On 11/14/2017 07:44 PM, Matthew Wilcox wrote: > On Mon, Nov 13, 2017 at 02:46:25PM -0800, Dave Hansen wrote: >> On 11/13/2017 02:20 PM, Dave Hansen wrote: >>> On 11/09/2017 05:09 PM, Tycho Andersen wrote: >>>> which I guess is from the additional flags in grow_dev_page() somewhere >>>> down >>>> the stack. Anyway... it seems this is a kernel allocation that's using >>>> MIGRATE_MOVABLE, so perhaps we need some more fine tuned heuristic than >>>> just >>>> all MOVABLE allocations are un-mapped via xpfo, and all the others are >>>> mapped. >>>> >>>> Do you have any ideas? >>> >>> It still has to do a kmap() or kmap_atomic() to be able to access it. I >>> thought you hooked into that. Why isn't that path getting hit for these? >> >> Oh, this looks to be accessing data mapped by a buffer_head. It >> (rudely) accesses data via: >> >> void set_bh_page(struct buffer_head *bh, >> ... >> bh->b_data = page_address(page) + offset; > > We don't need to kmap in order to access MOVABLE allocations. kmap is > only needed for HIGHMEM allocations. So there's nothing wrong with ext4 > or set_bh_page().
Yeah, it's definitely not _buggy_. Although, I do wonder what we should do about these for XPFO. Should we just stick a kmap() in there and comment it? What we really need is a mechanism to say "use this as a kernel page" and "stop using this as a kernel page". kmap() does that... kinda. It's not a perfect fit, but it's pretty close.
