linux-kernel - Re: [PATCH v2 2/5] mm/page_alloc: place pages to tail in __putback_isolated

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Message-ID: <ddeba755-eed5-d412-ffa0-4d1a6a4bc297@suse.cz>
Date:   Tue, 20 Oct 2020 19:18:00 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-hyperv@...r.kernel.org,
        xen-devel@...ts.xenproject.org, linux-acpi@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Oscar Salvador <osalvador@...e.de>,
        Wei Yang <richard.weiyang@...ux.alibaba.com>,
        Pankaj Gupta <pankaj.gupta.linux@...il.com>,
        Michal Hocko <mhocko@...e.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Michal Hocko <mhocko@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Mike Rapoport <rppt@...nel.org>,
        Scott Cheloha <cheloha@...ux.ibm.com>,
        Michael Ellerman <mpe@...erman.id.au>
Subject: Re: [PATCH v2 2/5] mm/page_alloc: place pages to tail in
 __putback_isolated_page()

On 10/5/20 2:15 PM, David Hildenbrand wrote:
> __putback_isolated_page() already documents that pages will be placed to
> the tail of the freelist - this is, however, not the case for
> "order >= MAX_ORDER - 2" (see buddy_merge_likely()) - which should be
> the case for all existing users.
> 
> This change affects two users:
> - free page reporting
> - page isolation, when undoing the isolation (including memory onlining).
> 
> This behavior is desireable for pages that haven't really been touched
> lately, so exactly the two users that don't actually read/write page
> content, but rather move untouched pages.
> 
> The new behavior is especially desirable for memory onlining, where we
> allow allocation of newly onlined pages via undo_isolate_page_range()
> in online_pages(). Right now, we always place them to the head of the
> freelist, resulting in undesireable behavior: Assume we add
> individual memory chunks via add_memory() and online them right away to
> the NORMAL zone. We create a dependency chain of unmovable allocations
> e.g., via the memmap. The memmap of the next chunk will be placed onto
> previous chunks - if the last block cannot get offlined+removed, all
> dependent ones cannot get offlined+removed. While this can already be
> observed with individual DIMMs, it's more of an issue for virtio-mem
> (and I suspect also ppc DLPAR).
> 
> Document that this should only be used for optimizations, and no code
> should rely on this behavior for correction (if the order of the
> freelists ever changes).
> 
> We won't care about page shuffling: memory onlining already properly
> shuffles after onlining. free page reporting doesn't care about
> physically contiguous ranges, and there are already cases where page
> isolation will simply move (physically close) free pages to (currently)
> the head of the freelists via move_freepages_block() instead of
> shuffling. If this becomes ever relevant, we should shuffle the whole
> zone when undoing isolation of larger ranges, and after
> free_contig_range().
> 
> Reviewed-by: Alexander Duyck <alexander.h.duyck@...ux.intel.com>
> Reviewed-by: Oscar Salvador <osalvador@...e.de>
> Reviewed-by: Wei Yang <richard.weiyang@...ux.alibaba.com>
> Reviewed-by: Pankaj Gupta <pankaj.gupta.linux@...il.com>
> Acked-by: Michal Hocko <mhocko@...e.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>