Re: [PATCH v4 10/45] libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE

[Alexander Potapenko]

> > > Unfortunately, it can not be dynamically enabled because the size of
> > > 'struct page' is unfortunately recorded in the metadata of the device.
> > > Recall this is for supporting platform configurations where the capacity
> > > of the persistent memory exceeds or consumes too much of System RAM.
> > > Consider 4TB of PMEM consumes 64GB of space just for 'struct page'. So,
> > > NVDIMM subsystem has a mode to store that page array in a reservation on
> > > the PMEM device itself.
> >
> > Sorry, I might be missing something, but why cannot we have
> >
> > #ifdef CONFIG_KMSAN
> > #define MAX_STRUCT_PAGE_SIZE 128
> > #else
> > #define MAX_STRUCT_PAGE_SIZE 64
> > #endif
> >
>
> Possibly because this needs to be a fixed size on permanent storage
> (like an inode on a disk file system)
>

Ah, thank you, that makes sense.
Then I'm assuming some contents of struct pages are also stored in the
persistent memory. What happens if sizeof(struct page) stays below
MAX_STRUCT_PAGE_SIZE, but the layout changes?
E.g. doesn't it cause problems if the user enables/disables CONFIG_MEMCG?