Jan Kara <jack@xxxxxxx> writes: > On Thu 21-02-19 19:11:14, Aneesh Kumar K.V wrote: >> On 2/21/19 5:42 PM, Jan Kara wrote: >> > Hi Aneesh, >> > >> > On Thu 21-02-19 12:52:39, Aneesh Kumar K.V wrote: >> > > We found this while testing dax with XFS, but i guess this is true for >> > > other file systems too. The stack trace looks as >> > > >> > > [c00000000007610c] set_pte_at+0x3c/0x190 >> > > LR [c000000000378628] insert_pfn+0x208/0x280 >> > > Call Trace: >> > > [c0000002125df980] [8000000000000104] 0x8000000000000104 (unreliable) >> > > [c0000002125df9c0] [c000000000378488] insert_pfn+0x68/0x280 >> > > [c0000002125dfa30] [c0000000004a5494] dax_iomap_pte_fault.isra.7+0x734/0xa40 >> > > [c0000002125dfb50] [c000000000627250] __xfs_filemap_fault+0x280/0x2d0 >> > > [c0000002125dfbb0] [c000000000373abc] do_wp_page+0x48c/0xa40 >> > > [c0000002125dfc00] [c000000000379170] __handle_mm_fault+0x8d0/0x1fd0 >> > > [c0000002125dfd00] [c00000000037a9b0] handle_mm_fault+0x140/0x250 >> > > [c0000002125dfd40] [c000000000074bb0] __do_page_fault+0x300/0xd60 >> > > [c0000002125dfe20] [c00000000000acf4] handle_page_fault+0x18 >> > > >> > > >> > > Now that is WARN_ON in set_pte_at which is >> > > >> > > VM_WARN_ON(pte_hw_valid(*ptep) && !pte_protnone(*ptep)); >> > > >> > > Multiple architecture optimize set_pte_at based on the assumption that >> > > we will never use set_pte_at to update a valid pte entry. This helps in >> > > avoid flushing tlb etc. We should be using ptep_set_access_flags for >> > > this. >> > >> > Hum, I didn't know about this assumption and neither did lot of other >> > people reviewing DAX patches. Is this documented somewhere? > > Any answer here? > Not in the form of kernel documentation. But we had related fixes which got applied earlier cee216a696b2004017a5ecb583366093d90b1568 56eecdb912b536a4fa97fb5bfe5a940a54d79be6 Which also imply we possibly want to document this properly. We use: For a RO -> RW: update ptep_set_access_flags >From an invalid pte to valid: set_pte_at For read/modify/write sequence we have: ptep_prot_modify_start/ptep_prot_modify_commit ptep_set_access_flags do carry a comment which explains it is used when relaxing the access. -aneesh
