Re: [patch][rfc] mm: hold page lock over page_mkwrite

[Jamie Lokier <jamie@shareable.org>]

Nick Piggin wrote:
> > Suppose the systems has two pages to be written.  The first must
> > _reserve_ 40 pages of scratch space just in case the operation will
> > need them.  If the second page write is initiated concurrently with
> > the first, the second must reserve another 40 pages concurrently.
> > 
> > If 10 page writes are concurrent, that's 400 pages of scratch space
> > needed in reserve...
> 
> You only need to guarantee forward progress, so you would reserve
> 40 pages up front for the entire machine (some mempools have more
> memory than strictly needed to improve performance, so you could
> toy with that, but let's just describe the baseline).
> 
> So allocations happen as normal, except when an allocation fails,
> then the task which fails the allocation is given access to this
> reserve memory, any other task requiring reserve will then block.
> 
> Now the reserve provides enough pages to guarantee forward progress,
> so that one task is going to be able to proceed and eventually its
> pages will become freeable and can be returned to the reserve. Once
> the writeout has finished, the reserve will become available to
> other tasks.
> 
> So this way you only have to reserve enough to write out 1 page,
> and you only start blocking things when their memory allocations
> wolud have failed *anyway*. And you guarantee forward progress.

Ah.  *light bulb*  In the writing tasks, memory allocation is
forbidden, but blocking is allowed.  That's what I was missing :-)

Forward progress marches on.

-- Jamie

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