On Fri, Aug 27, 2021 at 09:05:21PM +0800, Li, Zhijian wrote:
>
> on 2021/8/27 20:10, Jason Gunthorpe wrote:
> > On Fri, Aug 27, 2021 at 08:15:40AM +0000, lizhijian@xxxxxxxxxxx wrote:
> > > i looked over the change-log of hmm_vma_handle_pte(), and found that before
> > > 4055062 ("mm/hmm: add missing call to hmm_pte_need_fault in HMM_PFN_SPECIAL handling")
> > >
> > > hmm_vma_handle_pte() will not check pte_special(pte) if pte_devmap(pte) is true.
> > >
> > > when we reached
> > > "if (pte_special(pte) && !is_zero_pfn(pte_pfn(pte))) {"
> > > the pte have already presented and its pte's flag already fulfilled the request flags.
> > >
> > >
> > > My question is that
> > > Per https://01.org/blogs/dave/2020/linux-consumption-x86-page-table-bits,
> > > pte_devmap(pte) and pte_special(pte) could be both true in fsdax user case, right ?
> > How? what code creates that?
> >
> > I see:
> >
> > insert_pfn():
> > /* Ok, finally just insert the thing.. */
> > if (pfn_t_devmap(pfn))
> > entry = pte_mkdevmap(pfn_t_pte(pfn, prot));
> > else
> > entry = pte_mkspecial(pfn_t_pte(pfn, prot));
> >
> > So what code path ends up setting both bits?
>
> pte_mkdevmap() will set both _PAGE_SPECIAL | PAGE_DEVMAP
>
> 395 static inline pte_t pte_mkdevmap(pte_t pte)
> 396 {
> 397 return pte_set_flags(pte, _PAGE_SPECIAL|_PAGE_DEVMAP);
> 398 }
>
> below is a calltrace example
>
> [ 400.728559] Call Trace:
> [ 400.731595] dump_stack+0x6d/0x8b
> [ 400.735536] insert_pfn+0x16c/0x180
> [ 400.739596] __vm_insert_mixed+0x84/0xc0
> [ 400.744144] dax_iomap_pte_fault+0x845/0x870
> [ 400.749089] ext4_dax_huge_fault+0x171/0x1e0
> [ 400.754096] __do_fault+0x31/0xe0
> [ 400.758090] ? pmd_devmap_trans_unstable+0x37/0x90
> [ 400.763541] handle_mm_fault+0x11b1/0x1680
> [ 400.768260] exc_page_fault+0x2f4/0x570
> [ 400.772788] ? asm_exc_page_fault+0x8/0x30
> [ 400.777539] asm_exc_page_fault+0x1e/0x30
>
>
> So is my previous change reasonable ?
Yes, can you send a proper patch and include the mm mailing list?
Jason
