Re: [PATCH 3/5] lib: lockless generic and arch independent page table (gpt) v2.

[Jerome Glisse]

On Thu, Nov 06, 2014 at 05:32:00PM -0500, Rik van Riel wrote:
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> On 11/03/2014 03:42 PM, j.glisse@xxxxxxxxx wrote:
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 
> > Page table is a common structure format most notably use by cpu
> > mmu. The arch depend page table code has strong tie to the
> > architecture which makes it unsuitable to be use by other non arch
> > specific code.
> > 
> > This patch implement a generic and arch independent page table. It
> > is generic in the sense that entry size can be u64 or unsigned long
> > (or u32 too on 32bits arch).
> > 
> > It is lockless in the sense that at any point in time you can have
> > concurrent thread updating the page table (removing or changing
> > entry) and faulting in the page table (adding new entry). This is
> > achieve by enforcing each updater and each faulter to take a range
> > lock. There is no exclusion on range lock, ie several thread can
> > fault or update the same range concurrently and it is the
> > responsability of the user to synchronize update to the page table
> > entry (pte), update to the page table directory (pdp) is under gpt
> > responsability.
> > 
> > API usage pattern is : gpt_init()
> > 
> > gpt_lock_update(lock_range) // User can update pte for instance by
> > using atomic bit operation // allowing complete lockless update. 
> > gpt_unlock_update(lock_range)
> > 
> > gpt_lock_fault(lock_range) // User can fault in pte but he is
> > responsible for avoiding thread // to concurrently fault the same
> > pte and for properly accounting // the number of pte faulted in the
> > pdp structure. gpt_unlock_fault(lock_range) // The new faulted pte
> > will only be visible to others updaters only // once all concurrent
> > faulter on the address unlock.
> > 
> > Details on how the lockless concurrent updater and faulter works is
> > provided in the header file.
> > 
> > Changed since v1: - Switch to macro implementation instead of using
> > arithmetic to accomodate the various size for table entry
> > (uint64_t, unsigned long, ...). This is somewhat less flexbile but
> > right now there is no use for the extra flexibility v1 was
> > offering.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> 
> Never a fan of preprocessor magic, but I  see why it's needed.
> 
> Acked-by: Rik van Riel <riel@xxxxxxxxxx>

v1 is not using preprocessor but has a bigger gpt struct footprint and also
more complex calculation for page table walking due to the fact that i just
rely more on runtime computation than on compile time shift define through
preprocessor magic.

Given i am not a fan either of preprocessor magic if it makes you feel any
better i can resort to use v1, both have seen same kind of testing and both
are functionaly equivalent (API they expose is obviously slightly different).

I am not convince that what the computation i save using preprocessor will
show up in anyway as being bottleneck for hot path.

Cheers,
Jérôme

> 
> 
> - -- 
> All rights reversed
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