > - On powerpc book3s/32 (hash MMU), the flag _PAGE_HASHPTE must be preserved among writes. set_pte_at() preserves it, WRITE_ONCE() doesn't. > > set_pte_at() also does a few other mandatory things, like calling pte_mkpte() > > So, the WRITE_ONCE() must definitely become a set_pte_at() Sure, will do. These are part of the clear tests that populates a given entry with a non zero value before clearing and testing it with pxx_none(). In that context, WRITE_ONCE() seemed sufficient. But pte_clear() might be closely tied with proper page table entry update and hence a preceding set_pte_at() will be better. There are still more WRITE_ONCE() for other page table levels during these clear tests. set_pmd_at() and set_pud_at() are defined on platforms that support (and enable) THP and PUD based THP respectively. Hence they could not be used for clear tests as remaining helpers pmd_clear(), pud_clear(), p4d_clear() and pgd_clear() still need to be validated with or without THP support and enablement. We should just leave all other WRITE_ONCE() instances unchanged. Please correct me if I am missing something here. > > Christophe >
