lkml.org 
[lkml]   [2021]   [Feb]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
    /
    Date
    From
    SubjectRe: [PATCH V2 1/2] arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory
    On Tue, Feb 02, 2021 at 12:32:15PM +0000, Will Deacon wrote:
    > On Tue, Feb 02, 2021 at 09:41:53AM +0530, Anshuman Khandual wrote:
    > > pfn_valid() validates a pfn but basically it checks for a valid struct page
    > > backing for that pfn. It should always return positive for memory ranges
    > > backed with struct page mapping. But currently pfn_valid() fails for all
    > > ZONE_DEVICE based memory types even though they have struct page mapping.
    > >
    > > pfn_valid() asserts that there is a memblock entry for a given pfn without
    > > MEMBLOCK_NOMAP flag being set. The problem with ZONE_DEVICE based memory is
    > > that they do not have memblock entries. Hence memblock_is_map_memory() will
    > > invariably fail via memblock_search() for a ZONE_DEVICE based address. This
    > > eventually fails pfn_valid() which is wrong. memblock_is_map_memory() needs
    > > to be skipped for such memory ranges. As ZONE_DEVICE memory gets hotplugged
    > > into the system via memremap_pages() called from a driver, their respective
    > > memory sections will not have SECTION_IS_EARLY set.
    > >
    > > Normal hotplug memory will never have MEMBLOCK_NOMAP set in their memblock
    > > regions. Because the flag MEMBLOCK_NOMAP was specifically designed and set
    > > for firmware reserved memory regions. memblock_is_map_memory() can just be
    > > skipped as its always going to be positive and that will be an optimization
    > > for the normal hotplug memory. Like ZONE_DEVICE based memory, all normal
    > > hotplugged memory too will not have SECTION_IS_EARLY set for their sections
    > >
    > > Skipping memblock_is_map_memory() for all non early memory sections would
    > > fix pfn_valid() problem for ZONE_DEVICE based memory and also improve its
    > > performance for normal hotplug memory as well.
    >
    > Hmm. Although I follow your logic, this does seem to rely on an awful lot of
    > assumptions to continue to hold true as the kernel evolves. In particular,
    > how do we ensure that early sections are always fully backed with

    Sorry, typo here: ^^^ should be *non-early* sections.

    Will

    \
     
     \ /
      Last update: 2021-02-02 13:38    [W:3.441 / U:0.012 seconds]
    ©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site