Re: [PATCH v2] mm/page_alloc: do not boost watermarks in kdump capture kernels

[Johannes Weiner]

On Wed, Sep 16, 2026 at 07:25:45PM +0800, Yuanhe Shu wrote:
> A watermark boost is not confined to one watermark: wmark_pages() adds
> it to min, low and high alike, so every watermark check sees it,
> including should_reclaim_retry() and the last ditch ALLOC_WMARK_HIGH
> attempt in __alloc_pages_may_oom().  Once the boost exceeds the memory
> still free the allocator gives up and invokes the OOM killer, and a
> capture kernel that is still booting has nothing to kill: the boot
> panics and the vmcore is lost.
> 
> Seen on an arm64 machine with 64K pages, CONFIG_PAGE_BLOCK_MAX_ORDER=10
> (pageblock = 64M) and crashkernel=512M, running a distribution kernel
> based on 7.0.14.  A high order UNMOVABLE allocation fell back to a
> MOVABLE pageblock while the capture kernel was still in do_initcalls():
> 
>   Node 0 DMA free:68096kB boost:65536kB min:68160kB
>   low:68800kB high:69440kB managed:479168kB
>   Out of memory and no killable processes...
>   Kernel panic - not syncing: System is deadlocked on memory
> 
> The zone was not short of memory.  Subtracting the boost gives
> min:2624kB low:3264kB high:3904kB, so the 68096kB still free sat 17
> times above the high watermark and the allocator would not even have
> entered its slow path.  The boost supplied 65536kB of the 68160kB min
> and by itself put the zone 64kB under water.  It is that large because
> boost_watermark() clamps it with max(pageblock_nr_pages, max_boost);
> watermark_boost_factor alone would have allowed 5824kB.
> 
> Commit 14f69140ff9c ("mm: limit boost_watermark on small zones") already
> tried to protect capture kernels, but it infers them from the zone size
> and skips the boost only below four pageblocks.  arm64 64K pageblocks
> were 512M then, so the guard reached zones up to 2G;
> CONFIG_PAGE_BLOCK_MAX_ORDER can cap them at 64M, which shrinks the guard
> to zones under 256M and lets this 468M zone through.
> 
> kdump is a property of the kernel, not of the zone, so test for it
> directly.  A capture kernel exits within seconds and never uses the
> fragmentation avoidance the boost buys.  Normal kernels are unaffected:
> the size based check still covers their genuinely tiny zones.
> 
> Passing sysctl.vm.watermark_boost_factor=0 to the capture kernel does
> not cover this window: sysctl.* parameters are written through procfs
> by do_sysctl_args(), which runs after do_initcalls() where the panic
> above happened, and watermark_boost_factor has no early_param of its
> own.
> 
> Fixes: 1c30844d2dfe ("mm: reclaim small amounts of memory when an external fragmentation event occurs")
> Cc: stable@xxxxxxxxxxxxxxx # v5.0
> Cc: Henry Willard <henry.willard@xxxxxxxxxx>
> Cc: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>

Reviewed-by: Johannes Weiner <hannes@xxxxxxxxxxx>