Re: [RFC PATCH 1/4] mm: introduce a safer interface to check whethera page is managed by SLxB

[Jiang Liu]

Hi Chris,
	Really appreciate your suggestions and I will work out a new version to
fix callers of PageSlab() instead of changing the slab allocator.
	Thanks!
	Gerry

On 07/06/2012 09:50 PM, Christoph Lameter wrote:
> On Fri, 6 Jul 2012, Jiang Liu wrote:
> 
>> 	Originally the patch is aimed to fix an issue encountered when
>> hot-removing a hot-added memory device. Currently memory hotplug is only
>> supported with SPARSE memory model. After offlining all pages of a memory
>> section, we need to free resources used by "struct mem_section" itself.
>> That is to free section_mem_map and pageblock_flags. For memory section
>> created at boot time, section_mem_map and pageblock_flags are allocated
>> from bootmem. For memory section created at runtime, section_mem_map
>> and pageblock_flags are allocated from slab. So when freeing these
>> resources, we use PageSlab() to tell whether there are allocated from slab.
>> So free_section_usemap() has following code snippet.
>> {
>>         usemap_page = virt_to_page(usemap);
>>         /*
>>          * Check to see if allocation came from hot-plug-add
>>          */
>>         if (PageSlab(usemap_page)) {
> 
> Change this to PageSlab(usemap_page) || PageCompound(usemap_page) and then
> the code segment will work. Fallback to the page allocator always implied
> the use of compound pages. It would be cleaner if memory hotplug had an
> indicator which allocation mechanism was used and would use the
> corresponding free action. Slab allocators could put multiple objects into
> the slab page (if the structures are sufficiently small). So this is not
> that good of a solution.
> 
> 
>> 	And when fixing this issue, we found some other usages of PageSlab() may
>> have the same issue. For example:
>> 	1) /proc/kpageflags and /proc/kpagecount may return incorrect result for
>> pages allocated by slab.
> 
> Ok then the compound page handling is broken in those.
> 
>> 	2) DRBD has following comments. At first glance, it seems that it's
>> 	dangerous if PageSlab() to return false for pages allocated by slab.
> 
> Again the pages that do not have PageSlab set were not allocated using a
> slab allocator. They were allocated by calls to the page allocator.
> 
>> 	(With more thinking, the comments is a little out of date because now
>> 	put_page/get_page already correctly handle compound pages, so it should
>> 	be OK to send pages allocated from slab.)
> 
> AFAICT they always handled compound pages correctly.
> 
>> 	3) show_mem() on ARM and unicore32 reports much less pages used by slab
>> 	if SLUB/SLOB is used instead of SLAB because SLUB/SLOB doesn't mark big
>> 	compound pages with PG_slab flag.
> 
> Right. That is because SLUB/SLOB lets the page allocator directly
> allocator large structures where it would not make sense to use the slab
> allocators. The main purpose of the slab allocators is to allocate
> objects in fractions of pages. This does not seem to be a use case for
> slab objects. Maybe it would be better to directly call the page allocator
> for your large structures?
> 
>> 	For example, if the memory backing a "struct resource" structure is
>> allocated from bootmem, __release_region() shouldn't free the memory into
>> slab allocator, otherwise it will trigger panic as below. This issue is
>> reproducible when hot-removing a memory device present at boot time on x86
>> platforms. On x86 platforms, e820_reserve_resources() allocates bootmem for
>> all physical memory resources present at boot time. Later when those memory
>> devices are hot-removed, __release_region() will try to free  memory from
>> bootmem into slab, so trigger the panic. And a proposed fix is:
> 
> Working out how a certain memory structure was allocated could be most
> easily done by setting a flag somewhere instead of checking the page flags
> of a page that may potentially include multiple slab objects.
> 


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