Re: [RFC][PATCH] release mmap_sem before starting migration (WasRe: Need to take mmap_sem lock in move_pages.

[Swamy Gowda]

KAMEZAWA Hiroyuki wrote:
> On Wed, 4 Feb 2009 10:39:19 -0500 (EST)
> Christoph Lameter <cl@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> > On Wed, 4 Feb 2009, KAMEZAWA Hiroyuki wrote:
> >
> > > mmap_sem can be released after page table walk ends.
> >
> > No. read lock on mmap_sem must be held since the migrate functions
> > manipulate page table entries. Concurrent large scale changes to the 
> > page
> > tables (splitting vmas, remapping etc) must not be possible.
> Just for clarification:
>
> 1. changes in page table is not problem from the viewpoint of kernel.
>   (means no panic, no leak,...)
> 2. But this loses "atomic" aspect of migration and will allow unexpected
>   behaviors.
>   (means the page-mapping status after sys_move may not be what user 
> expects.)
>
>
> Thanks,
> -Kame
But I can't understand how user can see different page->mapping , since 
new page->mapping still holds the anon_vma pointer which should still 
contain the changes in the vma list( due to split vma etc). But, 
considering it as a problem how is it avoided in case of hotremove?



--Swamy


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