Re: THP handling with driver compound page on fault

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On Sat, Jan 30, 2021 at 01:28:44AM +0000, Matthew Wilcox wrote:
> On Fri, Jan 29, 2021 at 05:22:24PM -0800, Minchan Kim wrote:
> > Hi Mattew,
> > 
> > On Sat, Jan 30, 2021 at 01:00:02AM +0000, Matthew Wilcox wrote:
> > > On Fri, Jan 29, 2021 at 04:13:07PM -0800, Minchan Kim wrote:
> > > > A custom driver overrides (vm_operations_struct.fault) and map their
> > > > compound page(__GFP_COMP) to page table on userprocess on demand.
> > > 
> > > You're looking to backport:
> > > 
> > > commit d01ac3c35214ce362f50cada37cb7bab8c801896
> > > Author: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
> > > Date:   Thu Oct 15 20:05:26 2020 -0700
> > > 
> > >     mm/memory: remove page fault assumption of compound page size
> > > 
> > 
> > I guess you meant the check below.
> > 
> > 	if (compound_order(page) != HPAGE_PMD_ORDER)
> > 
> > What happens if driver allcoated HPAGE_PMD_ORDER size?
> 
> ... then it should be fine to map it with a PMD entry?

I don't think so because following logic assumes it's THP page,
for example, page_add_file_rmap, count_vm_event(THP_FILE_MAPPED)
in do_set_pmd.

Isn't it bug?




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