Re: [PATCH v2] nvdimm: Support sizeof(struct page) > MAX_STRUCT_PAGE_SIZE

[Alexander Potapenko]

On Wed, Jan 25, 2023 at 9:23 PM Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> Commit 6e9f05dc66f9 ("libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE")
>
> ...updated MAX_STRUCT_PAGE_SIZE to account for sizeof(struct page)
> potentially doubling in the case of CONFIG_KMSAN=y. Unfortunately this
> doubles the amount of capacity stolen from user addressable capacity for
> everyone, regardless of whether they are using the debug option. Revert
> that change, mandate that MAX_STRUCT_PAGE_SIZE never exceed 64, but
> allow for debug scenarios to proceed with creating debug sized page maps
> with a compile option to support debug scenarios.
>
> Note that this only applies to cases where the page map is permanent,
> i.e. stored in a reservation of the pmem itself ("--map=dev" in "ndctl
> create-namespace" terms). For the "--map=mem" case, since the allocation
> is ephemeral for the lifespan of the namespace, there are no explicit
> restriction. However, the implicit restriction, of having enough
> available "System RAM" to store the page map for the typically large
> pmem, still applies.
>
> Fixes: 6e9f05dc66f9 ("libnvdimm/pfn_dev: increase MAX_STRUCT_PAGE_SIZE")
> Cc: <stable@xxxxxxxxxxxxxxx>
> Cc: Alexander Potapenko <glider@xxxxxxxxxx>
> Cc: Marco Elver <elver@xxxxxxxxxx>
> Reported-by: Jeff Moyer <jmoyer@xxxxxxxxxx>
Acked-by: Alexander Potapenko <glider@xxxxxxxxxx>