Re: [PATCH] 2.2.17pre7 VM enhancement Re: I/O performance on

[Andrea Arcangeli]

On Thu, 29 Jun 2000, Stephen C. Tweedie wrote:

>"progress" does not mean freeing a page.  Simply finding a page which
>can be unmapped from a process's page tables is considered "progress".  

I agree, the current swap_out design is too much fragile.

btw, in such area we also have a subtle hack/magic: when we unmap a clean
page we consider that a "fail" ;), while instead we really did some kind
of progress. We need to consdier that a fail because we need more margin
(even if that can lead in unmapping way too much stuff) because such page
could be shared by more than one pte for example (I remeber I tried to
consider that a progress and the allocations started failing). Dirty ptes
in a MAP_SHARED segment instead always return success so they don't have
such magic hack (even if they could be mapped by more than one pte too)
and that makes them even more fragile... (of course unmapping an huge
clean vma and faulting in the cache later is not good but it can be
handled, while writing to disk at once an huge MAP_SHARED dirty vma would
probably kill the machine for some time ;)

Andrea