Re: [PATCH v7] mm: Add PM_THP_MAPPED to /proc/pid/pagemap

[David Hildenbrand]

On 23.11.21 01:01, Mina Almasry wrote:
> Add PM_THP_MAPPED MAPPING to allow userspace to detect whether a given virt
> address is currently mapped by a transparent huge page or not.  Example
> use case is a process requesting THPs from the kernel (via a huge tmpfs
> mount for example), for a performance critical region of memory.  The
> userspace may want to query whether the kernel is actually backing this
> memory by hugepages or not.
> 
> PM_THP_MAPPED bit is set if the virt address is mapped at the PMD
> level and the underlying page is a transparent huge page.
> 
> A few options were considered:
> 1. Add /proc/pid/pageflags that exports the same info as
>    /proc/kpageflags.  This is not appropriate because many kpageflags are
>    inappropriate to expose to userspace processes.
> 2. Simply get this info from the existing /proc/pid/smaps interface.
>    There are a couple of issues with that:
>    1. /proc/pid/smaps output is human readable and unfriendly to
>       programatically parse.
>    2. /proc/pid/smaps is slow because it must read the whole memory range
>       rather than a small range we care about.  The cost of reading
>       /proc/pid/smaps into userspace buffers is about ~800us per call,
>       and this doesn't include parsing the output to get the information
>       you need. The cost of querying 1 virt address in /proc/pid/pagemaps
>       however is around 5-7us.
> 
> Tested manually by adding logging into transhuge-stress, and by
> allocating THP and querying the PM_THP_MAPPED flag at those
> virtual addresses.
> 
> Signed-off-by: Mina Almasry <almasrymina@xxxxxxxxxx>
> 
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Cc: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> Cc: David Rientjes rientjes@xxxxxxxxxx
> Cc: Paul E. McKenney <paulmckrcu@xxxxxx>
> Cc: Yu Zhao <yuzhao@xxxxxxxxxx>
> Cc: Jonathan Corbet <corbet@xxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Xu <peterx@xxxxxxxxxx>
> Cc: Ivan Teterevkov <ivan.teterevkov@xxxxxxxxxxx>
> Cc: Florian Schmidt <florian.schmidt@xxxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> 
> 
> ---
> 
> Changes in v7:
> - Added clarification that smaps is only slow because it looks at the
>   whole address space.
> 
> Changes in v6:
> - Renamed to PM_THP_MAPPED
> - Removed changes to transhuge-stress
> 
> Changes in v5:
> - Added justification for this interface in the commit message!
> 
> Changes in v4:
> - Removed unnecessary moving of flags variable declaration
> 
> Changes in v3:
> - Renamed PM_THP to PM_HUGE_THP_MAPPING
> - Fixed checks to set PM_HUGE_THP_MAPPING
> - Added PM_HUGE_THP_MAPPING docs
> ---
>  Documentation/admin-guide/mm/pagemap.rst | 3 ++-
>  fs/proc/task_mmu.c                       | 3 +++
>  2 files changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/admin-guide/mm/pagemap.rst b/Documentation/admin-guide/mm/pagemap.rst
> index fdc19fbc10839..8a0f0064ff336 100644
> --- a/Documentation/admin-guide/mm/pagemap.rst
> +++ b/Documentation/admin-guide/mm/pagemap.rst
> @@ -23,7 +23,8 @@ There are four components to pagemap:
>      * Bit  56    page exclusively mapped (since 4.2)
>      * Bit  57    pte is uffd-wp write-protected (since 5.13) (see
>        :ref:`Documentation/admin-guide/mm/userfaultfd.rst <userfaultfd>`)
> -    * Bits 57-60 zero
> +    * Bit  58    page is a huge (PMD size) THP mapping
> +    * Bits 59-60 zero
>      * Bit  61    page is file-page or shared-anon (since 3.5)
>      * Bit  62    page swapped
>      * Bit  63    page present
> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
> index ad667dbc96f5c..d784a97aa209a 100644
> --- a/fs/proc/task_mmu.c
> +++ b/fs/proc/task_mmu.c
> @@ -1302,6 +1302,7 @@ struct pagemapread {
>  #define PM_SOFT_DIRTY		BIT_ULL(55)
>  #define PM_MMAP_EXCLUSIVE	BIT_ULL(56)
>  #define PM_UFFD_WP		BIT_ULL(57)
> +#define PM_THP_MAPPED		BIT_ULL(58)
>  #define PM_FILE			BIT_ULL(61)
>  #define PM_SWAP			BIT_ULL(62)
>  #define PM_PRESENT		BIT_ULL(63)
> @@ -1456,6 +1457,8 @@ static int pagemap_pmd_range(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  
>  		if (page && page_mapcount(page) == 1)
>  			flags |= PM_MMAP_EXCLUSIVE;
> +		if (page && is_transparent_hugepage(page))
> +			flags |= PM_THP_MAPPED;
>  
>  		for (; addr != end; addr += PAGE_SIZE) {
>  			pagemap_entry_t pme = make_pme(frame, flags);
> 

Thanks!

Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb