Re: [Lsf-pc] [LSF/MM TOPIC] Killing reliance on struct page->mapping

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On 2/1/18 8:57 AM, Jerome Glisse wrote:
> On Thu, Feb 01, 2018 at 08:34:58AM -0700, Jens Axboe wrote:
>> On 1/31/18 11:13 AM, Jerome Glisse wrote:
>>> That's one solution, another one is to have struct bio_vec store
>>> buffer_head pointer and not page pointer, from buffer_head you can
>>> find struct page and using buffer_head and struct page pointer you
>>> can walk the KSM rmap_item chain to find back the mapping. This
>>> would be needed on I/O error for pending writeback of a newly write
>>> protected page, so one can argue that the overhead of the chain lookup
>>> to find back the mapping against which to report IO error, is an
>>> acceptable cost.
>>
>> Ehm nope. bio_vec is a generic container for pages, requiring
>> buffer_heads to be able to do IO would be insanity.
> 
> The extra pointer dereference would be killing performance ?

No, I'm saying that requiring a buffer_head to be able to do IO
is insanity. That's how things used to be in the pre-2001 days.

-- 
Jens Axboe




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