RE: [PATCH] Avoiding mmap fragmentation - clean rev

Wolfgang Wander
2005-05-20T12:39:14Z

Chen, Kenneth W writes:
> While working on porting the munmap free area coalescing patch on top of
> 2.6.12-rc4-mm2 Kernel, this change from wolfgang looked very strange:
>
> > @@ -1209,8 +1218,14 @@ void arch_unmap_area(struct vm_area_stru
> > * Is this a new hole at the lowest possible address?
> > */
> > if (area->vm_start >= TASK_UNMAPPED_BASE &&
> > - area->vm_start < area->vm_mm->free_area_cache)
> > - area->vm_mm->free_area_cache = area->vm_start;
> > + area->vm_start < area->vm_mm->free_area_cache) {
> > + unsigned area_size = area->vm_end-area->vm_start;
> > +
> > + if (area->vm_mm->cached_hole_size < area_size)
> > + area->vm_mm->cached_hole_size = area_size;
> > + else
> > + area->vm_mm->cached_hole_size = ~0UL;
> > + }
> > }
>
>
> First, free_area_cache won't get moved on munmap. OK fine. Secondly,
> if area that we just unmapped is smaller than cached_hole_size, instead
> of doing nothing (the condition of largest know hole size below current
> cache pointer still holds at this time), the new code will reset hole
> size to ~0UL, which will trigger a full scan next time for any mmap
> request.
>
> Wolfgang, did you tweak this area? Or this is just a simple typo or
> something? AFAWICS, this patch will trigger a lot more innocent full scan
> than what people claim it is.

Thanks for checking Ken. I believe that this logic becomes mostly
obsolete with your munmap patch anyhow.

You are perfectly right that the reset to ~0UL is not strictly
required however since the munmapped area can be joined with
neightborings hole to form something larger (which size *I* cannot
determine at this time) I wanted to be better safe and restart any
search request from base.

If the unmapped area sits between base and free_area_cache we
can then increase the cached_hole_size to the area_size if it
is indeed larger than the current cached_hole_size.

In both cases it would be nice to just calculate the real cached
hole size with some vma_find calls instead...

Wolfgang