Re: [PATCHv14 5/9] efi: Add unaccepted memory support

[Mel Gorman <mgorman@techsingularity.net>]

On Tue, Jun 06, 2023 at 05:26:33PM +0300, Kirill A. Shutemov wrote:
> efi_config_parse_tables() reserves memory that holds unaccepted memory
> configuration table so it won't be reused by page allocator.
> 
> Core-mm requires few helpers to support unaccepted memory:
> 
>  - accept_memory() checks the range of addresses against the bitmap and
>    accept memory if needed.
> 
>  - range_contains_unaccepted_memory() checks if anything within the
>    range requires acceptance.
> 
> Architectural code has to provide efi_get_unaccepted_table() that
> returns pointer to the unaccepted memory configuration table.
> 
> arch_accept_memory() handles arch-specific part of memory acceptance.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
> Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>

By and large, this looks ok from the page allocator perspective as the
checks for unaccepted are mostly after watermark checks. However, if you
look in the initial fast path, you'll see this

        /* 
         * Forbid the first pass from falling back to types that fragment
         * memory until all local zones are considered.
         */     
        alloc_flags |= alloc_flags_nofragment(ac.preferred_zoneref->zone, gfp);

While checking watermarks should be fine from a functional perspective and
the fast paths are unaffected, there is a risk of premature fragmentation
until all memory has been accepted. Meeting watermarks does not necessarily
mean that fragmentation is avoided as pageblocks can get mixed while still
meeting watermarks. This will be tricky to detect in most cases so it may
be worth considering augmenting the watermark checks with a check in this
block for unaccepted memory

        /*
         * It's possible on a UMA machine to get through all zones that are
         * fragmented. If avoiding fragmentation, reset and try again.  */
        if (no_fallback) {
                alloc_flags &= ~ALLOC_NOFRAGMENT;
		goto retry;
	}

I think the watermark checks will still be needed because hypothetically
if 100% of allocations were MIGRATE_UNMOVABLE then there never would be
a request that fragments memory and memory would not be accepted.

It's not a blocker to the series, simply a suggestion for follow-on
work.

-- 
Mel Gorman
SUSE Labs