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SubjectRe: [PATCH v2 13/15] slab: use struct page for slab management
From
2013/10/18 Christoph Lameter <cl@linux.com>:
> On Wed, 16 Oct 2013, Joonsoo Kim wrote:
>
>> - * see PAGE_MAPPING_ANON below.
>> - */
>> + union {
>> + struct address_space *mapping; /* If low bit clear, points to
>> + * inode address_space, or NULL.
>> + * If page mapped as anonymous
>> + * memory, low bit is set, and
>> + * it points to anon_vma object:
>> + * see PAGE_MAPPING_ANON below.
>> + */
>> + void *s_mem; /* slab first object */
>> + };
>
> The overloading of mapping has caused problems in the past since slab
> pages are (or are they no longer?) used for DMA to disk. At that point the
> I/O subsystem may be expecting a mapping in the page struct if this field
> is not NULL.

I search the history of struct page and find that the SLUB use mapping field
in past (2007 year). At that time, you inserted VM_BUG_ON(PageSlab(page))
('b5fab14') into page_mapping() function to find remaining use. Recently,
I never hear that this is triggered and 6 years have passed since inserting
VM_BUG_ON(), so I guess there is no problem to use it.
If this argument is reasonable, please give me an ACK :)

Thanks.


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