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From:   Yu Zhao <yuzhao@google.com>
Date:   Wed, 25 Jan 2023 13:54:42 -0700
Message-ID: <CAOUHufashDpjnj=XxaR3jsAxPT6tOuv+Uv9ZuJ_8=vLS_HrDWw@mail.gmail.com>
Subject: Re: [PATCH v2] nvdimm: Support sizeof(struct page) > MAX_STRUCT_PAGE_SIZE
To:     Dan Williams <dan.j.williams@intel.com>
Cc:     nvdimm@lists.linux.dev, stable@vger.kernel.org,
        Alexander Potapenko <glider@google.com>,
        Marco Elver <elver@google.com>, Jeff Moyer <jmoyer@redhat.com>,
        linux-mm@kvack.org, kasan-dev@googlegroups.com,
        linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
        gregkh@linuxfoundation.org
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On Wed, Jan 25, 2023 at 1:23 PM Dan Williams <dan.j.williams@intel.com> 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@vger.kernel.org>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Marco Elver <elver@google.com>
> Reported-by: Jeff Moyer <jmoyer@redhat.com>

Thanks -- that BUILD_BUG_ON() has been a nuisance for some of my debug configs.

Acked-by: Yu Zhao <yuzhao@google.com>