COW page cache for file hole?

[Dave Young]

Hi,

During test fengguang's readahead alloc-noretry patch, I have some thoughts

In the 1000 dd case, page cache of sparse file hole are all zero indeed.
So what about make a global zero page for that purpose, fs level know
it is a hole,
when write occurs on that page we can alloc a new page for that.

I have no enough knowledge to implement it so just give out the
question without patch, sorry.
BTW, was there any attempt for this before?

-- 
Regards
dave
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