linux-kernel - Re: [PATCH] [RFC] init/main: fix broken buffer_init when DEFERRED_STRUCT_PAGE

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Date:   Wed, 25 Nov 2020 16:20:16 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Lin Feng <linf@...gsu.com>, linux-mm@...ck.org
Cc:     linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net
Subject: Re: [PATCH] [RFC] init/main: fix broken buffer_init when
 DEFERRED_STRUCT_PAGE_INIT set

On 11/23/20 12:05 PM, Lin Feng wrote:
> In the booting phase if CONFIG_DEFERRED_STRUCT_PAGE_INIT is set,
> we have following callchain:
> 
> start_kernel
> ...
>    mm_init
>      mem_init
>       memblock_free_all
>         reset_all_zones_managed_pages
>         free_low_memory_core_early
> ...
>    buffer_init
>      nr_free_buffer_pages
>        zone->managed_pages
> ...
>    rest_init
>      kernel_init
>        kernel_init_freeable
>          page_alloc_init_late
>            kthread_run(deferred_init_memmap, NODE_DATA(nid), "pgdatinit%d", nid);
>            wait_for_completion(&pgdat_init_all_done_comp);
>            ...
>            files_maxfiles_init
> 
> It's clear that buffer_init depends on zone->managed_pages, but it's reset
> in reset_all_zones_managed_pages after that pages are readded into
>   zone->managed_pages, but when buffer_init runs this process is half done
>   and most of them will finally be added till deferred_init_memmap done.
> In large memory couting of nr_free_buffer_pages drifts too much, also
> drifting from kernels to kernels on same hardware.
> 
> Fix is simple, it delays buffer_init run till deferred_init_memmap all done.

Hmm nobody should need bh_cachep to allocate buffer heads until then, right.

> But as corrected by this patch, max_buffer_heads becomes very large,
> the value is roughly as many as 4 times of totalram_pages, formula:
> max_buffer_heads = nrpages * (10%) * (PAGE_SIZE / sizeof(struct buffer_head));
> 
> Say in a 64GB memory box we have 16777216 pages, then max_buffer_heads
> turns out to be roughly 67,108,864.
> In common cases, should a buffer_head be mapped to one page/block(4KB)?
> So max_buffer_heads never exceeds totalram_pages.
> IMO it's likely to make buffer_heads_over_limit bool value alwasy false,
> then make codes 'if (buffer_heads_over_limit)' test in vmscan unnecessary.
> Correct me if it's not true.

Maybe we could compile that out with CONFIG_HIGHMEM then?

> So this patch will change the original behavior related to
> buffer_heads_over_limit in vmscan since we used a half done value
> of zone->managed_pages before, or should we use a smaller factor(<10%) in
> previous formula.
> 
> Signed-off-by: Lin Feng <linf@...gsu.com>

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

> ---
>   init/main.c     | 2 --
>   mm/page_alloc.c | 3 +++
>   2 files changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/init/main.c b/init/main.c
> index 20baced721ad..a3f7c3416286 100644
> --- a/init/main.c
> +++ b/init/main.c
> @@ -58,7 +58,6 @@
>   #include <linux/rmap.h>
>   #include <linux/mempolicy.h>
>   #include <linux/key.h>
> -#include <linux/buffer_head.h>
>   #include <linux/page_ext.h>
>   #include <linux/debug_locks.h>
>   #include <linux/debugobjects.h>
> @@ -1034,7 +1033,6 @@ asmlinkage __visible void __init __no_sanitize_address start_kernel(void)
>   	fork_init();
>   	proc_caches_init();
>   	uts_ns_init();
> -	buffer_init();
>   	key_init();
>   	security_init();
>   	dbg_late_init();
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index eaa227a479e4..2931d706fb52 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -70,6 +70,7 @@
>   #include <linux/psi.h>
>   #include <linux/padata.h>
>   #include <linux/khugepaged.h>
> +#include <linux/buffer_head.h>
>   
>   #include <asm/sections.h>
>   #include <asm/tlbflush.h>
> @@ -2103,6 +2104,8 @@ void __init page_alloc_init_late(void)
>   	files_maxfiles_init();
>   #endif
>   
> +	buffer_init();
> +
>   	/* Discard memblock private memory */
>   	memblock_discard();
>   
>